handbook of stable isotope analytical techniques || sulfur

C H A P T E R 8

SULFUR

8-0. INTRODUCTION

Aston (1942) reported three sulfur isotopes: 32S, 33S and 34S. Nier (1938), by examining the

existence of all isotopes of sulfur, found the fourth isotope, 36S, by using SO2 gas for

measurement in an MS (SO2þ, SOþ and Sþ ions were considered; contribution on the mass

of SO2þ and SOþ by 18O, 17O and 16O was acknowledged).

A review on sulfur isotope analytical techniques in a very complete range of sample

materials and environments can be found in Volume I, Part 1, Chapter 26. Further, a somewhat

older but extensive review on the sulfur cycle, including sulfur isotopes, was presented by

Goldhaber & Kaplan (1974). Krouse (1980) reviewed sulfur in the environment.

Sakai (1968) reviewed the sulfur isotopic properties of sulfur compounds in hydro-

thermal processes.

8-0.1. GENERAL CONSIDERATIONS

This chapter is divided into sub-chapters, presenting analytical methods of sulfur

isotope determination on sulfides, native (elemental) sulfur, sulfates, sulfur bound in

organic matter and sulfur contained in metals. Systematics in these chapters are according

to the method of conversion into a gas for MS measurement, either SO2 or SF6, or to

direct methods such as TIMS, SIMS or ICP-MS. Both, off-line and on-line approaches

were presented from early stages on. No division on this factor is made in these

chapters. It was mentioned by Sprinson & Rittenberg (1948) that H2S can be used for

MS measurement, but it is less attractive to use than SO2 (disadvantages: high toxic level

of H2S, correction needed for H/D interference and the chemical reactivity with, for

instance, Fe and O2).

Many methods for preparation of SO2 from a variety of sample materials for stable

isotope analysis were derived from quantitative sulfur analytical procedures. Some examples

of literature references, used for the stable isotope techniques described below, are

Kirshenbaum & Grosse (1950) Pepkowitz & Shirley (1951), Siegfriedt et al. (1951), Agazzi

et al. (1951), Johnson & Nishita (1952), Kirsten (1953), Larsen et al. (1959), Ricke (1960),

Sen Gupta (1963), Zhabina & Volkov (1978) and Helmchen (1979).

Tabatabai (1982) reviewed the quantitative analytical techniques for analyzing different

forms of sulfur in soils. A monograph on preparation and isotope measurement by MS for

621

sulfur isotopes was published by Ustinov & Grinenko (1965; in Russian) and in an internal

report at McMaster University by Rees (undated).

Some analysts (e.g. McKibben & Eldridge, 1989; Rees & Holt, 1991; Rees, undated) prefer

first to convert sulfides into Ag2S, because this compound is easily converted (oxidation) into SO2

with good yields. Some sulfides gave problems (bad yields) in direct combustion or produce

vapors of elements such as As and Sb, which sublimate into colder parts of the preparation system

during combustion. Arsenopyrite has to be pretreated if analyzed in on-line systems for this reason

(F. Saupe, personal communication 1995). The advantage of conversion of sulfides into Ag2S is

the removal of impurities and silicates from the raw sample materials (Rees & Holt, 1991;

Rees, undated). Conversion of sulfates into SO2 also suffers from low yields. Conversion methods

for sulfates to produce complete yields are discussed in Chapter 8-3.

Soil

Eschka method

Parr Bomb oxidation

K2 CO3-treated filter

Precipitation as BaSO4

Off-line

H2S SO2

BaSO4 BaSO4

Combustion in a flushof oxygen in thepresence of V2O5

Precipitation as CdS ‘Roasting’ over CuO

Precipitation as Ag2S GC separation on Poropac QS

Conversion to SO2 in a silica glass vial in the presence of V2O5

Mass spectrometry Mass spectrometry

Conventional dual-inletcomparison of thesample versus standardsignal

Integration of area ofa standard peak andthe peak of the sample

Calculation of δ 34S value Calculation of δ

34S value

Atmospheric SO2 Plants

On-line

Reduction withKiba reagent

Figure 8-0.1 Chart showingsulfur isotopeanalyticalprocedures (afterGiesemannetal.,1994).

622 Handbook of Stable Isotope Analytical Techniques

Rees (1978, undated) brought forward that SO2 causes a memory effect in the MS by

slow flushing from the inlet line. The use of fluorination techniques, producing SF6 gas,

which is not polar, overcomes this memory problem.

A chart showing the strategy for sulfur isotope analysis of soil samples, plant

samples or atmospheric SO2 by off-line or on-line method is presented in Figure 8-0.1

(Giesemann et al., 1994).

8-0.2. MEASUREMENT OF SULFUR ISOTOPES ON UNSPECIFIEDMATERIALS

Use of inductively ion plasma-quadrupole MS (ICP-QMS) – Takahashi et al. (2002)

presented an abstract on the use of ICP-QMS (quadrupole MS) techniques for the

determination of sulfur isotopes (32S, 33S, 34S). The type of the material studied was not

reported (BEC and DL were named; probably rock samples). Xe was used to reduce

polyatomic ions (flow rate of 0.2 mL/min); He or H2 flow was not used. S isotopic levels of

ppb range were measured and standard deviation of the 34S/32S ratio was about 0.3%.

Table 8-0.1 shows ‘natural abundance’ of sulfur isotopes and contributions by major

background ions to given amu levels.

The measurement of sulfur on 32S and 34S suffers interference with polyatomic ions (see

Table 8-0.2) and with doubly charged ions (64Ni2þ, 64Zn2þ, 68Zn2þ) and further by low ion

Table 8-0.1 Polyatomic ions and sulfur isotope relative contributions to given amu levels bythe ICP-QMS method as presented by Takahashi et al. (2002)

30 amu 31amu 32 amu 33 amu 34 amu 35 amu14N16Oþ 99.39% 0.41% 0.2%16O2

þ 99.52% 0.08% 0.4%16O2H

þ 99.51% 0.09% 0.4%Sþ 94.93% 0.73% 4.29%

Table 8-0.2 Spectral interferences for sulfur isotopes at m/z 32 and 34 measured by ICP-MS(after Prohaska et al., 1999)

Isotope Abundance% Interference m/Dm

32S 95.0 16O–16O 180114N–18O 106115N–16O–1H 104014N–16O–1H–1H 770

34S 4.2 33S–1H 297732S–1H–1H 171116O–18O 129716O–17O–1H 100016O–16O–1H–1H 904

Sulfur 623

formation yield. Measurement of sulfur isotopes by ICP-MS usually is applied on 32S16Oþ

and 34S16Oþ ions at high mass resolution, up to m/Dm = 10,000 (Prohaska et al., 1999).

Use of multi-collector-ICP-MS (MC-ICP-MS) – Procedures for accurate and precise sulfur

isotope analysis by MC-ICP-MS were recently described by Clough et al. (2006) and

Mason et al. (2006). A two-standard bracketing procedure for precise analysis of sulfur

isotopic analysis by MC-ICP-MS was described by Santamaria-Fernandez & Hearn (2008).

8-0.3. TIMS METHODS

Both positive (Paulsen & Kelly, 1984) and negative (Wachsmann & Heumann, 1992)

TIMS methods for measuring sulfur isotopes were described in the literature. Because

Paulsen & Kelly (1984) concentrated on sulfur in metals, the TIMS method is discussed in

Chapter 8-5. Although Wachsmann & Heumann (1992) did not specify the source of sulfur

(they described the sulfur-containing compounds they loaded on the filament, without

discussion on preparation from the original sulfur-containing sample), their presentation of

the negative TIMS technique is reported in Chapter 8-5 for practical reasons.


8-1. SULFIDES

8-1.1. COMBUSTION METHODS

8-1.1.1. O2 combustion

Thode et al. (1949, 1953) shortly described the use of a stream of O2 for combustion of

sulfides (and native S) into SO2 for MS analysis. The O2 used for combustion was passed

through activated charcoal at dry ice temperature and concentrated sulfuric acid to remove

any moisture and hydrocarbon contamination. Thode et al. (1953) dried the SO2 over P2O5.

CO2 was separated from SO2 by increasing the temperature of the frozen (liquid

nitrogen) SO2 to �85�C (dry ice and acetone mixture) and by pumping.1

Rafter (1957a, 1965) presented a device for the combustion of sulfides into SO2 gas

(Rafter combusted Ag2S, obtained from sulfate reduction followed by sulfide precipitation)

(Figure 8-1.1). A sulfide sample (0.03–0.04 g Ag2S in Rafter’s case) was placed in a

microcombustion boat, which was placed in the silica glass combustion tube. The sample

Sample tubes

Vacuum pumps

C

P2O5 trap

Silica geltrap

gm

FurnaceMagnet

O2 cylinder

tc

sb

P2O5 trap

Figure 8-1.1 Device for sulfide combustion by a stream of air (after Rafter, 1957a). C=SO2collecting trap; gm= gas flow manometer (can be used as inlet for glassblowing); sb= sampleboat; tc= thermocouple.

1 Note that this temperature of �85�C will release both CO2 and SO2! Although this temperature is given for

separation of the two gases in Thode et al., (1949), it must be in error. In Thode et al., (1953) it was described that

at dry ice temperature the SO2 would stay frozen and CO2 was removed by pumping, with a vapor pressure of

0.5 mm of Hg (0.067 hPa) at dry ice temperature for SO2–it was claimed that no isotopic fractionation was caused

by pumping small SO2 fractions (!). Both B. Mayer and P. de Groot place question marks by the described

procedure.

Sulfur 625

boat was pushed by a glass-coated metal piece (placed behind the boat) and by a magnet

from the outside near the entrance of the furnace, while the combustion tube end was

closed with a rubber bung. The system was evacuated and oxygen from the cylinder was

flushed at a rate of 100 mL/min. Oxygen flow was monitored by bubbling in a beaker with

water. When the furnace was at 600�C, a dewar with liquid nitrogen was placed over the

SO2 collecting trap (C in Figure 8-1.1), and the sample boat was pushed into the hot spot of

the furnace by the magnet. The furnace temperature was increased to 1050�–1100�C for

combustion (5 min). The furnace was turned off, and oxygen was flushed for 3 more min.

SO2 was frozen from trap C into one of the break seal tubes by liquid nitrogen and was

sealed. SO2 was measured for its sulfur isotopic composition by crushing the break seal tube

(see Appendix C5) when connected to an MS.

The microcombustion boats were cleaned by boiling them in strong nitric acid,

followed by boiling in several changes of distilled water, dried in a stove and heated to

800�C in a furnace under an oxygen flow. The boats were stored dust free in petri dishes.

If CO2 volume mixed with the SO2 was large, sulfur was recovered as Ag2S after acid

treatment, or converted into sulfate (by ‘Na2O2-fritting’) and precipitated as BaSO4

(Rafter, 1965).

Thode et al. (1961) described the oxidation of FeS (troilite) in meteorites by a stream of

O2. A sulfide sample was placed in a quartz boat and burnt in a stream of O2 inside a silica

glass tube at 1350�C. O2 was purified by passage through ascardite (NaOH on an asbestos

bearer), anhydrone (MgClO4) and concentrated sulfuric acid. Hydrocarbons were elimi-

nated by passing the O2 through a tube filled with zircon at 1350�C. SO2 formed by the

oxidation was collected in an ampoule by freezing and was measured at an MS.

Oana & Ishikawa (1966) gave a short description of their SO2 extraction system for

sulfides and native sulfur (see Chapter 8-2) (Figure 8-1.2). Samples were combusted in a

stream of CO2-free air at 1 atm and 1250�C. SO2 was purified cryogenically.

Oana & Ishikawa (1966) and Mizutani & Oana (1973) separated CO2 from SO2 by

replacing a liquid nitrogen dewar with a dewar with n-pentane slush (melting n-pentane at

�130.8�C; liquid nitrogen cooled), where CO2 sublimed and SO2 stayed frozen in the trap.

This procedure is often referred in literature as a standard procedure. CO2 was pumped away

during its release. A variation on this method is a double-walled trap (Figure 8-1.3), where the

Samplevessel

vacuum

Air

Sample boat

n-pice

d.-i.acetone

Liquidnitrogen

trap

Liquidnitrogen

trap

Figure 8-1.2 S isotope combustion line with air (after Oana & Ishikawa, 1966). d.i.= dry ice;n-p=n-pentane.


outer volume contains n-pentane frozen by liquid nitrogen. When slow heated to the melting

point of n-pentane by removing the liquid nitrogen (reaching the 130.8�C point), CO2

sublimes and SO2 stays frozen.

Kusakabe (2005) described a closed n-pentane trap method. The advantage is that no

n-pentane evaporates and therefore no fire risk also exists as with the open vessel technique.

It was noted that closed traps were used before by others; no detailed description of the

technique existed however.

Filly et al. (1975) used a technique with pure O2 as oxidant for sulfides to produce SO2

for MS measurement. A similar technique was used by Ricke (1960) at the University of

Gottingen, Germany, with V2O5 tested as oxidant (at 1050�C), but was abandoned by Filly

et al. (1975) because very pure sulfides (i.e., without carbonate or hydroxy-bearing

compounds) were required.

A sample (no weight given) was loaded in a Mo boat, inserted in the vacuum system of

an extraction line (see Figure 8-3.4), and was evacuated, degassed and reacted with pure O2

at �1200�C by using a torch. A dry ice trap removed H2O from the SO2, and a second trap

with dimethylbutene slush (at �140�C) collected SO2. Excess O2 and CO2 were not

trapped, and were removed by pumping. The SO2 was collected in a sample vessel and

measured in an MS.

Watanabe (1975) described a combustion system (O2) for sulfides (Ag2S) (Figure 8-1.4).

A sample was placed in a Pt basket in the combustion bottle containing an atmosphere of

O2. O2 was purified in a column containing a molecular sieve cooled by liquid nitrogen

before admission into the combustion system. H2O from the combustion gases was

removed in a P2O5 trap. SO2 and CO2 were separated in a second trap cooled by liquid

nitrogen and excess O2 was pumped away (until pressure <1� 10�3 mmHg). CO2 was

liberated from an SO2–CO2 mixture by swapping the liquid nitrogen with an isobutanol

slush (�108�C). CO2 was removed by pumping.2 SO2 was collected in a sample bottle for

further MS measurement.

Vacuum system

Funnel

n -Pentane

Freezefinger

Figure 8-1.3 Freezing device for separation of CO2 from SO2 for S isotope determination.Variation on the method byOana & Ishikawa (1966).

2 Pumping at the SO2–CO2 mixture, while CO2 is released, has a risk of removing a small portion of the SO2

causing isotopic fractionation in the sulfur isotopic composition (note by PdG).

Sulfur 627

A brief description and review on high temperature combustion of sulfide samples in a

stream of O2 was given by Rees & Holt (1991) and Rees (undated). Samples were placed in

a silica glass boat and, under a flow of O2, were converted at 1350�C into SO2. Avoidance

of SO3 formation and isotopic fractionation between SO3 and SO2 were extensively

discussed by Rees & Holt (1991).

8-1.1.2. Combustion with O2–N2 gas mixtures

Sakai & Yamamoto (1966) used both O2 and an O2–N2 mixture for sulfide oxidation to

SO2. The reason for using an O2–N2 mixture was to decrease violence of the reaction.

Some materials burn violently with a flame if pure O2 is used. The gas mixture was purified

by passing it through ascarite and Mg perchlorate traps. SO2 was collected in liquid nitrogen

traps – the pressure in the system (Figure 8-1.5) must be kept low (�few mmHg) to prevent

O2 freezing in the traps. CO2 was removed from the SO2 by liberation at the melting point

of n-pentane (�131�C) by the method of Oana & Ishikawa (1966).

Molecularsieve

column

O2

Pt basket

Oxygencombustion

bottleP2O5trap

Pump

Samplingbottle

Figure 8-1.4 Device for combustion of Ag2S to SO2 in an atmosphere of O2 (afterWatanabe,1975).

Furnace withMo sample boat

O2

To pumps

Sampleampoule

Tovacuumpumps

Figure 8-1.5 Vacuum device for preparation and purification of SO2 from sulfides by O2combustion (after Sakai & Yamamoto,1966).


Yields with this method were �98%. However, yields decreased rapidly (as low as 60%)

after continuous use of the same silica glass tube in the furnace, probably caused by

devitrification. Precision was generally better than –0.2‰.

The use of a mixture of O2–N2 for high-temperature combustion of sulfides was also

mentioned by Rees & Holt (1991).

Repeatabilities of methods as described in this section were reported to range from 0.5

to 0.2‰.

8-1.1.3. Air-O2 combustion

Kiyosu (1973) used O2 from a stream of CO2-free air to oxidize Ag2S at 1200�C to

SO2. The sulfides (ZnS and PbS were named) were reacted with 6 N HCl to evolve

H2S, which was collected as CdS (Cd-acetate solution) and converted into Ag2S for

analysis.

8-1.1.4. O2 combustion + tungstic anhydride–Cu oxide-reducedCu reactor in an elemental analyzer

Giesemann et al. (1992) combusted Ag2S or BaSO4 samples in an elemental analyzer (EA)

[comparable setup with Figure 8-1.6 Pichlmayer & Blochberger (1988) already described

the coupling of EA with IRMS for S isotope analysis] in a flush of O2. The gases, carried by

a stream of He (flow 60 mL/min), were passed through a reactor filled with tungstic

anhydride þ CuO þ Cu. The produced SO3 was decomposed by passing the gases over

CuO and Cu, while Cu also traps surplus of O2. An anhydrone (MgClO4) trap removed

Elemental analyzer

Referencegas

Wasteline

IRMSTCD

GCcolumn

H2O trap

Combustion/reductionreactor

He-carrier gasO2 pulse

Autosampler

Opensplit

Figure 8-1.6 EA device for on-line S isotope analyses on sulfide and sulfate samples in Zncapsules withV2O5 oxidator (after Giesemann et al., 1994).

Sulfur 629

water traces. Finally, SO2 was separated from the other gases by GC (Poropak QS column;

80–100 mesh; use of silica gel was reported by Shanks et al., 1998). By an open split, 0.1%

of the gas produced by combustion, leaving the EA, was sent into the ion source of the

IRMS (m/z 64þ 65 and 66 were measured). Linearity of the ion source and collectors and

pressure dependence related to the amount of sulfur analyzed and isotopic values were

tested. The aim was to develop a system for very small amounts of sulfur (�20 mg).

Replicate analyses gave reproducible results within –0.2‰.

Comparison between off-line (Kiba reduction method for sulfates; V2O5 combustion

for sulfides), and this on-line method shows strikingly different d34S values. An excellent

linear (nonparallel) correlation was found.

This method was discontinued after the introduction of better techniques (e.g.

Giesemann et al. 1994; personal communication Giesemann, 1999: see Section 8-1.2.4).

8-1.1.5. Considerations on O2 combustion methods

Hagerman & Faust (1955) reported that combustion of inorganic materials (highly resistant

to pyrolysis decomposition) by oxygen from air failed to provide a satisfactory preparation

method for sulfur isotopic determination. The method was inadequate for refractory sulfide

minerals.

Rafter (1957a, c) analyzed sulfur compounds by converting them into BaSO4,

reduced the BaSO4 into sulfide, and converted the sulfide into Ag2S. Ag2S is the most

stable sulfide considering oxidation and is extremely insoluble (hence easily filtered from a

solution).

Rafter (1957c) described the decomposition of Ag2S in stages by increasing heating

temperature, and the formation of Ag2SO4 in oxidizing conditions. Complete decomposi-

tion of Ag2S and Ag2SO4 occurs above 1085�C.

Lead sulfide (galena) was reported (Rafter, 1957c) to give low yields, because of the

formation of a quantitative volume of PbSO4 during oxidation of the sulfide.

Robinson & Kusakabe (1975) reported that at atmospheric pressure in a stream of

oxygen the complete combustion of Ag2S requires a temperature of >1000�C, leading to

production of 5% SO3 and 95% SO2.

8-1.2. OXIDATION METHODS

8-1.2.1. PbO oxidation

Nord & Billstrom (1982) and Ueda & Krouse (1986) reported the use of PbO as oxidant in

a sulfide oxidation system, producing SO2, as was used by Vinogradov et al. (1956; in

Russian). Rees (undated), Ricke (1964) and Rees & Holt (1991) refer to the same work

and report a reaction temperature of 850–900�C and the use of a vacuum system. Ricke

(1964) reported the reaction equation

3FeS2þ 16PbO! 6SO2þ Fe3O4þ 16Pb [8-1.1]


Ricke (1964) mentioned that Vinogradov et al. (1956) purified the PbO–sulfide mixture by

heating in a furnace at 90�C. Higher temperatures caused isotopic fractionation.

The use of PbO was abandoned because of the harmful nature of Pb and the difficulty in

removing Pb from the silica glass vessel walls (if the combustion tubes were re-used). No

further details were found for this method.

8-1.2.2. V2O5 oxidation

Hagerman & Faust (1955) presented a method for sulfide oxidation/combustion with V2O5

as oxidizing agent together with a stream of air. A sulfide sample (0.3–0.6 g) was placed on a

layer of silica and V2O5 in a ceramic combustion boat and coated by another layer of V2O5.

The sample boat was placed in the combustion tube and pushed to within 50 mm from the

combustion furnace, which was heated at 900–950�C. A burner, adjusted to a temperature

of 900–950�C, was slowly moved from behind the sample boat toward the furnace in

25–30 min time period for total oxidation. SO2 was produced and, in the Hagerman &

Faust (1955) system, trapped as sulfuric acid for quantitative sulfur determination. For sulfur

isotopic analysis, SO2 has to be trapped, purified in the usual (cryogenic) way and measured

for its isotopic composition on an MS.

Gavelin et al. (1960) used a similar system. They mentioned the coproduction of CO2,

nitrogen compounds, chlorine compounds and small amounts of SO3 in commercially

obtained V2O5. To purify V2O5, it was heated at 620–640�C for �75 h. CdS was

converted to SO2 in a mixture of 1 part CdS to 4 parts V2O5. Of this mixture, 0.1 g was

placed in a Pyrex glass tube, evacuated on a vacuum system and heated by a Bunsen flame at

�600�C.The liberated SO2 was measured using an MS.

Ricke (1964) heated sulfide samples (0.14 mmoles) and portions of V2O5

(0.44 mmoles) separately under vacuum for 12 h at 400�C to eliminate contamination

(e.g. water). After cooling and addition of N2, a portion of sulfide was mixed (in a glove

box under N2 atmosphere) with a portion of V2O5 in a silica glass ampoule, after which

the ampoule was closed by fusion. Further procedure was very similar to the above

described ones.

The reaction process between sulfides and V2O5 can be presented as

MeSþ 3V2O5 ! MeOþ SO2þ 3V2O4 [8-1.2]

Schneider (1970) used a similar method by mixing samples with V2O5 and (after evacuation

at 150�C for 12 h) reaction at 1000�C.

Schneider (1970) discussed possible errors in his procedure for extracting sulfide as

CdS from rock powders. Also, extraction time has to be limited. If CdS stays in contact

with the Cd-formate solution, used for precipitation of CdS from H2S gas, for too long

a time, formation of an unknown compound, which cannot be removed by washing,

heating or evacuation, influences oxidation of the CdS by formation of significant CO2

quantities.

Giesemann et al. (1992) used a V2O5 oxidation technique for Ag2S recovered (by

Cd-acetate precipitation of H2S) from sulfates or sulfides. Dried Ag2S was sealed with V2O5

in evacuated silica glass vials and oxidized at 1000�C for 90 s in a muffle furnace.

Sulfur 631

8-1.2.3. V2O5 + SiO2 oxidation

Ueda & Krouse (1986) used V2O5 in combination with SiO2 in an extraction system similar

to that described by Yanagisawa & Sakai (1983; method for thermal decomposition of

BaSO4 – see Chapter 8-3.11) to convert sulfides and sulfates into SO2.

Sulfide samples (equivalent to 20 mmol S) were ground with a mixture of 50 mg V2O5

and 50 mg SiO2, placed at the bottom of a silica glass tube and covered by a quartz wool

plug. The mixture, connected to the vacuum system, was heated to 950�C for 15 min and

the SO2 was collected continuously in a cold trap. Sulfides start to oxidize to SO2 from

750�C and reach a maximum at 950�C. If sulfide mixtures were analyzed, time separate

maxima for the different phases at fixed temperatures were obtained. After reaction, the

SO2 was purified from CO2 and H2O by cryogenic methods. Precision of the method was

better than –0.2‰ (both for sulfides and sulfates) for pure samples.

Han et al. (2002) presented an improved V2O5þ SiO2 reaction method for sulfur

isotope determination on sulfide and sulfate samples. A glass stopcock with Teflon plug

and fitted with O-rings for vacuum conditions is closed at the bottom part in such a way it

forms a closed vessel (Figure 8-1.7). A second tube, closed at one end and with a smaller

diameter than the stopcock vessel inner diameter, contains, starting from the bottom side, a

mixture of V2O5þ SiO2þ sulfide or sulfate sample (in 10:10:1 weight ratio; sample weight

between 20 and 40 mg), a plug of quartz wool and finally a plug of Cu turnings on top of all.

The stopcock with reaction tube was evacuated ( to �6 hPa) and the part of the stopcock

with the reaction tube was placed in a tube furnace and heated at 450�C for 30 min to

further degas the device. The stopcock vessel was closed from the pumping system (vacuum

line) and temperature was raised to 850�C for 30 min for reaction. The stopcock vessel was

Teflon plug

Copper turnings

Quartz wool

Sample mixture(V2O5 + SiO2 + sample)

O-ring

O-ring

O-ring

Figure 8-1.7 Schematic diagram of the combined stopcock ^ reaction vessel device for sulfurisotopic analysis of sulfides and sulfates (after Han et al., 2002).


allowed to cool down to room temperature and the SO2 formed by the reaction was

transferred from the stopcock vessel into the inlet system of an MS. The stopcock vessel can

be used repeatedly after being washed with HCl and distilled water. If CO2 was also formed

with the SO2, these gases can be separated by cryogenic way or by GC. A reproducibility

better than 0.10‰ was reported for this method.

8-1.2.4. V2O5 in an elemental analyzer

Giesemann et al. (1994) described an ‘on-line’ EA system for S isotopes analyses on sulfide

and sulfate samples (Figure 8-1.6). A sample was wrapped with 0.1 mg V2O5 in a Zn

capsule, and thermally decomposed and oxidized at 1000�C (this is furnace temperature;

flash combustion yields considerably higher temperatures for short periods) in a 5-mL pulse

of O2 in an EA (Carlo Erba NA 1500). A He flow (60 mL/min) carried the gases obtained

by the reaction through an oxidation/reduction reactor filled with tungsten anhydride,

copper oxide and reduced copper. The CuO and reduced Cu reduced traces of SO3 into

SO2 and removed all surplus O2 from the gas stream. Water vapor was trapped by

anhydrous Mg(ClO4)2, and the gases were sent through a GC column filled with Poropak

QS (at 80�C) – CO2 and N2 were separated from SO2. An open-split interface and a

capillary transferred 1% of the gas to the ion source of an IRMS for isotopic measurement

(MS was differentially pumped). SO2 was measured on the 64 and 66 masses. Pulses of

reference gas (bottle gas) were introduced between the EA and the open-split interface.

A similar approach was reported by Grassineau et al. (1998) by describing flash

combustion of sulfide samples packed in Sn capsules in an oxidative environment. No

particulars were given by Grassineau et al. (1998) on the oxygen donor compound in the

reactor, but the use of VG-Isochrom-EA 1500 Series N-C-S device was mentioned.

Reaction took place by ‘flash combustion’ (furnace set at 1050�C), followed by reduction

on Cu (e.g. to catch remaining O2 or to reduce N-oxides), and the effluent gases were

carried through a GC by a He flow. Total analytical time for a sample was only 400 s.

At the end of a daily run (after 60–70 samples) a small ‘saturation peak’3 was noticed and

was observed earlier each day as the Cu reactor became more saturated, causing a 34S

enrichment in reported d34S values. Flushing with He between runs for �5–6 h erased the

effect and increased the Cu column’s life. For daily calibration, reference material values

having a reproducibility of –0.2‰ were used. Sample size for marcasite and pyrite is

1–1.2 mg, for chalcopyrite, pyrrhotite and sphalerite 1.6–1.8 mg and for galena 3.5–4 mg,

all corresponding to 15–20 hPa of SO2 gas.

The method was improved and can be considered a routine-based technique; see

Grassineau et al. (2001) in section 8-1.2.7 for a description.

8-1.2.5. CuO oxidation

Grinenko (1962; mentioned in Ueda & Krouse, 1986; Rees & Holt, 1991) used CuO as

oxidant for sulfides to produce SO2 (Figure 8-1.8). No further particulars can be given

because the paper is in Russian.

3 By ‘saturation peak’ is meant a peak resulting from material depositing over time in the reactor during the analysis

procedure and causing a memory effect (note by PdG).

Sulfur 633

Monster (1973) made a comparison between CuO and Cu2O as oxidants for sulfides

(and compared these results with the O2 gas oxidation method). He used a device as is

shown in Figure 8-1.9 for oxidation with both oxidants.

A thorough mixture of CuO and sulfide (all sulfides were converted into Ag2S by

Monster) with weight ratio of 1:4.5 to 1:6 was made. The sample was placed in a silica glass

‘combustion boat’ (tube: 40 mm long, 9 mm o.d., one end closed); a small plug of quartz wool

(degreased at high temperature in a muffle furnace) keeps the sample in place. The ‘combus-

tion boat’ was placed in a glass tube connected to the vacuum device of Figure 8-1.8 and was

evacuated and degassed. After 4 min the helical trap was isolated and cooled by liquid

nitrogen, and the sample boat was pushed into the furnace for oxidation (at 850–860�C),

and the produced gas was trapped cryogenically. After 8 min the stopcock to the cold traps for

cryogenic purification of the SO2 (of CO2) was opened. The purified SO2 was measured for

yield and was collected in a break seal for measurement on an MS. For the use of Cu2O as

oxidant, see Section 8-1.2.6. Monster (1973) did not see any advantage in high combustion

temperatures as are needed for Cu2O and reported problems of reaching complete yield at the

range of temperatures (850–860�C) which he used for oxidation.

Furnace

Cold trapVacuum pumps

Sample tube

Boat withsample

Figure 8-1.8 CuOcombustion line for sulfides (after Grinenko,1962).

Cold trap

SO2from

furnace

Break seal manifold

c.v.

Cold traps

Figure 8-1.9 Vacuum device for extraction of SO2 from sulfides by CuO and Cu2Ocombustion (afterMonster, 1973). c.v.= calibrated volume.


CO2 was produced during the combustion as a minor by-product. If the sample size is

decreased significantly, the CO2 volume is not similarly lowered in size, and the SO2/CO2

ratio may decrease considerably because of the considerably lower SO2 volume. Monster

(1973) stated that water in the extraction system must be avoided as much as possible.

Monster (1973) proposed to flame the whole extraction system (under vacuum) before each

extraction was carried out.

The most likely reaction of CuO with Ag2S is given by the reaction

4CuOþ Ag2S! SO2þ 2Cu2Oþ 2Ag [8-1.3]

Fritz et al. (1974) used CuO, instead of the toxic V2O5, for combustion of sulfides

producing SO2 for MS measurement (Figure 8-1.10). Samples were well mixed with

copper oxide (100–200 mg; at oxygen – sulfur ratio of 2:1 to 5:1) and loaded in an open-

ended quartz tube, with quartz wool plugs at both ends to keep the sample in its place. The

quartz tube with the sample þ oxide mixture was brought into the system, evacuated and

pushed into the combustion furnace (�1000�C) by a magnetic rod. SO2 was trapped by

liquid nitrogen during the 3 min combustion. Excess oxygen was pumped away. SO2 was

distilled over to a next trap, cooled by liquid nitrogen, by replacing the liquid nitrogen at

the first trap by a cooling mixture at �40�C. A mixture of SO2 and CO2, collected this

way, was measured for total yield.4 CO2 was separated from the SO2 by vaporizing the

CO2 by placing a dewar with an ethanol–liquid nitrogen slush. CO2 was pumped away (if

still in contact with the frozen SO2, the level of the dewar with the slush should be raised to

avoid pumping away SO2). CuO from commercial suppliers (Analar grade) is sufficiently

pure to be used as it is, without preparation.5

Westgate & Anderson (1982) slightly modified the method by Fritz et al. (1974).

Samples, all in the form of Ag2S, were prepared by mixing 25 mg of sulfide with 50 mg

of CuO. This mixture was placed in a silica glass boat and placed in a silica glass tube at the

extraction line as is shown in Figure 8-1.11. The extraction line was evacuated and the

Magnetic rod 1st trap2nd trap

Sampletube

Highvacuum

Highvacuum

PrimarypumpPrimary

pump

CuO +sample

Microcombustionfurnace

Figure 8-1.10 Combustion device for sulfides to sulfur dioxide for sulfur isotopemeasurement (after Fritz et al., 1974).

4 No explanation is given by Fritz et al. (1974) why they include CO2 in the yield measurement. PdG and a

reviewer (B. Mayer) would measure yield after purification of SO2.5 Adsorbed water from air should be avoided as much as possible, for example, by storage in a desiccator

(note by PdG).

Sulfur 635

mixture combusted at 950�C for 8 min. Water was removed by an isopropyl alcohol/dry

ice slurry and CO2 was removed by an ethanol/liquid nitrogen slurry at �125�C. The SO2

yield was measured and SO2 was transferred for the S-isotopic analysis on a MS.

8-1.2.6. Cu2O oxidation

Kaplan et al. (1970) reported the use of Cu2O as oxidant for sulfur isotope analysis on

sulfides (Ag2S). Cu2O was produced from CuO by heating at 800�C in ‘partial

vacuum’.

Monster (1973) tested sulfide combustion for S isotope determination with both

CuO and Cu2O. The procedure for Cu2O combustion was similar to the procedure

such as described for the CuO in Section 8-1.2.5, with the only difference being the use

of a higher combustion temperature (range of 860–980�C). Monster (1973) reported that

Cu2O slowly oxidized to CuO under atmospheric conditions (no long storage periods of

Cu2O was advised). No pure Cu2O is obtainable from commercial companies – pine oil

(up to 0.3%) is generally added to the Cu2O as ‘preservative’ against oxidation [see also

Fritz et al. (1974), Robinson & Kusakabe (1975) and Rees (undated), about oil

contamination]. The most likely reaction to occur for combustion of Ag2S with

Cu2O is given by the reaction

2Cu2Oþ Ag2S! SO2þ 2Agþ 4 Cu [8-1.4]

In a comparison of CuO and Cu2O as oxidants for combustion of Ag2S, presented in a

graph of temperature versus %-yield SO2 (Figure 8-1.12), it is shown that to obtain a

complete SO2 yield Cu2O requires a higher combustion temperature than CuO.

Fritz et al. (1974) also tested Cu2O as oxidant for their sulfide combustion method (see

description above). They concluded that Cu2O obtained from commercial suppliers con-

tains up to 0.3% oil, and thus gives an unwanted large contamination of H2O and CO2.

Moveable furnace

Silica glass boat LV

LVHV

Cold traps

To manometer

Sampletube

Cu wool

Figure 8-1.11 Sulfide combustion device (after Westgate & Anderson, 1982). HV= highvacuum; LV= low vacuum.


If sample gas can be cleaned from this contamination first, then the Cu2O is considered as

useful as CuO by Fritz et al. (1974).

Robinson & Kusakabe (1975) preferred the use of Cu2O above CuO (after Grinenko,

1962: in Russian) for sulfide oxidation. They stressed that production of SO3 or sulfates

must be avoided in any method by use of Cu2O. Equilibrium isotopic fractionation

between SO2 and SO3 causes a small depletion of 34S (theoretically about 2–4‰ at

1100�C) in the SO2 (Thode et al., 1961; Sakai & Yamamoto, 1966; Robinson & Kusakabe,

1975). Cu2O must be prepared from CuO (as mentioned above: commercial Cu2O

contains oil contamination which cannot be removed totally) (Kaplan et al., 1970; Robinson

& Kusakabe, 1975). Cu2O is the stable phase of copper oxide at 900�C and a PO2 between

10�7.5 and 10�2 bar (Robinson & Kusakabe, 1975; see their Figure 1 p. 1180). Cu2O can be

produced from Analar grade CuO by heating it at a temperature of about 800–900�Cunder vacuum for at least 2 h (Robinson & Kusakabe, 1975; Schoenau & Bettany, 1988).

Approximately twice the stoichiometric amount of Cu2O for a sulfide mineral is necessary to

avoid incomplete yields of SO2.

Normally a small volume of CO2 is present in the SO2-containing gas mixture,

originating from carbon impurities in the sample and/or absorbed gas on the sample and

Cu2O (Robinson & Kusakabe, 1975), and from the silica glass (see Appendix C14). Any

CO2 can be removed effectively from the SO2 by use of a n-pentane liquid nitrogen trap

(see Oana & Ishikawa, 1966). Further procedure is comparable with that of Fritz et al.

(1974) as described above.

A similar device, compared with the ones described above, was published by Nord &

Billstrom (1982) (Figure 8-1.13). A weighed amount of sulfide sample (equivalent to 5 mg

of sulfur) was mixed with 200 mg Cu2O for combustion, resulting into �160 mmole SO2

gas. Smaller amounts of sample may lead to lower precision.

All silica glass material has been heated to 1000�C and then treated with HF acid to

remove traces of carbon. The gas produced by the combustion process passed a first cold

CuO

Cu2O

100

90

80

70

60

50

40

30

20

10

400 500 600 700 800 900 1000°C%

Figure 8-1.12 Relationshipbetween SO2 yield (in%) and temperature (�C) for combustion ofsulfides with CuO and Cu2O as oxidants (after a sketch byMonster, 1973).

Sulfur 637

trap cooled by ethanol/dry ice to remove any water from the gas. A n-pentane trap was

used to remove CO2 from the SO2. Samples with yields <95% were discarded. Most parts

of the glass system were heated to 70�C to reduce absorption of SO2 to the walls, and to

improve the flow speed of the SO2.

For the conversion of H2S into SO2 by a Cu2O furnace (Chapter 8-3.7), see Ueda &

Sakai (1983).

8-1.2.7. WO3 + Al2O3 oxidation

Grassineau et al. (2001) oxidized sulfide (and sulfate) samples (powder to coarse grain size)

in a reactor, packed with WO3þ Al2O3 and pure Cu wires, at 1030�C and flushed with He

(80–120 mL/min). Before starting analysis, the system was stabilized for 1 h. Samples were

weighed in Sn capsules, crimped and dropped into the oxidation furnace (having an oxygen

atmosphere). Excess of O2 was removed by the Cu, and reaction products were separated

by chromatography: SO2, CO2, N2 (all separated in 0.8-m PTFE GC column packed with

Porapak 50–80 mesh at 50�C), H2O (trapped in 10-mL column with Mg-perchlorate).

Cycle time was 450 s; arrival time of SO2 was 180 s and peak width was 200 s. Standards

were analyzed in a similar way, ‘bracketing’ unknown samples. Reference gas pulses

(30 s pulse) of tank SO2 were introduced at the open split for comparison with the sample

and reference SO2. Memory effects of SO2 were negligible, because of He flow which has a

‘cleaning’ effect. Regular calibration was needed for correcting machine drift. Oxygen

isotope contribution to mass 66 must also be corrected for (see Volume I, Part 2, Chapters 44

and 45).

Saturation of the Cu by the O2 was causing a saturation peak at the end of the

measuring cycle each day, an effect that appears earlier when the Cu reactor is becoming

older. Flushing with He for 6 h between runs erases this effect and increases the expected

life of the Cu reactor. SO2 was measured (on m/z = 64 and 66) with a typical precision

(repeatability!) of –0.1‰.

8-1.2.8. Oxidation without specification of agent

Carmody et al. (1998) described a method for oxidation (no specification on oxidation

agent was given, probably Cu2O) of sulfide and sulfate samples (extracted from sulfur-

Furnacehv

fv

wtpt

hvhvhvhv

fvfvfvfv

SamplecontainerGlass

filter

SampleCu2O

mixture

Figure 8-1.13 Device for sulfide combustionwith Cu2O to produce SO2 forMSmeasurement(after Nord & Billstro« m, 1982). fv= to fore vacuum pump; hv= to high vacuum pump;pt= n-pentane trap; wt=water trap.


containing compounds in water samples). Details on the method are given in Chapter

8-3.11 Deviations from the ‘sulfate-system’ were omitting of the Cu wire in the reaction

tube, a lower reaction temperature (1050�C instead of 1150�C) and a shorter reaction time

(10 instead of 15 min).

8-1.3. KIBA REAGENT REDUCTION – ‘STRONG’ PHOSPHORICACID WITH Sn(II)

Pyrite-containing samples (e.g. whole rock samples, sediments and soils; see

Chapter 8-3.7) can be reacted with the Kiba reagent [phosphoric acid with Sn(II); Ueda &

Sakai, 1983]. It is important to have more than 1/20 of the original 100 mg tin(II) chloride

dihydrate in the reagent–preferably between 1/10 and 1/5 of the original volume for complete

reaction. Sulfur is released as H2S by this reaction, precipitated as sulfide (e.g. Ag2S, CdS or

ZnS) and must be converted into SO2, or less commonly into SF6, for MS measurement.

Ueda & Sakai (1983) do this by reaction in a Cu2O furnace in two steps, as is described

in Chapter 8-3.7.

Ueda et al. (1991) used Kiba reagent to react under vacuum with 2–7 g of powdered

sample at 90�C for 1 day to partially remove carbonate–CO2. The sample, representing a

mixture of sulfide and sulfate, was then heated to 280�C for 1 h to extract H2S from sulfides

and SO2 from sulfates. Under the strongly reducing conditions, up to 5% of the sulfate

may be reduced to H2S, and therefore the total H2S formed may be depleted in 34S up to

5‰, compared with the parent sulfate. After separation of H2S and SO2, the H2S, after

being precipitated as Ag2S, was oxidized to SO2 by reaction with Cu2O (uncertainty on

yield –5%).

Bottomley et al. (1992) used two batches of Kiba reagent for samples containing

carbonate minerals to prevent excessive foaming. A first portion of Kiba reagent was reacted

with the sample at 90�C for �1 h to dissolve most of the carbonate. Then a second portion

was added to liberate the sulfur from the sulfide (at 280�C for 1 h). H2S liberated by this

process was swept through a Cd-acetate trap to deposit CdS, which was converted into

Ag2S.

8-1.4. JOHNSON–NISHITA REDUCTION–DISTILLATIONAPPARATUS

Schoenau & Bettany (1988) used a procedure similar to those of Fritz et al. (1974) and

Robinson & Kusakabe (1975) for generating SO2 from Ag2S produced from soil samples by

a Johnson–Nishita reduction–distillation apparatus (Figure 8-1.14). Precision6 of this system

was given as a standard deviation of 0.18‰.

6 What is meant here is repeatability of the procedure. No propagation of standard uncertainties or eventual other

uncertainties related to corrections or preparations are included (note by PdG and B. Mayer).

Sulfur 639

8-1.5. REDUCTION WITH CrCl2 SOLUTION

Newton et al. (1995), following Canfield et al. (1986; see Chapter 13-1.5), reacted

pyrite samples in a sealed vessel under a continuous flow of pure N2, with 40 mL of 1 M

CrCl2 (acidified with 10% HCl) and 20 mL of concentrated HCl according to the

reaction

4Hþþ 2Cr2þ FeS2 ! 2H2Sþ 2Cr3þ Fe2þ [8-1.5]

H2S was trapped either in a 3% Zn-acetate–10% ammonium hydroxide solution or in a

0.1 M of CuCl2–10% ammonium hydroxide solution to form ZnS or a CuS/Cu2S mixture

(and not a stoichiometric CuS precipitate), respectively. The ZnS or CuS/Cu2S mixture

was analyzed for the S isotope composition following the methods described in this chapter.

Tuttle et al. (1986) and Bates et al. (1993) applied Cr(II) reduction on disulfides

(e.g. pyrite, markasite) in oil shales, converting the sulfur in H2S, which was collected as Ag2S.

8-1.6. LiAlH4 REACTION

Smith et al. (1964) reacted rock samples with LiAlH4 to form H2S, which was

collected as Ag2S or CdS. LiAlH4 (1 g) was dissolved in tetrahydrofuran (50 mL). A device

as is shown in Figure 8-1.15 was used for reaction. The sulfide sample was brought into the

reaction device together with the LiAlH4–tetrahydrofuran solution. The reaction bulb was

cooled in an ice bath. Water was added slowly in drops injected through the serum cap and

the solution was stirred continuously (e.g. by magnetic stirrer). After a violent reaction was

subsided (after adding �5 mL water), 50 mL of water was added to the reaction flask. The

Figure 8-1.14 Example of a Johnson^Nishita reduction (digestion) distillation apparatus(after Johnson &Nishita, 1952).


ice bath was removed and 50 mL of 30% perchloric acid was added to the reaction mixture.

To have a complete yield of H2S, the reaction bulb was heated to the boiling point (1/2 h;

stirring constantly). The H2S was trapped in a gas scrubber filled with 250 mL of a

Cd-sulfate solution (prepared with 9 g of 3CdSO4•8H2O dissolved in 250 mL H2O). For

a similar method, see Westgate & Anderson (1982).

8-1.7. FLUORINATION METHODS – SF6

The important advantages of SF6 over SO2 for isotopic measurement are as follows:

• SF6 is non-polar, hydrophobic and chemically inert (rivalling N2) and gives low memory

effects.

• Fluor has only one stable isotope, thus avoiding any correction as needed for the

O isotope composition in SO2 (see Volume I, Part 2, Chapters 44 and 45 for discussion

on oxygen correction for sulfur isotope measurement on SO2).

• SF6 has no problems for MS measurement with interfering molecules having similar masses.

• Besides the 34S/32S ratio, the 33S/32S and 36S/32S ratios can also be measured on SF6 gas

more easily and more precisely (if the collector assay of the MS permits it, last ratios also

can be measured on SO2).

The disadvantage is

• The dangerous chemicals needed for the fluorination reaction. (see Section 8-1.8.3 for

comparison between SO2 and SF6 preparations by laser techniques).

8-1.7.1. BrF3 as fluorination agent

Puchelt et al. (1971) presented a method with BrF3 as fluorination agent (Figure

8-1.16). They poured into a Ni tube, after Baertschi & Silverman (1951), 1–2 mL

BrF3 while flushing with dry N2 and cooling at the bottom with dry ice. A sample,

Reactionflask

Serum cap

Water

Water

N2

Hot plate andmagnetic stirrer

Gas scrubber

Coarse frit

Vacuum

Rubber tubing

Figure 8-1.15 Device forpyrite (sulfide)extractionbyLiAlH4 reaction(afterSmithetal.,1964).

Sulfur 641

containing approximately 10 mg of sulfur, was wrapped in thin Al foil and dropped on

the frozen BrF3. The reaction tube was connected to the vacuum system and evac-

uated. replacing the cold trap by a furnace, the reaction tube was heated overnight at

200�C. The Teflon gasket of the connecting flange between the reaction tube and the

vacuum line was cooled by tap water. The equation for the fluorination reaction of

pyrite is

2FeS2 þ 10BrF3 ! 4SF6þ 2 FeF3þ 5 Br2 [8-1.6]

After reaction, the reaction tube and the Na(OH)-containing traps were cooled by dry

ice and the glass traps by liquid nitrogen. The reaction vessel was slowly opened to the

vacuum system, and products such as HF, HBr and Br2 were trapped by the Na(OH).

SF6 was collected in the first liquid nitrogen traps (this takes some time because SF6

releases slowly at dry ice temperatures, delaying a complete collection in the first liquid

nitrogen traps). The SF6 was further purified by repeated sublimation. A last purification

was done on a GC with molecular sieve (5 A – at 100�C) with a He carrier flow. It was

frozen out of the He stream in a liquid nitrogen trap. The SF6 was collected and

measured by an MS as SF5þ.

8-1.7.2. BrF5 as fluorination agent

Thode & Rees (1971) used BrF5 as fluorination agent on Ag2S samples of �3–7 mg size.

The samples were wrapped in Al foil and reacted with an approximate 20�molar excess of

BrF5 for 16 h at 300�C. SF6 was distilled by a series of dry ice–acetone and liquid nitrogen

cold traps. After the final cold trap, the SF6 was introduced by a He carrier into a GC (0.25

inch o.d. Cu tubing packed with 5 A molecular sieve) to separate any contaminants.

Heþ SF6 was transferred through a liquid nitrogen trap placed after the GC to collect

SF6. He was pumped away. After the Heþ SF6 passed the GC, the gas flow was diverted to

discard any other products. SF6 was transferred to a Pyrex break seal for MS measurement.

Tests for incomplete reaction (second reaction with new batch of BrF5), blanks (reaction on

empty Al foil) or loss of SF6 (second pass through GC for gases normally discarded) were

carried out, resulting in yields �0.1% of original amount of SF6.

SF5þ peaks were measured at an MS for 34S, and some samples for 33S and 36S. Precision

of the method was given to be better than –0.05‰.

H2O

Reactionvessel

Monel trapswith NaOH

Samplevessel

Vacuum

Figure 8-1.16 Device for BrF3^sulfide reaction for S isotope measurement (after Pucheltet al., 1971).


If mass-dependent isotope fractionation takes place, a relationship between 34S, 33S and36S is defined as

D33S ¼ �33S� 0:515 �34S [8-1.7]

�34S ¼ ð1:94 – 0:01Þ �33S ðHulston & Thode; 1965bÞ [8-1.8]

�33S ¼ 0:512 – 0:002 ln �34S ðHoering; 1990Þ [8-1.9]

D36S ¼ �36S� 1:90 �34S [8-1.10]

�36S ¼ ð1:90 – 0:01Þ �34S ðHulston & Thode; 1965bÞ [8-1.11]

for d values of less than 15‰ (Hulston & Thode, 1965b).

Bains-Sahota & Thiemens (1987) showed that the d33S/d34S ratio is dependent on the

reduced mass of specific molecules. Thus different sulfur species have slightly different

values, ranging from 0.501 to 0.518.

Hoering (1990) presented a short review on earlier publications of fluorination techniques

for sulfides. He summarized a method in which sulfur-containing samples were converted into

sulfide, preferably Ag2S. Samples were reacted with BrF5 for 4 h at 400�C in a vacuum extraction

line after Clayton & Mayeda (1963) used for fluorination of silicates (see Chapter 6-1.4/5). SF6

was separated from the other reaction products by cryogenic distillation, chemical trapping and

GC. It was noted that SF6 has a boiling point and melting point similar to CO2. Yields (after

purification) were normally >80% – most losses occurred during the purification steps rather

than by the fluorination reaction. Precision for d34S was better than �= –0.1–0.2‰. High

precision for the 36S/32S ratio was still problematic because of impurities with interfering masses

e.g. C3F5þ overlapping the 36SF5

þ peak (at mass 131).

Gao & Thiemens (1991) used, after fluorination of a sulfide, a single pass (32S, 33S, 34S)

or multiple pass (36S) through a GC column (1/8 in. o.d.; packed with Poropak QS

(80–100 mesh), flushed by a He flow (35 mL/min), for purification of SF6. The column

was baked at 100�C for at least 5 h between samples.

8-1.7.3. F2 as fluorination agent

Hulston & Thode (1965b) used F2 as fluorination agent to convert sulfides into SF6. It was

noted that yields not greater than 75% were obtained with both F2 and BrF3. SF6 was

purified in a GC column, after a method developed by Hoering (personal communication

by Hulston & Thode, 1965b). The column (Cu tubing, 1/4 in. o.d.) was packed with

molecular sieve 5 A (20–40 mesh) with the first half of the column heated to 150�C and the

second half at room temperature.

Leskovsek et al. (1969, 1974) described a method for fluorination (F2) of sulfides

(Figure 8-1.17). A powdered sample was loaded in a Ni reaction vessel and was evacuated.

A specific volume of F2 was introduced by a pressure-dose system. The reaction vessel was

closed and heated at 150�C for reaction. The best result was obtained at 150�C, at F2

overpressure and a 2 h reaction time; the reaction was complete within 20 min (Leskovsek

et al., 1974). After completion of the reaction, the vessel was cooled by liquid nitrogen, and

Sulfur 643

noncondensable gases were pumped away.7 In Leskovsek et al. (1974) excess of fluorine was

absorbed in a soda lime trap. The liquid nitrogen at the reaction vessel was swapped by a

�80�C trap to release SF6. SF6 was concentrated in a trap with KOH or NaOH and was

collected in ampoules for isotopic measurement.

Besides SF6, phases such as SF4 and S2F10 can form, eventually causing an unwanted

isotope fractionation. Tests (by Leskovsek et al., 1974) showed no formation of significant

quantities of these phases during the fluorination process. A range of different sulfide

minerals was tested with this method and only cinnabar was giving unsatisfactory results –

cinnabar needs conversion into Ag2S before fluorination. Precision of the method was

generally better than –0.2‰ for yields >95%; lower yields cause isotopic fractionation.

8-1.8. LASER METHODS

For discussion on laser types and characteristics, see for instance Gibson & Carr

(1989), Fallick et al. (1992), Wiechert & Hoefs (1995), Rumble & Sharp (1998) and Shanks

et al. (1998). Information on lasers in relationship to O isotope analysis, but to a high extent

valid for S isotope analysis too, can be found in Chapter 6-1.5.1.

Specific characteristics related to S isotope analysis are given below in Section 8-1.8.3.

Laser methods are, together with SIMS, the two technological advances with promise for

high-spatial resolution stable isotope analysis. SIMS offers higher spatial resolution

(10–20 mm) with relatively higher uncertainties, while the laser technique spatial resolution

(100–200 mm) is an order larger at precisions very close to that of conventional techniques.

Laser systems, combined with GC-(CF)-IRMS, have a high potential to analyze very

small samples (as small as 20–30 nmoles) at relatively high precision. Laser spot sizes can be

Vacuum

Soda limetrap

Reaction vessel

Rest F2 – soda lime trap

Pyrexsamplevessel

F2 cylinder

Figure 8-1.17 Device for F2 fluorination of sulfides for S isotope measurement on SF6 gas(after Leskovsek et al., 1974).

7 NB: Never pump F2 directly by oil-based pumps without a trap to remove fluor components – fluorination

agents react explosively with oil. Use of fluorinated oil (e.g. Fomblin, Krytos; inert to fluorination agents) in

pumps can avoid accidents (note by PdG).


reduced, GC-techniques can be used to purify the SO2 and potential sample throughput is

relatively high (see Shanks et al., 1998).

8-1.8.1. Oxidation of sulfides with O2 by laser methods

Jones et al. (1987a) gave a very short description of sulfide combustion by a batch O2 and a

Nd-YAG laser as heat source, releasing SO2 gas. The laser was operated at a wavelength of

1.02 mm or 0.532 mm in either continuous or pulsed mode. The system was monitored by

video camera and single grains were reacted by selection of grains with a microscope stage

integrated in the analytical system. The d34S value was measured in an MS without further

treatment of the gas.

A laser device (Nd-YAG IR laser), described in Chapter 4-5.3.2 for the carbonate

ablation method by Smalley et al. (1989), can also be used for S isotope analysis of sulfides.

Sulfides were combusted in an O2 atmosphere (pressure 500–750 hPa) into SO2. A Cu

furnace converted SO3 into SO2. In the same system, SO2 was purified and measured

on-line in an MS.

Crowe et al. (1990), Crowe & Valley (1992) and Crowe & Vaughan (1996) described a

method using a laser microprobe for oxidation of sulfide samples (Figure 8-1.18). In a

vacuum system, a Nd-YAG laser was focused on a sulfide sample in an atmosphere of O2.

A confocal He–Ne laser was used for aiming the invisible Nd-YAG laser beam on the

sample, and a video camera was used to monitor the laser combustion process. The reaction

chamber was of Pyrex with a quartz window for the laser beam. All optical elements in the

laser beam path were ‘v-coated’ for optimal transmission of the beam and reflectivity.

Samples can be illuminated from above (reflected light) and from below (transmitted light).

To MS inlet

Cold fingers

Videocamera

Monitor

Nd-YAG laser

O2

Cryogenic trap

To vacuum To vacuum

Microscopeoculars

Laser beam

Reflected light

Transmitted light

10 × ol

Reactionchamber

To vacuum

Figure 8-1.18 Diagram of theWisconsin laser microprobe system for combustion of sulfideswithO2 (after Crowe et al., 1990; Crowe &Valley,1992). ol=objective lens.

Sulfur 645

A flexible SS tube connection to the vacuum system allowed movement of the reaction

chamber. For S isotope measurement, an MS is connected to the vacuum preparation

system.

Samples were cut into small slabs. The slabs were cleaned in an ultrasonic bath, first with

deionized water, followed by acetone. They were dried in a heated (250�C) vacuum vessel

(<10�3 Torr) until they were degassed (Crowe et al., 1990). Crowe & Valley (1992)

cleaned the slabs in an ultrasonic bath, first with trichloroethylene (5 min), followed by

distilled water (5 min); they dried and degassed them in a vacuum desiccator at 120�C (12–

24 h). The degassed sample slabs were placed in a heated (80�C) reaction chamber and

evacuated. O2 was introduced into the reaction chamber with a PO2of 40–70 Torr. During

laser combustion, the reaction chamber was open to the cryogenic trap (liquid nitrogen) to

remove SO2 continuously.

To combust, the laser was switched on for 1–3 s on continuous mode at 36 A, burning a

pit in the sample (pit diameter: 50–70 mm; Crowe & Vaughan, 1996). Laser combustion,

with time, depends on mineralogy, grain size and laser power setting. No combustion

occurs at <30 A, while at an amperage of 39 A the laser beam has a greater density, which is

undesirable. During the reaction, noncondensable gases formed and excess of O2 was

present, which were slowly pumped off with complete freezing of SO2 in the cryogenic

trap. The cryogenic trap was heated to room temperature and a dry ice/ethanol slurry was

placed on the cryogenic trap for 5 min to separate H2O from the SO2. CO2 was minimal

and caused no necessity to separate it from the SO2. The SO2 yield was measured and SO2

was cryofocused in a cold finger before introduction directly into the inlet of a connected

MS. Crowe & Vaughan (1996) froze the condensable gas in a liquid nitrogen trap and

increased temperature to �10�C to keep water trapped and to release all SO2 – water may

cause undesirable protonated SO2 and may form H2SO4 in the presence of SO2.

Heating (80�C) of the complete extraction device strongly reduced memory by avoid-

ance of absorption of SO2 on the walls of the device (‘sticky gas’; Crowe & Vaughan, 1996.)

If powders were used instead of coarse-grained or slab material, a lower d34S value of

0.3–1.1‰ was obtained for sphalerite, pyrite and galena powders (with similar precision).

d34S values of powders of fine-grained aggregates of pyrrhotite and chalcopyrite, however,

were 0.1–0.5‰ higher compared with similar coarse-grained material. It was stated (Crowe

et al., 1990; Crowe & Vaughan, 1996) that the effect of powdering samples increases the

depletion in 34S of the SO2 produced by the laser, although this is not supported by their

reporting on different effects for various sulfides.

Pressure of O2 in the reaction chamber was another factor of importance: if the PO2is

too low, SO or SO3 ions can be formed, causing isotopic fractionation. It was concluded

that no SO is produced by the laser beam.

Precision of the laser–O2 combustion method was better than –0.14‰ for sphalerite

and pyrrhotite and –0.44‰ for chalcopyrite (1� level).

Fallick et al. (1990) mentioned detailed experiments with Ar ion and Nd-YAG con-

tinuous wave lasers on sulfides in an oxygen atmosphere where SO2 was produced for stable

isotope measurement. The higher spatial resolution of the laser technique results in larger

ranges of isotopic values compared with classical methods on the same samples and may

provide a better constraint on different sulfur sources.

Kelley & Fallick (1990) used an Ar-ion laser (4W continuous power; TEMoo mode

operation; spot ø 100 mm) for S isotope analyses on sulfides. Use of a microscope stage


reduced the intensity of the beam by 25%. The power of the beam was barely sufficient for

sulfide combustion. Samples of 0.5–15 mg were analyzed. The smallest sample SO2 was

frozen in the cold finger volume of an MS, with a size down to 1 mmol of gas. SO2

produced during the laser combustion was purified cryogenically. Results had a standard

deviation of –0.25‰. It was found that d34S corrections must be made, which are mineral

specific. Minerals analyzed were acanthite, cinnabar, chalcopyrite, covellite, galena, molyb-

denite, pyrite, sphalerite and troilite, including interlaboratory standards CP1 (chalcopyrite)

and NZ1 (acanthite). Attempts to analyze elementary sulfur weve not successful (yields of

<0.1%), The majority of the samples sublimed on the walls of the reaction chamber

forming a coating.

Exchange of the O isotopes (important for correction) may occur during the short period

of laser heating. Little or no fractionation was observed by Kelley & Fallick (1990). Formation

of SO3 during the laser combustion was causing little isotopic fractionation (relationship of

SO3/SO2 ratio); the variation between 3 and 250 Torr O2 pressure caused less than 0.5‰

variation in d34S. Further, metals may form oxides during the combustion process and cause a

complex isotopic fractionation mechanism. However, no significant isotopic fractionation

was detected from metal oxide formation during the laser combustion process.

Fallick et al. (1992) discussed the advantages and disadvantages of laser-heated O2

combustion of sulfides for S isotope analyses. Spot analyses (�100 mm spatial resolution)

were made possible with a laser device, although isotopic fractionation by laser–solid

interaction may occur. Several cases, with different types of lasers and materials, were

presented. Another important factor is the MS capability to measure small samples. With

modern MS machines using a microinlet system, smaller samples can be measured (range of

0.1–0.05 mmol SO2; favorable conditions �5 nmol SO2) with a precision better than

–0.05‰ or, for favorable conditions of nmol sample size, �–0.5‰.

Huston et al. (1995) used an Nd-YAG laser microprobe on polished sections containing

sulfides. Samples were ablated in an atmosphere of O2. In many aspects their system worked

similar to others described here. They introduced two modes of SO2 collection: a static and

a dynamic collection. The two different connecting parts for the different modes are shown

in Figure 8-1.19. In the static mode, after ablation, the gas containing the SO2 was passed

through an ethanol trap (�100�C; gas stays for 7.5 min) to remove water, and a liquid

nitrogen trap for SO2 collection (7.5 min trapping time), and finally O2 was pumped away.

In the dynamic mode, after ablation, the gas was bled slowly through a steel capillary (1 m

length, 1/32 in. ø), kept for 5 s in the water trap and pumped through the liquid nitrogen trap

for 10 min, thus also pumping away the O2. Because of the capillary, the flow rate was slow

enough to condense all SO2 from the gas stream. Purification of the gas was automated in

both modes of the system. In the static mode, roughly 60–70% of the SO2 gas was

condensed. In the dynamic mode, SO2 collection was 90–95% but resulted in a consistent

isotopic fractionation of 0.16 – 0.14‰ (n = 5).

It was noted that the quality of the polishing of the sample surface affected the

correction factors needed for the method (after comparing with reference materials).

A precision of 0.4–0.5‰ (1�) was reported. This is higher than that reported by many

others using comparable methods; Huston et al. (1995) claimed that these better results

generally are based on data where ‘bad numbers’ (non-optimized results) were omitted.

Kakegawa & Ohmoto (1999) used an Nd-YAG laser microprobe, fired (output power

�5 W; TEM00 mode) at 100 mmø area for 10 s under an O2 atmosphere (PO2=�8 Torr)

Sulfur 647

for in situ analyses of separated single sulfide grains from shales. H2O, CO2 and various

hydrocarbons were produced, besides SO2. Hydrocarbons were converted (pyrolyzed) into

H2O and CO2 in a Pt furnace (850�C). SO2 was separated from H2O and CO2 cryogeni-

cally. Reproducibility of –0.2–0.3‰ was achieved by this method. All d34S values mea-

sured by this laser method were corrected by the addition of 1‰ to the measured values.

Sulfides were analyzed by the laser-oxidation technique by moving the laser beam

perpendicular to the spatial resolution direction. By a spot velocity of �3.10�5 m/s

(= �2 mm/min), a trench was formed (depth �50 mm; width �50 mm; length dependent

on gas volume required, generally �1 mm).

A major problem of small samples is the purification of the gas.

It was stressed by Fallick et al. (1992) that a laser basically is nothing else than a heating

tool, and chemistry still is crucial to obtain precise and accurate isotopic data.

Taylor & Beaudoin (1993) discussed the difference between laser analytical techniques

by O2 combustion resulting in SO2 and fluorination resulting in SF6. SO2 is compromised

by mineral and instrumental isotopic fractionations as large as 8‰ (Kelley & Fallick, 1990;

Taylor & Beaudoin, 1993), with reported precisions between 0.15‰ and 0.43‰ (1�).

Reported precisions for SF6 were 0.32–0.65‰ using BrF5 and 0.13–0.3‰ using F2 with

GC purification.

No difference in results was obtained if different laser types were used (e.g. Ar-ion, Nd-

YAG, CO2 lasers), because the laser is merely a heat source to promote reaction, but the

level of irradiance is more important (Wright, 1995). Isotopic fractionations occur during

Monitor

Ablation site

Microscope

Ocular

Objective

Turningmirror

Glass samplechamber

Sample

Vacuum

Vacuum

Nd-YAG laserFused glasswindow

To MS

Laser focusingelement

Electronic time shutter

X-Y-Z translatorCapillary(dynamiccollection)

Liquid nitrogen trap

Ethanol trap(–100°C)

Static collection

Figure 8-1.19 Diagram showing the laser microscope for oxidation of sulfides into SO2, andthe two modes of SO2 collection (static and dynamic) as were used at the University ofTasmania (after Huston et al., 1995).


laser combustion, and for each laser probe system this should be determined separately.

Normally, S isotopic fractionations are small in these systems over a considerable range of

isotopic values (Wright, 1995). Moreover, it should be tested if mineral-dependent isotopic

fractionation exists for each type of laser and/or system.

8-1.8.2. Fluorination of sulfides by laser methods

Laser fluorination gives analytical results that are less fractionated in sulfur isotopes than laser

O2 combustion, but is slower in performance because of the GC step for SF6 purification

(Rumble et al., 1991a). However, Shanks et al. (1998) stated that the presence of fluid or

mineral inclusions in natural samples may lead to the production of HF, CF4, ClF3, SiF4,

AlF3 and SO2F2 or SOF2 and other solid, liquid or gaseous contaminants.

The sulfur-containing compounds in such a mixture will fractionate the isotopic

composition of SF6 to some extent (if repeatable, correction can be made accordingly),

and GC purification (= removal of these compounds from the SF6) is of more importance

for repeatable and precise results (note by PdG).

BrF5 as fluorination agent – Rumble et al. (1991b) used a 25 W CO2 laser at 10% of its

power with BrF5 as fluorination agent to produce SF6 from sulfides. Excess BrF5 was

separated from SF6 by cryogenic procedures and GC separation before introduction of SF6

in an MS.

The Canon Diablo Troilite (CDT) standard was measured as 0.55 – 0.65‰.

F2 as fluorination agent – Rumble et al. (1991a) tested the use of F2 as fluorination agent

for S isotope analysis on sulfides. The laser device used is highly comparable with that of

Sharp (1990) (see also Chapter 6-1.5). Fluorine gas is purified and stored on Asprey salt

(Asprey, 1976; see also Chapter 6-1.4.3). The procedure includes laser heating of the sulfide

sample in a reaction chamber filled with F2. The chamber was open to a liquid nitrogen trap

to collect SF6 during the conversion process. Excess F2 after reaction was removed by

reaction with KBr. SF6 was purified by reaction with moist KOH to eliminate traces of F2

and HF, followed by GC. SF6 was measured in the ionized form SF5þ in an MS.

The laser (20 W CO2 laser) was used in pulsing mode with pulse spacing of 10 ms and

pulse width of 10 ms at a laser power setting of 20%. Complete fluorination is reached with

this setting. If initially a high laser power is used for powders, scattering of the sample

through the reaction chamber might occur.

Precision of measurements by this method were as follows: for troilite and sphalerite,

better than –0.06–0.15‰ for d33S and d34S, and –6–11‰ for d36S; for pyrite and synthetic

Ag2S better than –0.16–0.3‰ for d33S and d34S and –27–74‰ for d36S. Deviations from

accepted standard values were between 0.16‰ and 0.53‰.

A similar system was described by Rumble et al. (1993) and Rumble & Hoering (1994)

(Figure 8-1.20a, b, c). The system they presented can be used for laser fluorination of silicate

samples for O isotope determination too (after design by Sharp, 1990; see Chapter 6-1.5).

They tested the system for SF6 volumes of >3 mmol (no need for microvolume cold

finger at MS), and for purification of SF6 a GC, modified after a design by Bains-Sahota &

Thiemens (1988) and Gao & Thiemens (1991), was used. A single pass through the GC

(1/4 in. o.d., 6 ft long; packed with Chromosorb 106 porous polymer – 80–100 mesh;

80�C; He flow 30 mL/min; Figure 8-1.20c) was sufficient for d34S and d33S determination,

but not for d36S determination. It is important to achieve 100% yield of the SF6 gas, because

Sulfur 649

Laser beam

(b)

Observer

Sapphirewindow

Quick flange

KALREZO-ring

BaF2 window

GC

He carrier in(c)

Injection trap

Collection traps

Furnace

Gas flow

Wire trap

He carrier outHe flow outReference in

Liquidnitrogen

VV

Dry ice

Sample inletand highvacuum

To manometerand high vacuum

Detector

Figure 8-1.20 (a) Laser device for fluorination (F2, BrF5) of sulfides or silicates (after Rumbleet al., 1993; Rumble & Hoering, 1994). (b) Detail of the reaction chamber of Figure 8-1.14a(after Rumble et al., 1993; Rumble & Hoering, 1994). (c) Gas chromatograph part for SF6purification in laser fluorination line for sulfur isotope measurement (after Rumble et al.,1993). V= 4-port, 2-position ball valves. cms= cold trap & molecular sieve; f= furnace;RC= reaction chamber; ST=sample tube.

SF6

KOH(a)

F2

f

cms

RC

High vacuum

Highvacuum

f

f

f

RC

KBr

K2NiF6 • KF

O2BrF5

ST


an isotopic effect by the GC (gas–solid GC) otherwise causes erroneous isotopic values

(Moiseyev & Platzner, 1976).

The system was tested for fine powders and (thin) sections (polished and unpolished)

samples. Coarse powders were not advised because of jumping by laser heating. Samples

were loaded into the reaction chamber under a mild overpressure by N2 flow. F2 pressure

must be adjusted to a volume representing at least two to five times stoichiometric excess

for complete fluorination of the sample. After introducing F2 with the sample in the

reaction chamber, it must be controlled if no fluorination at room temperature occurs.

Laser power was adjusted to the lowest level needed to achieve fluorination, resulting in the

best yields (near 100%).

For powders, power supply was at 20%, the pulse generator was set for 100 ms pulse space

and 10 ms pulse width and the laser beam was defocused to a 700 mm diameter. If powders of

more refractive minerals (e.g. pyrite, sphalerite) were analyzed, higher power was needed.

For in situ analysis, low (20%) laser power and a focused beam of minimal 150 mm

diameter and pulses of two single shots of 1 s were used. Power was increased until

fluorination starts. Laser shots were repeated until the desired pit size had formed.

Some minerals form a fluoride armor, which can be scattered by continued laser

irradiance.

Samples were measured on an MS on SF5þ ions for masses 127, 128, 129 and 131.

Two reaction chamber types were used in the system (shown in Figure 8-1.20b). A

BrF2 window on the vertical axis was invisible to the CO2 laser (25 W) beam, and a

sapphire window at inclined axis for view were fitted on the reaction chamber. The laser

was mounted on a stage which could be moved by joystick, and a small He–Ne laser was

used for aiming the beam.

A trap with a bed of KOH grains in an SS tube was used to remove halogens or acids

from the SF6 before final GC purification. A KBr trap was added for purification of O2 from

halogens.

BrF5 was tried for fluorination of sulfides, but produced erratic d34S values.

Extraction experiments were done with troilite (FeS), chalcopyrite (CuFeS2), pyrite

(FeS2), sphalerite (ZnS), galena (PbS), argentite (Ag2S) and greenockite (CdS), for yield,

precision of replicate analysis and interlaboratory comparison for powders of samples.

Precision of repeated CDT analysis measured against SF6 gas from a commercial

cylinder, used as a laboratory standard, was better than –0.1‰ for d34S and –0.09‰ for

d33S. Both delta values were zero, showing a good accuracy for the method. Powdered

minerals have a precision in the range of 0.1–0.2‰. Precision determination in minerals

was difficult to estimate because of the heterogeneous character of most materials measured.

Isotopic fractionation can be caused by formation of SO2F2 or SOF2 during laser

fluorination of disseminated sulfides in a matrix of silicates (Rumble & Hoering, 1994;

Shanks et al., 1998).

If a CO2 laser is used, isotopic separation in SF6 occurs with laser irradiation

(Ambartsumyan et al., 1975a, b, c, 1976; Lyman et al., 1975; Letokhov & Moore, 1977;

Letokhov, 1979; Rumble & Hoering, 1994). More general laser isotope separation effects

were described in Letokhov (1983, 1989, 1999), Bagratashvili et al. (1985) and Petit (1999).

Spot analysis by laser fluorination shows a better precision than by laser oxidation.

Comparison with results by other methods shows that values from laser fluorination agree at

þ0.1 –þ0.8‰ difference.

Sulfur 651

Taylor & Beaudoin (1993) and Beaudoin & Taylor (1994) described a CO2 laser–F2

system for S isotope analysis on sulfide samples (named MILES: Micro-Isotopic Laser

Extraction System), both on powdered samples and for in situ analyses (Figure 8-1.21).

Their procedure was basically the same as described above.

Prefluorination was advised by Beaudoin & Taylor (1994) to lower the blank (very

important for small samples).

Powder samples were treated with a low irradiance power beam, which is slowly

increased to maximum power to avoid sputtering of the powder particles by sudden

absorption of a high power laser beam.

SF6 was purified by a variable cryogenic trap (see Appendix C7.1). A sample was

condensed at �192�C for 5 min, before the trap was warmed slowly to �172�C and

(small) amounts of impurities were pumped away; small gas samples (<1 mmol) of SF6

were purified between �170�C and �155�C. SF6 was released at approximately �140�Cand impurities were released at �130�C and pumped away.

The F2 was released and absorbed by the Asprey salt method described in Chapter

6-1.4.3. Residual and excess F2 was reacted with activated charcoal into inert CF4 follow-

ing the equation

CðsolidÞþ 2F2ðgasÞ ! CF4ðgasÞ [8-1.12]

This is an extremely exothermic reaction and only small quantities of F2 must be allowed to

react for safety reasons.

The laser system was designed with a microscope and video camera and monitor. The

laser beam was coaxial with the microscopic view path by mirrors. An He–Ne red laser to

aim the invisible CO2 laser beam was omitted – the laser can be aimed by help of the

camera and sample locations can be marked on the monitor screen.

A detailed diagram of the multisample chamber used by Taylor & Beaudoin (1993) is

shown in Figure 8-1.22.

F2 reservoir Activated charcoal waste trap Turbo molecularpump

F2 pump

Furnace

Capillary inlet

Filter

Microvolumevtct Sampletube

Figure 8-1.21 Schematic diagram of the MILES laser fluorination system for sulfur isotopeanalysis (after Beaudoin &Taylor, 1993,1994). vtct= variable temperature cold trap.


Stoichiometric SF6 yields were between 76% and 96% for sample sizes between 19.2

and 31.5 mmol. Analytical precision for routine operation was better than 0.03–0.09‰ (1�)

for powders of pyrite, galena and sphalerite for an d34SCDT range of �33 to þ29‰.

Precision and accuracy were reported in Beaudoin & Taylor (1994) to be both �0.2‰.

A similar upgraded system is described by Taylor in Volume I, Part 1, Chapter 20 and also

in Taylor (2004).

‘Wildfires’ – Rumble et al. (1993) were warned against uncontrolled, destructive burn of

the sample after a single laser shot until the fluorination agent is exhausted. These are

thought to occur due to the presence of impurities on the metal surface (e.g. organic

solvents).

Purification of SF6 by GC – See Appendix C7.4 for a description of SF6 purification

methods.

F2 pressure effects – At lower pressures of F2, the d34SCDT is a function of this pressure [for

the MILES system described by Beaudoin & Taylor (1994), this is below Pf2 <20 kPa].

Isotopic exchange between SF6 and sulfur-containing compounds can occur by reac-

tions such as

32SF6 $ 34SF4 þ F2 [8-1.13]

32SSF6 þ C$ 34SCF2 þ 2F2 [8-1.14]

32SF6 þ 0:5O2 $ 34SOF2 þ 2F2 [8-1.15]

32SF6 þO2 $ 34SO2F2 þ 2F2 [8-1.16]

The number of C-O-F-S compounds formed will be a function of the partial pressure of F2,

and of the trace quantity of free O2 and C during reaction. Prefluorination should

efficiently remove water and surfacial C.

Side view

Sample trayNi

Crucible

Au wire

ZnSewindow

Sample tray

Flange

tkr (brass)

Kal-RezO-ring

TeflonO-ring

Top view

Figure 8-1.22 View of the sample chamber and tray of the system by Taylor & Beaudoin(1993). tkr= threaded keeper ring.

Sulfur 653

Interference by other components – Shanks et al. (1998) reported the occurrence of severe

isotopic fractionation caused by formation of SO2F2 and/or SOF2 compounds with the

laser SF6 technique if oxygen is available in the system.

8-1.8.3. Laser specifics

Laser crater characteristics:

O2 combustion by laser – Factors such as pit morphology and pit depth were studied by

Crowe et al. (1990) in order to obtain the best analytical results. Reducing the beam

diameter was increasing the density of the beam and caused deeper pits. Shorter burning

times with a dense beam were creating shallow pits, which gave better results compared

with deeper ones. Burning of deeper pits gave less complete oxidation, probably due to

lower supply of O2 in the deeper levels. Beam diameter of 70 mm and burn times of 1–2 s in

the continuous mode produced suitable pit morphologies and reproducible results. Thin

sample slices placed on an SS holder were another way to prevent deep pits. Rims,

immediately surrounding the laser pit, show S-deficient effects causing isotopic fractiona-

tion (S depletion up to 200-mm wide halo surrounding the crater).

Laser fluorination – Rumble et al. (1993) presented an extensive description of laser

craters by fluorination. Craters (diameter 0.3–1.3 mm) had a central hole that was coated

with metal fluorides. Blankets extended several crater diameters away from the crater. The

hole was surrounded by a raised rim and ejecta blankets composed of metal fluorides. If

ejected and rim material was removed, small amounts of up to 300 mm from the crater

fluoride products were found in cracks and cleavage planes. Chalcopyrite undergoes

incongruent, two-stage fluorination (rim of 10 mm FeF2 separating crater wall from FeF3

lining in the central hole; no Cu remained in the fluorination products). Craters had a sharp

bordered lining of bornite (Cu3FeS3), eventually causing isotopic fractionation.

Halos around pits with O2 laser combustion were reported (Crowe et al., 1990; see

above), while with laser fluorination such halos were not found.

Isotopic fractionation – decomposition effects by laser beam adsorption – Specific bands in the

infrared (IR) spectrum (named n3 and n4) were described to be adsorbed strongly by SF6 gas

(Brunet & Perez, 1969). Ambartsumyan et al. (1975a, b, c, 1976) reported on the dissocia-

tion and isotope separation induced by a strong IR field, acting on various composite

vibrations of molecules of SF6 (and CCl4).

General comments on laser applications – different lasers compared – Operating an IR laser in

pulsed mode (e.g. Nd-YAG laser in pulsed or Q-switched modes: Shanks et al., 1998)

ablates the sample without appreciable heating. A rapid fluency of energy in the pulsed

mode ablation favors formation of SO in respect to SO2 (due to kinetics of oxidation of

sulfides). SO2 production is effected by focussing the laser for 0.1–0.2 s on the target area,

with an O2 pressure of �0.05–0.1 atm, which promotes formation of SO2 and suppression

of SO or SO3 (Shanks et al., 1998).

Operating a Nd-YAG laser in Q-switched mode was reported to result in uneven

combustion and poor reproducibility. Metal oxide halos were commonly formed around

the ablation pit (Shanks et al., 1998).

The ‘laser SO2-technique’ is limited by availability of standards, but it has advantages

over the ‘laser SF6-technique’. No reaction of the sample takes place with O2 at ambient

temperatures (negligible blank levels), while samples may react with fluorination agents


(F2, BrF5, ClF3). With fluorination, ‘wildfires’ exist, which do not occur with the oxidation

technique (see discussion in Shanks et al., 1998).

The ‘laser SO2-technique’ requires a relatively simple set-up compared with the more

complicated setup for the fluorination technique.

An important advantage of ‘SF6’ over ‘SO2’ is that no corrections of other isotopic

variation are needed for SF6 (F has only one stable isotope), such as with the 17O and 18O

correction needed for SO2 (e.g. Shanks et al., 1998).

Precision of laser SF6 is slightly better than for laser SO2: –0.05–0.2‰ (1�) versus –0.2–

0.3‰ (1�), respectively (Beaudoin & Taylor, 1993; Rumble & Hoering, 1994; Shanks

et al., 1998).

Up to now, the use of Nd-YAG, Ar-ion and CO2 lasers for S isotope analysis was

reported, both for producing SO2 and SF6 gases for MS measurement.

8-1.9. LASER ABLATION MULTICOLLECTOR INDUCTIVELY IONPLASMA MASS SPECTROMETRY (LA-MC-ICP-MS)

Recent descriptions of methods for sulfur isotope analysis in sulfides by laser ablation

MC-ICP-MS were presented by Bendall et al. (2006) and Mason et al. (2006).

8-1.10. SECONDARY ION MASS SPECTROMETRY TECHNIQUES

A general description of the SIMS technique is given in Volume I, Part 1, Chapter 30.

Pimminger et al. (1984) measured the 34S, 33S and 32S abundances in galena crystals by SIMS

technique (Cameca ims-3f) at a high mass resolution (m/Dm�5000). Precision and accuracy of

2–3‰ for the d34S can be obtained (analyzed area of 8 mm). Most earlier measurements on S

isotopes by SIMS were made at low resolution setting, and interferences were either negligible

or were eliminated by energy filtering of secondary ions or by peak-stripping routine.

Samples were embedded in a low melting point alloy or in a vacuum-proof synthetic

resin. The polished surface was cleaned ultrasonically and a conductive film of Au (20–

30 nm thick) was deposited. 33S was also measured for an additional check for instrumental

isotopic effects. A focused beam (10–30 mmø) of Arþ or O2þ primary ions was rastered over

areas of 25� 25 mm to obtain a more constant ion emission. Measurement was on the

negative secondary ions (see for details Pimminger et al., 1984).

Deloule et al. (1986) measured S isotopes (32S and 34S; 33S was measured for analytical

control only) through single galena grains by SIMS. A resolution better than 4000 was used,

allowing to separate O2 and SH from S ions. Negatively charged secondary S ions were

used to avoid Zn2þ interferences.

Deloule et al. (1986) stated that ‘As the primary beam is not perpendicular to the sample

surface, this surface has to be strictly planar and perpendicular to the secondary optic axis to

obtain a spot independent of the sample position. For the same reason, the primary beam

acceleration voltage (normally relative stability is not better than –10�3) has to be constant

because the primary beam position and focus depend on it’. Different machine settings were

given in Deloule et al. (1986).

Sulfur 655

Analytical precision is in the range of –1‰ (or d34S = –1‰, 2� error) and reproduci-

bility is about –1‰ on daily duplicates, and –2‰ for duplicates on a 1-month period.

Eldridge et al. (1987) were among the first to report on S isotope analysis by SIMS on

a range of different sulfides and a sulfate (pyrite, chalcopyrite, pyrrhotite, galena, barite).

A specialized SIMS (SHRIMP = Sensitive High Mass Resolution Ion Micro Probe),

basically designed for U-Pbzircon age dating measurements, was used in their study. Samples

were mounted in polished (last step 1 mm diamond paste) epoxy discs covered by a

carbon layer to avoid charge buildup on the analyte. A negative O2 beam (10 kV, 3 nA)

was employed, and positive secondary ions 32Sþ, 34Sþ and also 33Sþ (for determination of

mass fractionation) were measured. 32Sþ secondary beam intensity ranged from 0.3 to

0.07 MHz for different matrices. For details of the machine settings, see Eldridge et al.

(1987).

Isobaric overlap by, for instance, 64Zn2þ or 16O2þ with 32Sþ was prevented by using a

mass resolution of 4500. Accuracy and precision were near –2‰ (2�), regardless of matrix

or of 34S abundance.

Differences in fractionation factors of different minerals were too large to allow the use

of a single standard for all minerals. However, systematic variation in the isotopic fractiona-

tion for a specific mineral suggests that it might be possible to correct (calculate) fractiona-

tions for a range of minerals while using a single standard. Differences in isotopic

fractionation were explained by different strengths of bonding between O–S and different

Me-S bonds (Me = metal) for 34S and 32S, whereby 34S is more strongly bonded than 32S

and 34S generally is released more easily from Me-S bonds than it is from O-S bonds

(Eldridge et al., 1987; see also Chaussidon & Demange, 1988).

Chaussidon et al. (1987) measured sulfide inclusions in diamonds by ion probe (mod-

ified Cameca ims 3f). The inclusions were mounted in epoxydiscs, polished and coated

with Au. An O� primary beam, rastered over 100 mm (analyzed area 60 mmø), and a

secondary positive beam were used without energy filtering. Mass resolution was 3000,

or 4500 interferences when interferences of hydrides were observed. Linearity was verified

on pyrite for a d34S range from �25‰ and þ20‰. Reproducibility was better than –1‰

(within run –0.7‰).

Chaussidon et al. (1989) used a different design of SIMS (Cameca ims3f). Samples, as

separate sulfides or as pieces of rock containing sulfides, were embedded in epoxydiscs,

polished and coated with Au. A negative primary ion beam (10–15 nA; 10 mmø) of O� was

applied. Secondary ions were measured over a 60 mmø area, with a centered beam.

Eventually, a crater of 60 mm was used to achieve a better reproducibility. Ratios of33S/32S and 34S/32S were measured with a positive secondary beam (no energy filtering;

mass resolution 3000 without observation of hydrides or 4000 with hydride detection).

Similar large isotopic fractionations were observed, such as described above by Eldridge

et al. (1987). Sulfur isotopes were measured on sulfides, such as pyrite, pyrrhotite, chalco-

pyrite and pentlandite.

Using calibration curves of sulfide standards, an accuracy of –5% in d34S was obtained

(on pentlandite and chalcopyrite). Variation of instrumental mass fractionation is <<10‰.

Knowledge of the correction factor to –5% results in a change of <<0.5‰ in the corrected

d34S value. Reproducibility on both standard minerals and samples is mostly �1‰.

Reliability of the d34S values is considered to be –1‰ for most samples >50 mm, but

falling to –3‰ for 10mm ones.


Eldridge et al. (1989) ‘updated’ their reporting on the use of the ‘SHRIMP’ for S isotope

measurement. The preparation of microphotographs of areas in the samples of interest prior to

analysis was applied. Coating of the polished sample by carbon, copper or gold was men-

tioned. Continuous reflected light viewing during measurement was permitted during analy-

sis. Primary ions were accelerated at 10 kV, focused on the target at a crater size of 15–30 mmø,

and with a primary beam current of 2–7 nA at the target. Difference of using a negative or

positive primary beam was discussed and is shown schematically in Figure 8-1.23a, b.

Interference by other compounds on the 32Sþ and 33Sþ ions is given in Table 8-1.1.

Zinner (1989) mentioned the use of SIMS (Cameca ims-3f and SHRIMP ion probes)

for measurement of sulfides with a Csþ beam, measuring 32S and 34S in terrestrial rocks.

Reference to Eldridge et al. (1989) was made for the general method on SIMS.

Graham & Valley (1992) used a 133Csþ (strongly electropositive ion, which gives

greater negative secondary ion intensity for electronegative elements such as sulfur: Reed,

1989) primary beam (energy 14.5 keV), defocussed to a 30–40 mm spot size. Beam currents

were in the range of 0.8–1.5 nA. Secondary beams were collimated and measured at a mass

resolution of �4000. The result of collimation is that only ions of the central 8 mm of the

beam spot were analyzed. Ratios were determined by peak switching, with recalibration of

the magnet at the beginning of each analysis.

To ensure electrical conductance, polished samples were Au-coated (0.1 mm) supple-

mented by a thin line of Ag colloid paint. Standards (e.g. Balmat sphalerite) were mounted

together with samples for comparison. Internal precision was –0.3–0.5‰ (depending on

the number of 34S/32S ratio measurements made). Large mass fractionations occur, typically

for each different mineral analyzed, for which corrections were made. Drift corrections

were also applied.

Graham & Valley (1992) discussed electron multiplier performances and effects of

rastering of the primary beam versus defocussing.

Taylor & Beaudoin (1993) discussed the advantage of SIMS in measuring through

zoned crystal (e.g. Deloule et al., 1986) or in crystal inclusions (Chaussidon et al., 1989).

Uncertainties are in the range of 1–2‰ [in the newest SIMS generation, such as the

Cameca ims 1270, this is around 0.5‰ (Deloule, personal communication)] and large

corrections up to 60‰ are needed.

Riciputi (1996) compared precision and accuracy of 34S/32S ratio measurements in

sulfides produced by two different techniques of SIMS: high mass resolution and extreme energy

filtering. The result was that in most cases the extreme energy filtering technique provided

more accurate and ‘robust’ measurements of ratios of light stable isotopes (including 34S/32S)

compared with the high mass resolution technique. This was even more apparent where

standards cannot be mounted with the samples, and must be measured in different runs.

Results of 34S/32S ratios can be obtained rapidly (12 min), with a spatial resolution of

�20 mm and at a precision and accuracy approaching the conventional analyses techniques

(i.e., �0.25‰, 1�).

Paterson et al. (1997) compared two methods of SIMS measurements. Traditional high

mass resolution approach and more recent extreme energy filtering. The first approach is

according to the method described above by Graham & Valley (1992) and is based on achieving

maximum secondary ion intensity. The second approach makes use of the fact that the energy

distribution of molecular ions is much narrower than that of atomic ions. Energy filtering uses

offsets of <100 eV to minimize molecular interferences. To suppress molecular ions and

Sulfur 657

O+

O+

O+

O+

O+

O+

O+

O+

O+

O+

O+

Pb–

Pb–

Pb–

Pb–

Pb–

Pb–

Pb–

Pb–

Pb–

Pb–

S–

O+

S–

S–S–

S–

S–

S–

S–

S–

S–

S–

S–

S–

S–

S–

S–

S–

e–

Galena lattice

Secondary ions to massanalyzer

(a)

(b)

(c)



Galena lattice

Galena lattice

Primary ions fromsource



e–e–

e–

e–

e–

Electronbeam in

Figure 8-1.23 Diagrams showing the effect of sputteringby aprimarybeamon the surface of agalena crystal. (a) A negatively charged oxygen beam is producing secondary positivelycharged Pbþ and Sþ ions. A high ratio of Pb to S shows the low ionization efficiency of sulfurin this mode. (b) A positively charged oxygen beam is producing secondary negatively chargedS� and PB� ions.The high ratio of S to Pb is showing a reversed pattern compared with‘a’. Alsonote the excitation of electrons from the surface shown in the diagram. (c) Same situation asshown in ‘b’, except an electron flood gun, bombarding a wide area, may replace the electronslost to the secondary ion extraction process and eliminate sample charging which reducesdrastically secondary ion yields.

eliminate them from the secondary mass spectrum, eliminate an offset to the secondary

accelerating voltage relative to the voltage of the electrostatic analyzer was employed. For

further discussion on the differences between the two approaches, see Paterson et al. (1997).

Although both the high mass resolution and extreme energy filtering techniques are

precise and accurate for analysis within a single sample mount, this is not the case when

comparisons are made between different sample mounts. Using high mass resolution,

differences in the measured isotope ratio of up to 7‰ have been found for some minerals.

Riciputi & Greenwood (1998) measured S isotopes in sulfides by SIMS (Cameca ims 4f;

use of primary 133Csþ beam, accelerated at 10 keV with currents from 4 to 15 nA, analyzing

a 15–40 mmø analytical spot). They reported on matrix effects, for sulfides, typically

carbonate or silicate minerals, where mass bias corrections were needed. Analysis of

8–25 mmø spots offers an excellent spatial resolution. If the element of interest in the target

mineral is not present in the matrix mineral(s), instrumental mass bias remains constant.

The isotopic compositions of S (and C) for each SIMS analysis were calculated by using

instrumental mass bias measured on pyrite (and dolomite) standards. The instrumental

fractionation factor (�inst) was calculated by comparing the measured isotope ratio of a

standard with its accepted value by using the equation

�inst¼ð34

S=32SÞmeasured

ð34S=32SÞaccepted

[8-1.17]

Isotope ratio measurements on unknown samples were corrected and converted into ‰

notation relative to an accepted standard by

�34Sð‰Þ ¼ ½ð34S=32

SÞmeasured=ffinst�ð34

S=32SÞCDT � 1

( )� 1000 [8-1.18]

where CDT is standing for Canon Diablo Troilite, the former sulfur standard. Presently

IAEA-S-1 (Ag2S) is the sulfur standard (see Volume,I Part2, Chapter 40).

McKibben & Riciputi (1998) reviewed the state of SIMS techniques and discussed

different instrumental parameters and their effects (see Table 8-1.2). In their compila-tion,

no results from the Cameca ims1270, where in general a far higher resolution can be

reached and a higher precision compared with the ‘3f’ and ‘4f’ machines, were added,

probably by lack of reporting then on these fairly new machines.

Additional studies reporting on SIMS techniques for S isotope analysis on sulfides

(–sulfates) can be found in MacFarlane & Shimizu (1991) and Greenwood et al. (1997).

Table 8-1.1 Showing interferences of compounds with sulfur isotope ions by SIMS techni-que (after Eldridge et al., 1989)

Naturalabundance ratio

Possible commonisobaric overlaps

Dm(milli-amu)

Required resolution(m/Dm)

32S/34S (=22:1) 16O2þon 32Sþ 17.75 180064Zn2þ on 32Sþ 7.52 4252H33Sþ on 34Sþ 11.416 2976

32S/33S (=127:1) H32Sþ on 33Sþ 8.438 3908

Sulfur 659

Table 8-1.2 Review of instrument parameters on recent SIMS studies of sulfur isotopes on sulfide and sulfate minerals (after McKibben &Riciputi, 1998)

Primarybeam (nA)

Spatialresolution(mm)

Secondarypolarity

Precision(2�)

Accuracy(2�)

Time peranalysis(minutes)

Technique Instrument Phaseanalyzed

References

O�(2.5–4.5) 30–60 Positive –2‰ –2‰ 30 HMR SHRIMP py, cp,po, sp,gl, ba

Eldridge et al. (1987,1988, 1989, 1993),McKibben &Eldridge (1989)

O�, O2þ

(10–40?)30–60

(rastered)Negative –1–2‰ –1–2‰ 35 HMR Cameca 3f ga, tr Deloule et al. (1986)

O� (10–15) 60–100(rastered)

Positive –0.7‰ –1‰ 45 HMR(3500)

Cameca 3f py, gl, pn,po, cp,ars, cin

Chaussidon et al. (1987,1989)

O� (20–60) 15–30 Positive –1.5‰ –1.5‰ 45 HMR(3500)

Cameca 3f cp, po, tr,pn

Chaussidon & Lorand(1990)

O� 30 (?) Negative –0.5‰ –0.5‰ 60 30 V offset Cameca 3f pr, tr, gl MacFarlane & Shimizu(1991)

Csþ

(0.8–1.5)30–40 Negative –0.6–1‰ –1–2‰ 12–25? HMR

(4000)Cameca 4f py Graham & Valley (1991)

Csþ

(0.5–1.5)30–40 Negative –0.7–1‰ –1–2‰ 15 HMR

(4500)Cameca 4f py, cp, sp,

pn, tr,po, an

Riciputi et al. (1995),Riciputi (1996),Paterson et al. (1994)

Csþ (2–4) 15–25 Negative –0.5‰ –0.5–0.7‰

12 EEF 350 Voffset

Cameca 4f Riciputi (1996),Paterson et al. (1994),Riciputi & Paterson(1996)

HMR = high mass resolution, EEF = extreme energy filtering.

Mineral abbreviations: an = anhydrite, as = arsenopyrite, ba = barite, cn = cinnabar, cp = chalcopyrite, gl = galena, pn = pentlandite, po = pyrrhotite, py = pyrite, tr = troilite, sp = sphalerite.

8-1.11. THERMAL IONIZATION MASS SPECTROMETER METHOD

Paulsen & Kelly (1984) used a positive ion TIMS method for measuring sulfur

isotopes. Wachsmann & Heumann (1992) described a negative ion TIMS method. See

Chapter 8-5.3 for details.

8-1.12. AMS METHOD

Accelerator mass spectrometry (AMS) is used by S.H. Sie (at CSIRO, Australia: 1999,

personal communication; Sie et al., 2002) for measurement of sulfur isotopes in sulfur

compounds, including sulfides. This method was in an experimental stage, with only a small

number of data. No details are given here at this stage.

8-1.13. ADDITIONAL MATTERS CONCERNING SULFIDE SULFURISOTOPE ANALYSIS

8-1.13.1. Acid dissolution methods

HCl dissolution method with Ag2S precipitation – A method to dissolve sulfides from rock

samples by HCl was used by Thode et al. (1954) for salt dome cap rock (3-h reaction) and

by Thode et al. (1961) for meteorites (troilite).

The sulfite in the solution was precipitated as CdS and converted into Ag2S or directly

precipitated as Ag2S. It was filtered and dried. Ag2S was analyzed for its isotopic composi-

tion by methods as described above. Results of this method (including the Ag2S conversion

into SO2) are comparable in accuracy and precision to the direct oxidation (combustion) of

the sulfide (troilite). Ricke (1964) mentioned the use of acids (nonspecified) by Gavelin

et al. (1960) to dissolve sulfides, and the H2S was precipitated as CdS.

Aqua regia dissolution of sulfides – Robinson & Kusakabe (1975) mentioned the possibility

of dissolving sulfides (e.g. pyrite) in aqua regia and conversion into BaSO4, which can be

analyzed according to methods described in Chapter 8-3. Rafter (1957a) tried aqua regia

oxidation of sulfides.

Sulfide mixtures – Ricke (1964) reported on separation methods for different sulfide and/

or sulfate mixtures.

Alt et al. (1989) used a stepwise extraction from powdered rock samples. Monosulfides

(e.g. pyrrhotite, chalcopyrite, pentlandite) were decomposed by reaction with 5 N HCl at

80�C in a closed bottle under a stream of N2. Zn was added for reduction of cuprous

sulfides. H2S from the reaction was converted into Ag2S by bubbling the gas through

AgNO3 solution. The HCl solution was filtered and BaSO4 was precipitated by addition of

BaCl2 solution, thus extracting the acid-soluble sulfate. Pyrite sulfur was extracted from the

residue by reaction with CrCl2–HCl solution at 80�C in a stream of N2 and evolved H2S

was precipitated as Ag2S.

Sulfur 661

8-1.13.2. Thermal dissociation (pyrolysis)

McEwing et al. (1980) handled troilite in an atmosphere of dry Ar to prevent oxidation in a

damp atmosphere. Sulfide was stored in a desiccator kept in a N2 dry box.

A weighed sample of troilite (100 mesh) was placed in a silica glass tube closed at one

end, connected to a vacuum device and evacuated. Overnight heating of the sample at

100�C under vacuum removed air and water vapor. A part of the tube between the furnace

and the connection to the vacuum device was surrounded with dry ice to ensure trapping of

reaction products in a small region of the tube. The temperature of the furnace was

increased to a level high enough to melt the sample and cause dissociation (�1196�C,

monitored by thermocouple). Reaction products were native sulfur and an opaque product

deposited in a narrow ring on the tube wall between the furnace and the dry ice, having a

shiny metallic lustre (assumed to be troilite, it is acid-soluble). These different fractions were

collected, converted into a measurable gas phase and measured for the S isotopic signatures

in an MS.

8-1.13.3. Experiments to extract sulfur from sulfides

Attempts were made to extract sulfur as H2S from pyrites by using a reaction with Zn dust

and HCl. Results were poor and no quantitative sulfur could be extracted with this method

(Rafter, 1957a). Other attempts by Rafter (1957a) were made, besides combustion or

oxidation methods as described in Sections 8-1.1,2, by H2S evolution and aqua regia

(=three parts HCl to one part HNO3) oxidation into sulfate.

8-1.13.4. SO2–SF6 comparison for MS measurement

Leskovsek et al. (1974) state that the memory effect of SO2 in a vacuum extraction device is

approximately twice that of SF6. A table comparing results for the same sulfide mineral

samples was given in Leskovsek et al. (1974; Table 2, p. 376).

Rees (1978) compares SO2 and SF6 gases for MS measurement. Because F has only one

stable isotope, no correction is needed for SF6, while for SO2 the 18O/16O and 17O/16O

ratios must be included in MS data corrections. An oxygen correction method for SO2

measurement was included in Rees (1978) in an appendix and is also discussed in Volume I,

Part 2, Chapters 44 and 45 (see also Section 8-1.12.7). Rees (1978) obviously preferred

calibrations measured with SF6 instead of SO2.

8-1.13.5. SO3 formation and avoidance of the problem

Thode et al. (1961) reported the formation of SO3 following the reaction equation

SO2þ1

2O2 $ SO3 [8-1.19]

The yield of SO3 decreases rapidly with temperature increase and is less than 5% at 1200�C.

Any quantitative production of SO3 while preparing SO2 might result in isotopic fractiona-

tion, because 34S is favored by SO3. The exchange is exemplified by the reaction equation

(Thode et al., 1961; Ricke, 1964)


S34O2þ S32O2 $ S32O3þ S34O2 [8-1.20]

with exchange constants of 1.037, 1.003 and 1.001 at 25�C, 900�C, and 1500�C, respec-

tively. A relatively low PO2during the oxidation process is also important to keep the SO3

production low in favor of SO2 (see equation [8-1.19] above).

A high temperature reaction for SO2 production is favorable in the sense of small

isotopic fractionation between SO3 and SO2 (Filly et al., 1975), although Sakai &

Yamamoto (1966) stated ‘ . . . it can be concluded that the isotope fractionation between

sulfur dioxide and trioxide is really a main cause of error introduced during the combustion

process at a high temperature’. Rees (undated) supported the last authors and claimed a

deviation in d34S at 1200�C combustion temperature shown in the expression

�34SðSO3Þ� �34SðSO2Þ » 3:5‰ [8-1.21]

SO2 gas containing SO3 can be reduced into pure SO2 in a furnace, included in the

extraction system, containing Cu turnings heated to 600�C (Bailey & Smith, 1972; Haur

et al., 1973; Coleman & Moore, 1978; Halas & Wolacewicz, 1981; Nord & Billstrom,

1982; Yanagisawa & Sakai, 1983). Halas & Szaran (1999) used Cu boats containing the

sample for the same purpose.

8-1.13.6. Rayleigh fractionation in ‘branched reaction’ systems

This situation exists when a sulfide structure consist of a combination of different crystal

structures and/or different chemical components which have specific, different physical

behavior when decomposing the mineral. A simple Rayleigh fractionation process cannot

be applied (McEwing et al., 1980). McEwing et al. (1980) called such systems ‘branched

reaction’ systems. They gave an example with pyrolysis of troilite, where an evaporation

and a dissolution isotope effect are involved. The magnitude of these isotope effects can be

determined by the equations

� ¼� �A

1� f

fþ 1

ln ð1� f Þ

� �� B [8-1.22]

� ¼� �A

1� f

fþ 1

ln ð1� f Þ

� �� C [8-1.23]

where � is the isotope effect in the evaporation of troilite and � is that in the dissociation of

troilite. dA represents the isotopic composition of the residual troilite, dB that of the

evaporated troilite and dC of the sulfur from the dissociated troilite. f is the fractional extent

of reaction. McEwing et al. (1980) gave a detailed derivation of equations as given in

[8-1.22] and [8-1.23] in an appendix (pp. 569–571 of their publication).

8-1.13.7. Oxygen isotope corrections in SO2 measurements

Correction for contribution of oxygen isotopes in the SO2 molecule for sulfur isotope

measurement must be made, for which Volume I, Part 2, Chapters 44 and 45 provide

excellent descriptions.

Sulfur 663

Synopsis of methods reported in Chapter 8-1

Section Method Analyte T �C Comments Status

8-1.1 Combustion methods (oxidation on O2

gas)

8-1.1.1 O2 combustion Sulfides 1050–1350 Separation of CO2 from SO2

required; eventually SO2

dried over P2O5; if much

CO2: SO2 recovered as

Ag2S þ converted to Ba-

sulfate

Older, obsolete

method

8-1.1.2 O2 þ N2 gas (purified) Sulfides 1050–1350 Less violent reaction than with

pure O2

Old, obsolete

method

8-1.1.3 Air-O2 gas Ag2S 1200 Sulfides (ZnS, PbS ) converted

into H2S, precipitated CdS,

converted into Ag2S

Rare, obsolete

method

8-1.1.4 O2 þW-anhydride þ CuO/Cu Sulfides (Ag2S) Not specific On-line method, CF-IRMS Modern but rare

and obsolete

method

8-1.1.5 Considerations on O2 combustion methods

8-1.2 Oxidation methods (oxidation

with oxygen donors)

8-1.2.1 PbO oxidation Sulfides 850–900 Pb is toxic and is sticking to

glass walls

Very rare and

obsolete

method

8-1.2.2 V2O5 oxidation Sulfides (eventually

converted to CdS or

Ag2S)

900–950 or 1000 Both off-line and on-line

possible

Older method

8-1.2.3 V2O5 þ SiO2 Sulfides 850–950 Can be used for sulfides and

sulfates

Older method; still

in use in some

labs

8-1.2.4 V2O5 in EA Sulfides 1000–1050 (flash

comb)

On-line EA-(CF)-IRMS Relatively modern

method

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8-1.2.5 CuO oxidation Sulfides (eventually

converted into Ag2S)

850–860, 950 or

�1000

By-product CO2 must be

separated from SO2

Older method

8-1.2.6 Cu2O oxidation Sulfides 800–980 Cu2O advantage over CuO:

less violent reaction

Standard

method (off-

line)

8-1.2.7 WO3þAl2O3þCu wires Sulfides (þsulfates) 1030 On-line (EA); He carrier;

SO2, CO2, N2 separated by

GC

Modern, relatively

rare method

8-1.2.8 Oxidation without specific oxidizing agent Sulfides (sulfates) 1050 Cu2O (?) oxidation without

Cu wire

Rare method

8-1.3 Kiba reagent Sulfide (e.g. pyrite) 280 H2S analyte gas precipitated as

Ag2S, CdS or ZnS

Less common

method

8-1.4 Johnson–Nishita reduction–distillation

apparatus

Ag2S (in soil) – Specialized tool Modern method,

uncommon

8-1.5 Reduction with CrCl2 solution Pyrite, markasite ‘Boiling’ H2S analyte gas: precipitated

as ZnS

Modern method,

rare

8-1.6 LiAlH4 reaction Sulfides (rock samples) n.s. H2S analyte gas: precipitated

as ZnS or CdS

Older, rare

method

8-1.7 Fluorination methods (SF6)

(advantages and disadvantages given in introduction to section)

8-1.7.1 BrF3 fluorination Sulfides 200 Ni metal vacuum device

required

Rare method

8-1.7.2 BrF5 fluorination Sulfides 300–400 Ni metal vacuum device

required; GC purification

of analyte gas (SF6)

Older method, less

common

8-1.7.3 F2 fluorination Sulfides 150 Ni metal vacuum device

required; GC purification

of analyte gas (SF6)

Older method, less

common

(Continued )

Sulfur

66

5

Synopsis of methods reported in Chapter 8-1 (Continued )

Section Method Analyte T �C Comments Status

8-1.8 Laser methods

8-1.8.1 Oxidation with O2 Sulfides – Batch or spot analysis; SO3

reduced to SO2 on Cu

Modern, less

common

method

8-1.8.2 fluorination BrF5 F2 (Asprey salt) Sulfides – Batch or spot analysis Modern method

8-1.8.3 Laser specifics

8-1.9 LA-MC-ICP-MS Sulfide (‘solids’) – Combined method, laser þICP technology

Modern method

high potential

8-1.10 SIMS Sulfide (solid mineral, or

rock)

– Expensive and specialized

tool; correction for mass

interferences required

Modern method

8-1.11 TIMS Sulfides – Sample preparation required;

positive and negative ion

approaches exist

Very uncommon

method

8-1.12 AMS ‘Sulfur compounds’ – Very expensive, specialized

tool

Extremely rare

method

8-1.13 Additional matters concerning sulfide sulfur isotope analysis

8-1.13.1 Acid dissolution methods

8-1.13.2 Thermal dissolution methods (pyrolysis)

8-1.13.3 Experiments to extract sulfur from sulfides

8-1.13.4 SO2–SF6 comparison for MS measurement

8-1.13.5 SO3 formation and avoidance of the problem

8-1.13.6 Rayleigh fractionation in ‘branched reaction’ systems

8-1.13.7 Oxygen isotope corrections in SO2 measurements

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8-2. ELEMENTAL SULFUR

Measurement procedures of elemental or native sulfur are basically similar to those used for

sulfides (Chapter 8-1). As also mentioned below, the main differences are the caution for

sulfur mobility when it is heated, and redepositing on cold spots in the analytical system and

considering some elemental sulfur extraction methods from ‘raw’ sample material.

Rafter (1957a) reported, ‘The main difficulty in the combustion of pure sulfur (native

sulfur) is the tendency of sulfur to sublime at cold spots. Methods of placing the sulfur at

colder spots just in- or outside the furnace, or quickly heating the furnace result in bad

yields’. Burning the sulfur in a silica glass tube outside the hot furnace with a gas burner

gives a 99% yield according to Rafter (1957a).

Using an ampoule combustion or oxidation method avoids the mobility of sulfur

because of the restricted volume and includes an even and complete heating of the sample.

Different analytical methods are reviewed below, some given with reference to the

analytical method for sulfides discussed in Chapter 8-1.

Extraction procedures – Extraction of native sulfur can be done with acetone. After

complete extraction, the acetone solution can be separated and evaporated to deposit the

sulfur, which is analyzed by methods reviewed below (Tuttle et al., 1986; Bates et al., 1993).

Native sulfur from spring water deposits (preserved in securely closed heavy duty plastic

bags) was separated from sulfate in the samples by CCl4 and water in a separator funnel (Van

Everdingen et al., 1982). CCl4 was evaporated and the native S recovered for analysis

(sulfate was precipitated with Ba2þ).

Powdered samples (lunar rock in the example) can be treated with a benzene–methanol

(9:1) mixture to extract native sulfur. The solution is shaken with freshly cleaned Cu strips

to precipitate the S into CuS (S present in concentration of �0.5 mg/ 10 mL solution)

(Kaplan et al., 1970).

8-2.1. COMBUSTION IN A STREAM OF OXYGEN

Thode et al. (1949) combusted native sulfur in a stream of O2 comparable with the

method used for combustion of sulfides (see Chapter 8-1).

Thode et al. (1954) selected sulfur crystals from a salt dome cap rock. The crystals were

ground and 25–50 mg was burned in a stream of O2. SO2 was collected in a liquid nitrogen

trap and CO2 was separated using an acetone dry ice bath. A similar procedure for analysis of

native S from a salt dome rock was described by Feely & Kulp (1957). Tank O2 for

combustion was purified by passing it through charcoal cooled by dry ice and through ascarite.

Filly et al. (1975) described a method, after Ricke (1960), for direct combustion of native

S (and organic sulfur and sulfides) in a stream of pure O2 (see also Thode et al., 1949 above).

Use of CO2-free air – Oana & Ishikawa (1966) measured S isotopes on native sulfur by a

combustion method using CO2-free air (see Chapter 8-1.1.1)

Sulfur 667

8-2.2. COMBUSTION WITH O2 AND WO3

Haystead (1990) reported measurements of S isotopes on native S in a Dumas

combustion EA–on-line system. See Chapter 8-4.2.10 for more details.

8-2.3. REACTION WITH Ag POWDER

Robinson & Kusakabe (1975) reacted elemental sulfur with powdered Ag in sealed

tubes to form directly Ag2S that was subsequently used for mass spectrometry after con-

version into SO2 (or SF6, alternatively).

8-2.4. CARBONYL IRON AND Zn POWDER REACTION

Gavelin et al. (1960) analyzed elemental sulfur by reaction with a mixture of carbonyl

iron and Zn powder in a ceramic crucible at a temperature of 700�C for 10 min. For details

on this method, see Chapter 8-3.4.

8-2.5. CHROMOUS CHLORIDE REDUCTION

Canfield et al. (1986) reported the reduction of elemental sulfur into H2S by

‘chromium reduction’ (after the method by Zhabina & Volkov, 1978), giving a recovery

percentage of sulfur of 92.0%. For details of the method, see Chapter 13-1.5.

Bates et al. (1993; oil shale samples) and Carmody & Seal (1999) applied Cr(II)

reduction on elemental sulfur, converting the sulfur in H2S, which was collected as Ag2S.

The sulfur was ground finely in an agate mortar, and 26 mg (<200 mesh) of ground

silica glass was added and mixed with the sulfur to help removal of the powder from the

mortar (silica glass is inert in the reaction with CrCl2). The mixture was weighed after

removal from the mortar and placed in a reaction vessel after the method as described

by Bates et al. (1993). Reaction of CrCl2 with elemental sulfur gives no isotopic fractiona-

tion, as it was the case for pyrites (Newton et al., 1995). A single AgNO3 trap to collect

all the H2S as Ag2S was found sufficient after testing with a series of traps (Carmody &

Seal, 1999).

8-2.6. COPPER SULFIDE SYNTHESIS

Carmody & Seal (1999) reacted fine and coarse fractions of the Soufre de Lacq samples with

Cu wire (cleaned with acetone). Sulfur and Cu were weighed in stoichiometric proportions.

A high digenite (Cu9S5; Deer et al., 1974: p. 454) solid solution, Cu2-xS with 0� x� 0.268, was


synthesized and placed in high-purity silica glass tubes. The tubes were evacuated, sealed and

placed in a furnace for reaction. The furnace was heated slowly to 750�C, and held at this

temperature for 3 h. The tubes were annealed at 300�C overnight and cooled to room tempera-

ture. The copper sulfide was oxidized by techniques given in Chapter 8-1 into SO2 for isotopic

measurement. Kaplan et al. (1970) used this reaction between Cu and S for extraction of native

sulfur (see ‘extraction procedures’ in the introduction to this chapter).

8-2.7. CONVERSION INTO BaSO4 BY HNO3 + Br2

Volcanic native sulfur (if not pure) was converted to sulfuric acid by reaction with a heated

mixture of HNO3 and Br2, followed by precipitation of BaSO4 (Ueda et al., 1979). The BaSO4

was reduced to BaS by heating at 1000�C and then converted to Ag2S by the addition of an

AgNO3 solution. Further procedures for analysis of Ag2S are described in Chapter 8-1.

This method was also reported in the review publication by Krouse & Tabatabai (1986).

Ding et al. (2000) described a method for conversion of native S into BaSO4. A sample

of 49 mg S with a small amount of sub-boiling distilled water was placed in a 400 mL

beaker. Concentrated HNO3 was added. The beaker was gently heated on a water bath and

a Br2 solution was added dropwise until all the S had disappeared. Heating was continued to

remove all excess Br2 and HNO3. H2SO4 remained and was brought at a volume of 50 mL

by the addition of sub-boiling distilled water, and was filtered and washed with sub-boiling

distilled water to bring the volume at 300 mL. The pH was adjusted to 6 and the solution

was boiled in a glass beaker. A boiling solution of BaCl2 (10%; 30 mol% in excess as

required by H2SO4 present) was added and boiling was continued for 45 min. The solution

was filtered and the BaSO4-containing precipitate was washed 10�with sub-boiling

distilled water. In a Pt crucible, the BaSO4 was heated in a muffle furnace at 800�C for

removal of organic material.

Conversion into BaSO4 by aqua regiaþ Br2 – Smith & Batts (1974) extracted native sulfur from

coal by boiling the sample three�with 50 mL benzene, evaporation to dryness (80�C) and

boiling with bromine and aqua regia until a clear solution was obtained. Sulfate in the solution

was recovered as BaSO4 (see Chapter 8-4.3) and used for mass spectrometric analysis.

Conversion into BaSO4 by ‘reversed’ aqua regiaþ Br2 – Halas (personal communication) uses a

mixture of 30 mL concentrated HNO3, 10 mL concentrated HCl (=‘reversed’ aqua regia) and

1 drop of Br2 to oxidize S into SO42�. A powdered S sample (�10 mg) is placed into a small

beaker, then the fluid mixture is added and the beaker is covered by a watch glass. Sulfur

‘dissolves’ within 1 h. Overnight, the beaker is placed in a water bath to evaporate volatiles

(HNO3, HCl, Br). Finally, BaSO4 is precipitated with addition of a 10% BaCl2 solution.

8-2.8. FLUORINATION METHODS

Leskovsek et al. (1969) described a fluorination method for SF6 gas for isotopic

measurement (Chapter 8-1.7.3), which is also suitable for native sulfur analysis.

Velinsky et al. (1990) used the BrF5 fluorination technique for native sulfur, measuring

S isotopes on SF6 gas (fluorination methods are described in Chapter 8-1.7 and 8-1.8.2).

Sulfur 669

Native S was combusted in a 300-mL ‘Thomas-Ogg Schooninger’ combustion flask, with a

glass stopper with a hook on the bottom, at 1200�C in an O2 atmosphere, forming SO2 and

SO3. A sample of 20 mL of 0.01 N NaOH was placed in the flask and 50 mg of dried extract

(see Chapter 13-1.5) was wrapped in black, ashless paper placed in a Pt basket and fitted to

the glass stopper. The flask was purged with O2 for 3 min and sealed (a few drops of water

around the lip of the stopper to help seal the flask). The black paper was ignited by aiming

an external IR beam on the paper. After combustion the flask was swirled gently, and

during 40 min, SO2 and SO3 will absorb into the NaOH solution. The solution was poured

into a 50-mL beaker and the flask was rinsed with distilled water, added into the beaker

carefully. To oxidize the sulfur oxides into SO42�, 20 drops of concentrated HNO3 were

added to the solution. The solution was evaporated to dryness at low heat. The precipitate

was dissolved by 40 mL of distilled water and 0.75 mL of 6 N HCl and heated to near

boiling. To this, 20 mL distilled water with BaCl2 [0.15 mL of 5% (w/v) solution for each

mg S] was added (still heated to near boiling) and stirred with a glass rod; BaSO4 precipitates

while the solution was kept at 50�C for a night to form large crystals. The BaSO4 was

filtered off or collected by centrifuge methods. At least 5 mg of BaSO4 is needed for further

procedures. BaSO4 was converted into BaS after a method by Krouse & Tabatabai, 1986

(mixed with graphite and roasted in an Ar-flushed furnace at 1000�C for 2 h). BaS, unstable

in air, was converted into H2S by 6 N HCl, and trapped as Ag2S for fluorination.

Oxidation of SO2–SO3 was first tried with a Br2-water solution, resulting in low and

variable recoveries.


8-3. SULFATES

8-3.0. INTRODUCTION

Conversion of sulfur into sulfate – Sulfur contained in samples in different forms

generally is converted into BaSO4, which then is further treated by methods such as are

presented below in this chapter.

Precipitation of dissolved sulfate – Dissolved sulfates are precipitated as BaSO4 by the

addition of BaCl2 (10%) to the solution.

Claypool et al. (1980) simply precipitated dissolved sulfate as BaSO4, filtered and dried

the precipitate for further analysis, if only S isotopes were of interest. When the O isotopic

composition was also to be measured, a pure BaSO4 needed to be recovered. For purifica-

tion of the sulfate, the acidic (HCl) solution containing the sulfate was passed through a

column containing a cation exchanger (Dowex 50 W-8). The effluent was acidified to

pH 2, heated to boiling, and the sulfate precipitated with a BaCl2 solution (Sakai, 1977;

Claypool et al., 1980).

Conversion of anhydrite into BaSO4 – Sullivan et al. (1994) gave an example of converting

anhydrite into BaSO4 by the following process:

BaSO4 was prepared from anhydrite by dissolution, ion-exchange filtration (BioRad,

analgrade cation-exchange resin: AG 50W-X8, 200–400 mesh) and addition of 5% BaCl2solution to the acidified (pH 2) and warm (70�C) sulfate-containing solution.

Cagatay & Eastoe (1995) extracted sulfate from gypsum by dissolution with dilute HCl

followed by conversion into BaSO4.

8-3.1. REDUCTION WITH CARBON (GRAPHITE)

Thode et al. (1949, 1953) briefly described a method of Ca-sulfate reduction by

roasting of equal amounts of sulfate and finely powdered graphite at 800–1000�C for �24 h

(also mentioned in Rafter, 1957b). The Ca-sulfide was dissolved in slightly acidic solution

to form Ca hydrosulfide and was filtered from the unreacted sulfate and carbon and

precipitated as PbS, or oxidized to elementary sulfur. These compounds were treated by

O2 combustion producing SO2 for S isotope measurement (see Chapter 8-1.1.1).

Rafter (1957a) presented a method for sulfate (Ba sulfate was used in the example)

reduction by graphite (at 1050–1100�C) (Figure 8-3.1). Rafter (1965) stated that incom-

plete reaction occurs if the reduction process did not reach 1100�C). The sulfur was leached

by distilled water and precipitated as Ag2S. An accurately measured sample (0.10–0.15 g) of

pure sulfate was ground in an agate mortar with an equal amount of spectrographic pure

graphite and was loaded into a Pt microcrucible [a silica glass crucible was used by Rafter

(1965), with an improved recovery of Ag2S samples]. The mortar was cleaned with another

0.1 g of graphite, which was added on top in the crucible. The crucible was put carefully in

Sulfur 671

the silica glass tube of the device shown in Figure 8-3.1. In a beaker with water, 5 drops of

starch and one drop of 0.1 N iodine were added to the water for detection and estimation of

SO2. The crucible was heated by a burner until 1 min after bubbling in the water had

stopped. Eventually, additional drops of 0.1 N iodine were needed for estimation of the

SO2 loss during the reduction process. By adding 0.5 N AgNO3, acidified with a few drops

of 1 N HNO3, sulfur was precipitated as Ag2S. Ag2S was collected by filtration, washed

with water and acetone and dried in a stove. From the weight of the Ag2S, the recovered

percentage of sulfur from the sulfate can be calculated.

It is very important for this method that pure sulfate is analyzed.

The Ag2S was oxidized by combustion in a stream of oxygen by Rafter (1957a; see

Chapter 8-1.1.1). Any other method for sulfide sulfur isotopic analysis, as described in

Chapter 8-1, can be used alternatively. With this method, a recovery of 94–96% of the

sulfate sulfur for isotopic analysis was reached. Tests with a similar device as described here,

to determine the loss of sulfur during the reduction process, were carried out by Rafter

(1957b).

Rafter (1957b) described a different device for sulfate reduction than was shown in Rafter

(1957a) (Figure 8-3.2), where the first reduction was taking place under a flow of N2 at

atmospheric pressure conditions. In the latter, reduction occurred in a vacuum system. A Pt

crucible with a sulfate–carbon mixture (0.2 g BaSO4, 0.2 g C) was placed in the combustion

tube of a vacuum system. The carbon was previously heated to 1000�C under vacuum. The

system was closed and evacuated, and the mixture was degassed. The reduction process was

monitored by increase in pressure in the manometer; a rapid increase in pressure occurs in a

temperature range of 850–1000�C. The H2S produced by the reduction process was led

through AgNO3 solution to deposit Ag2S (Figure 8-3.3). By mass balance, it was calculated

that 96% of the theoretical sulphur content from the sample sulfate was recovered as Ag2S.

Different tests with lower weight of sulfate–carbon mixture, other temperature settings and

heating speed were performed by Rafter (1957b) leading to comparable results in sulfur

N2

G-t

Pt-c

H2O +detect.

Figure 8-3.1 Device for sulfate reduction by graphite in a Gooch crucible and by heating witha burner (after Rafter, 1957a, b). G-t=Gooch thimble; Pt-c= platinummicrocrucible; H2Oþdetect. =water with starch and a drop of N/10 iodine for SO2 detection.


recovery such as described above. A liquid nitrogen (liquid oxygen was originally used) trap

freezes the CO and CO2 produced during the reduction process: release showed that the gas

was mostly CO. This can be formed according to the reaction

BaSO4þ 4C! BaSþ 4CO [8-3.1]

Carbon reduction with NaF – A slightly different method for carbon reduction was presented

by Filly et al. (1975). They prepared a mixture of 100 mg sulfate (barite), 20 mg of C

(prepared from organic matter) and 5 mg NaF. The addition of NaF ascertained a complete

reduction. The mixture was loaded in a Mo boat for reaction (see Figure 8-3.4), evacuated

and degassed, while moderately heated. Heating was done by hand torch between

1100�C and 1200�C. During the reaction, water was trapped in a dry ice trap. By this

reaction BaS and CO were formed (see above equation [8-3.1]). After reaction, the Mo

Furnace

vp

tc

Pt boat

Sampleampoule

Figure 8-3.2 Device for reduction of sulfate by graphite under vacuum conditions (afterRafter, 1957b). tc= thermocouple; vp=vacuum pumps.

N2

0.2 N AgNO3 solutions

Figure 8-3.3 H2S evolution train for the collection of silver sulfide by a silver nitrate solution(after Rafter, 1957b).

Sulfur 673

boat was recovered from the reaction tube and dropped in an Erlenmeyer flask with 10%

AgNO3. After 15 min Ag2S was filtered from the solution and excess C grains were

removed by flotation. The sulfide was analyzed for its S isotopic composition following

the methods described in Chapter 8-1. Reaction of CaSO4 by this method may produce

COS, risking isotopic fractionation.

Reduction by carbon in a muffle furnace – A method to reduce sulfate in a high-temperature

muffle furnace was described by Rafter (1957b). Because results were significantly worse

than for the other methods by Rafter (1957a, b) described above, no further details are

given.

Reduction with glassy carbon – Gygli (1993) reported the possibility to analyze oxygen in

NaSO4 (quantitatively in his report) by ‘pyrolysis’ and immediate reaction with glassy

carbon at high temperature (1300�C) by flushing with carrier gas H2–N2 mixture (1:9).

CO was formed, a gas that can be applied for both quantitative and isotopic determination.

on-line ‘pyrolysis’ (reduction with carbon) CF-IRMS systems with glassy carbon

reactors such as described in the chapters above (i.e. the in Chapters 1-2.13.3, 1-3.2.5,

5-2.6.2, 6-1.1.2/3, 6-2.3.7, 6-3.4.4, and 6-4.3) can also be applied for sulfur isotope

analysis on sulfates.

8-3.2. JOHNSON–NISHITA REDUCTION–DISTILLATION METHOD

Complete conversion of sulfate into H2S is possible with the Johnson–Nishita

reduction–distillation apparatus (Johnson & Nishita, 1952, see Chapter 8-1.4, Figure

8-1.14), without fractionation of S isotopes. To obtain sufficiently large volumes of sulfur

(in the form of H2S) for MS measurement, Schoenau & Bettany (1988) combined a series of

these apparatuses.

Sulfate in soils range typically from 100 to 1000 mg/kg soil. If 1 g of soil is used in each

of six apparatus and combined, this will result in 1.8 mg of S (13 mg of Ag2S)–more than

sufficient for an isotopic analysis.

O2

Mo boat

qrt

fg

Sample vessel

vac

Figure 8-3.4 Device for sulfide combustion by a mixture of HI, HCl and H3PO2 (after Fillyet al., 1975). fg= fritted glass; qrt=quartz glass reaction tube; vac = to vacuum pumps.


H2S evolved from reduction of sulfate during heating with HI reducing mixture and

was driven by a stream of N2 (200 mL/min; O2 free). H2S was collected as CdS in a solution

of Cd acetate. Total distillation takes approximately 40 min. After the distillation was

complete, AgNO3 (1 mL; 1 M solution) was added to the solution, and Ag2S was formed.

Centrifuging was used to concentrate the Ag2S, which was washed with pure ethanol,

centrifuged again, decanted and dried. The Ag2S was analyzed for S isotope composition

according to techniques as are discussed in Chapter 8-1.

8-3.3. REDUCTION WITH Fe METAL POWDER

Thode (1953) described the reduction of sulfate by mixing it with excess Fe powder

in a porcelain crucible and heating over a burner for 2 h. The slag was treated with HCl and

the released H2S was trapped in a lead sulfide solution, precipitating PbS. The PbS was, after

being washed and dried, analyzed by methods such as described in Chapter 8-1 [Thode

(1953) used a method of burning in a stream of O2].

Rafter (1957b, 1965) tested the Fe reduction method. Ba-sulfate was mixed with Fe

powder. The porcelain crucible was placed in a larger Pt crucible with tightly fitting lid and

heated in a furnace under a reducing atmosphere at 950–1050�C for 2 h. The result was

only a little reduction. The method was not considered very useful, but if applied never-

theless, samples should not be smaller than 20 mg (Rafter, 1965).

Grinenko (1962: in Russian) also reported on this method (see discussion in Rafter,

1965).

8-3.4. CONVERSION TO SULFIDE WITH Fe CARBONYL + Zn

Gavelin et al. (1960) tested a method for sulfate decomposition by mixing sulfates

(5� 10�3 mol) with an excess of carbonyl iron (4.5 g; contains – 0.003% S!) and Zn

powder (0.5 g; to accelerate the reduction). This mixture was placed in a porcelain boat

covered by a layer of carbonyl iron and the boat was covered by a lid. The boat with

sample–reagent mixture was placed in a vacuum system and heated by hand torch for

reaction. BaSO4 was heated at approximately 950�C, SrSO4 at 820�C and CaSO4 at 750�C.

The method was also used for analysis of sulfide (pyrite), elemental sulfur or sulfosalts.

Before and during reaction, oxygen-free hydrogen gas was flushed through the system. The

liberated H2S was absorbed in flasks (1) with 300 mL of solution containing 5% HCOOH

and 0.75% Cd(HCOO)2, and (2) with 75 mL of 1.5% neutral Cd(HCOO)2 solution, both

at 95–100�C to obtain CdS in crystalline form. After reaction, the contents of the two flasks

were combined and filtered through a glass filter. The CdS was washed seven times with a

3% HCOOH solution and three times with 95% C2H5OH and dried at 100–110�C. The

CdS was pulverized and dried again in an oxygen-free stream of N2 at 400–425�C.

A temperature >400�C was needed to avoid formation of phases such as Cd(HCOO)2or CdCO3. CdS was analyzed following methods described in Chapter 8-1 (Gavelin et al.,

1960) using the V2O5 oxidation method.

Sulfur 675

8-3.5. HI–H3PO2–HCl REDUCTION AND Ag2S PRECIPITATION

Thode et al. (1961), Marowsky (1969b), Filly et al. (1975), Rees et al. (1978) and

Rees & Holt (1991) used a boiling mixture of HI, HCl and H3PO2 to convert sulfates into

H2S (after a method by Pepkowitz & Shirley, 1951) (Figure 8-3.4).

The composition of the reducing mixture is 500 mL (=850 g) HI (density: 1.7; 57%

solution); 816 mL concentrated HCl; 245 mL H3PO2 (50% solution) (mixture composition

introduced by Auger & Gabillon, 1911). Boiling of this mixture for 45 min removed all

traces of sulfur as H2S.

Reaction equations for this process are

SO42�þ 10Hþþ 8I� ! 4I2þH2Sþ 4H2O [8-3.2]

and

PO2�3þH2Oþ I2 ! 2I� þ 2HþþPO3

�3 [8-3.3]

Reduction was carried out in a 200-mL flask with reflux condenser. H2S was removed from

the reaction flask by a stream of nitrogen, and the H2S was absorbed in a solution of

cadmium acetate (Cd(CH3CO2)2; 0.1 M solution), acetic acid and distilled water. CdS was

converted into Ag2S (it filters easier from a solution) by addition of 0.1 N AgNO3 to the

solution. Ag2S coagulated in the solution on heating and was filtered through glass wool,

washed twice with concentrated NH4OH and dried in an oven at 120�C. Procedures for

analysis of Ag2S are given in Chapter 8-1.

Watanabe (1975) used a reducing solution of 500 mL of 57% HI, 816 mL of 12 M HCl

and 245 mL of 50% H3PO2 acids to convert sulfate into sulfide. This reducing mixture

(25 mL) was transferred to a round-bottom flask (300 mL) with a reflux condenser. N2 was

bubbled (35 mL/min) through the mixture at 200�C for 1.5 h, after which it was cooled for

30 min (and spiked by Watanabe, 1975), and then again refluxed with a stream of N2 at

180�C for 1.5 h. Bubble the generated H2S through a mixture of 10 mL of 0.1 M cadmium

acetate and 20 mL of water to precipitate CdS. CdS was converted into Ag2S by adding

10 mL of 0.1 M silver nitrate. The mixture was ‘aged’ (annealing of crystals) by heating and

the Ag2S was filtered (washed with ammonia liquor followed by water). The precipitate

was dried at 120�C. Ag2S was used for further analysis (see Chapter 8-1).

Sasaki et al. (1979) noted that the reaction with the HI–HCl–H3PO2 mixture is very

slow for pyrite-containing samples.

8-3.6. HI–H3PO2–FORMIC ACID REDUCTION

Amaral et al. (1989), in a modified Johnson & Nishita (1952) system (see Section

8-3.2), reduced sulfate to H2S by a mixture of HI, 90% formic acid and 50% H3PO2 (ratio

4:2:1 by volume; mixture was refluxed for 1.5–2 h under N2 atmosphere). About 8 mL of

this mixture was added to each sulfate sample in a flask, boiled for 1 h while H2S was

removed by flushing the headspace with (O2 free) N2 and the H2S was precipitated as ZnS

by flushing through a Zn(OH)2 solution.


8-3.7. KIBA REAGENT METHOD: ACID DIGESTIONAND CONVERSION INTO SULFIDES

The use of stannous dichloride and phosphoric acid mixture for reaction with sulfates,

after which the H2S is converted into ZnS, was described by Volkov & Ostromov in 1958

as reported by Rafter (1965).

Sasaki et al. (1979) used ‘Sn(II) – strong phosphoric acid’ (Kiba reagent after Kiba et al.,

1955, 1957a, b; Ohashi, 1955) to produce H2S from sulfates. This method can be used for

organic and inorganic samples. A suitable amount of sample is dropped in the Kiba reagent

of the reaction flask (Figure 8-3.5a and b), closed and flushed by N2. The flask is heated

slowly to 280�C. Reaction starts at 120�C and is normally complete before reaching 280�C;

after reaching 280�C, the reaction is continued for a further 15 min. H2S produced by the

reaction is carried away by the N2 stream and passes a distilled water trap to remove any

chloride and is precipitated in a second trap with a Zn acetate solution as ZnS. The ZnS is

converted into Ag2S by adding a 0.1 N silver nitrate solution to the second trap. Boiling of

this mixture improves coagulation of coarse Ag2S, which is easier filtered (e.g. over glass

wool). Dense viscous residue on the glass walls can be collected by washing with hot water.

The Ag2S can be analyzed for its S isotopic composition by the methods described in

Chapter 8-1.

Ueda & Sakai (1983) also used the Kiba reagent method for sulfate sulfur analysis. They

presented a vacuum device (Figure 8-3.6) for reaction of sulfates, separation of the

produced gases and conversion of the H2S into SO2 on a Cu2O trap. Because all procedures

were carried out in a single vacuum system, small samples can also be analyzed. A powdered

rock sample was heated with Kiba reagent in the vacuum system (reaction vessel:

Mantle heaterCT AV

To pump

500-mLflask

N2

N2

Mantle heater

500-mLflask

(a) (b)

Figure 8-3.5 (a) Reaction line with Kiba reagent for sulfates (after Sasaki et al., 1979).AV=H2S absorbing vessel; CT=water chlorine trap. (b) Device for production of Kibareagent (after Sasaki et al., 1979).

Sulfur 677

Figure 8-3.6), usually at 280�C; sulfate, sulfide and carbonate in the sample evolve SO2,

H2S and CO2 by the following reactions:

SO 2�4 þ Sn2þ þ 4Hþ ! SO2þ Sn4þ þ 2H2O [8-3.4]

S2� þ 2Hþ ! H2S [8-3.5]

SO3�2þ 2Hþ ! SO2þH2O [8-3.6]

If the sulfate was reacted in vacuum and with a stream of N2 instead of in an atmospheric

pressure environment, H2S rather than SO2 was evolved according to the reaction

SO 2�4 þ 4Sn2þ þ 10Hþ ! H2Sþ 4Sn4þ þ 4H2O [8-3.7]

although in reality only a small fraction was reduced to H2S. A correction was needed for

this effect. At atmospheric pressure, sulfite was formed first, and proceeded to SO2 forma-

tion (oxidation instead of reduction).

A powdered rock sample (0.5–10 g) was added to 50–100 mL of Kiba reagent. The

reaction vessel was evacuated for 5–10 min, and the temperature was raised to 90�C for 1 h

while pumping respectively, passing any of the air or ‘degassing’ products through the first

two cold traps cooled by either trichloroethylene or dryice/acetone slushes. If no carbonate

is present, the heating to 90�C can be omitted. With liquid nitrogen cooling of the other

cold traps, the reaction vessel was heated slowly to 280�C (takes �30 min). Nonconden-

sable gas (N2 mainly) was formed, presumably from the Kiba reagent.

SO2, H2S and CO2 were separated from each other and from water vapor by successive

vacuum distillation (n-pentane trap method). SO2 was produced later than the other two

gases and can be separated in that way too. H2S was converted into SO2 by reaction with

Cu2O in vacuum: first at 250�C a reaction of H2S with Cu2O formed Cu2S and H2O, and

at elevated temperature (900�C) SO2 was released by the reaction of the Cu2O and Cu2S.

SO2 directly formed from the sulfates (see equations above) was passed through the Cu2O

Reaction vessel

Cu2O

CVM

SamplevesselCT MT

Highvacuum CT

CTCT

Figure 8-3.6 Vacuum device for decomposition of rocks and for the separation of evolvedgases (after Ueda & Sakai, 1983). CT= cold trap; CVM=constant volume Hg manometer;MT= multiple trap.


furnace, and exchanges oxygen with the Cu2O oxygen, giving a similar oxygen isotopic

value to both SO2 batches to make them comparable for MS measurement (Ueda & Sakai,

1983; Bottomley et al., 1992). Precision of the sulfate analytical method is better than

–0.3‰.

The Cu2O furnace was renewed after 10 samples, because of fine Cu2O powder

increasingly blocking the furnace, and also a black film starting to form on the cooler

parts of the glass. To prepare a furnace, Cu2O–CuO wires are repeatedly heated and cooled

between 900�C and 500�C, before being heated at 900�C in a vacuum system.

The CO2 produced from carbonates by the reaction can also be measured for carbon

isotope composition; oxygen isotopes are not reliable, because they exchanged with other

oxygen-containing compounds in the system and need different calibrations as the CO2 was

produced from carbonates by phosphoric acid digestion. Care must be taken with C isotope

data obtained in this way; small contaminations (e.g. introduced during milling of the

sample) may cause significant isotopic changes.

Sulfate–H2S1 is 12–4‰ lower in d34S than sulfate–SO2

8 when using the Kiba-reagent

method,9 due to isotopic fractionation in the decomposing sulfate. This result in a measured

d34S of �0.3–0.5‰ for sulfate–SO2, greater than the d34S of total sulfur of the sample.

Corrections need to be made according to the sulfate concentration in a sulfide–sulfate

mixture.

Ueda et al. (1991) used Kiba reagent to react under vacuum with 2–7 g of powdered

sample at 90�C for 1 day to partially remove carbonate–CO2. The sample, representing a

mixture of sulfide and sulfate, was then heated to 280�C for 1 h to extract H2S from sulfides

and SO2 from sulfates. Under the strongly reducing conditions, up to 5% of the sulfate may

be reduced to H2S (see also Chapter 8-1.3). If a mixture of SO2 and H2S is produced during

the Kiba reaction, a possible reaction between the two gases may cause lowering of yield

and isotopic fractionation:

SO2þ 2H2S! 3Sþ 2H2O [8-3.8]

H2S is preferably precipitated as CdS by adding 15g/L cadmium acetate þ 0.5 mL glacial

acetic acid, followed by conversion into Ag2S by addition of silver nitrate solution (0.1 N)

(e.g. Giesemann et al., 1994). Ag2S is one of the most stable sulfides and is not oxidized very

quickly.

Giesemann et al. (1992) presented a similar, off-line method using Kiba reagent, and

compared this method with an on-line combustion method (EA-GC-IRMS) for small

samples of sulfides and sulfates.

Preparation of the Kiba reagent

After Kiba et al. (1955): A quantity of 300 g orthophosphoric acid (extra pure grade; d = 1.7)

is placed in a flask and dehydrated on a hot plate until the temperature reaches 300�C.

During heating water vapor and phosphoric acid, ‘mist’ must be pumped away. About

20–80 g of tin(II)-chloride dihydrate (extra pure grade) is placed in a flask and covered by

200 g strong phosphoric acid obtained by heating. Clean CO2 is flushed slowly through

8 What is meant is H2S and SO2, respectively, which were formed by reaction between sulfate and Kiba-reagent;

see equations [8-3.4–8-3.7].9 This reaction is often referred to as ‘in-vacuo Kiba’ and differentiates from the Kiba reaction at atmospheric

pressure (note by B. Mayer).

Sulfur 679

the flask, while heating to 300�C. HCl is formed and carried away by the CO2.

A brownish color may appear (probably tin(II)-sulfide) and has to disappear after

prolonged heating. During the cooling of the contents, CO2 is flushed continuously

through the flask. The tin (II)-strong phosphoric acid can be stored in a desiccator (e.g.

over CaCl, silica gel or P2O5) for several months.

After Sasaki et al. (1979): In a 500-mL Pyrex flask (Figure 8-3.5b), 300 mL of

orthophosphoric acid (extra pure grade; d = 1.7) is dehydrated by heating to 250�C for

1 h. Vapors produced by the heating are pumped away immediately. After cooling to

150�C, 30–40 g of tin(II)-chloride (extra pure grade) is added to the strong phosphoric

acid. Under a current of N2 gas, the mixture is heated for 1 h to 280�C – HCl and any

sulfur impurity (as H2S) formed by this heating are carried away by the N2 stream. After

the heating, the acid must cool down to below 150�C before the N2 flow is stopped. The

amount of the tin(II)-chloride in the reagent may vary depending on the concentration

of sulfur in the sample of interest.

After Ueda & Sakai (1983): Phosphoric acid (reagent grade) is dehydrated by heating in a stream

of air until the acid reaches a temperature of 250�C and then 8–10 g stannous chloride

dihydrate is added to each 1000 g of acid. The mixture is heated to 280�C in a stream of N2

for removal of HCl and H2O. The result is a fluid with the consistency of a syrup. If more

stannous tin is added in the mixture, more sulfate will be reduced to H2S. The reagent stays

active for reduction for a month if stored with metal Sn in an airtight bottle.

After Hall et al. (1988): A quantity of 1 L orthophosphoric acid (H3PO4) is dehydrated by

boiling for 1 h at 280�C under vacuum. After cooling, 300 g SnCl2•2H2O is added and

the mixture is reheated to 280�C for 1 h while a moderate flow of Ar or N2 is passing

through the solution to remove HCl and sulfur impurities. The flow continues during

cooling, and the Kiba reagent is stored in a desiccator.

After van der Raaij et al. (1992): 89% orthophosphoric acid is dehydrated by heating to

300�C in a stream of dry N2 gas; 20 g of pure tin(II) chloride dihydrate is dissolved in

200 g of the strong phosphoric acid and heated to 300�C in the N2 gas stream; the

resultant solution is stored in a stoppered jar with a piece of Sn metal.

Zn acetate solution preparation – Dissolve 40 g zinc acetate dihydrate (pure grade) in 100 mL

distilled water. Add 30 mL glacial acetic acid and dilute to 1000 mL with distilled water.

8-3.8. THERMAL DECOMPOSITION IN A VACUUM SYSTEM

A method for thermal decomposition of sulfates was presented by Holt & Engelk-

emeir (1970). A 20 mg sulfate sample (BaSO4 in their method) was placed in a small silica

glass sample tube (7 mm o.d.; 22 mm long); a 6-mm layer of pulverized quartz was added on

top of the sulfate, and a small plug of quartz wool closed the tube to avoid spreading of

powder in the reaction tube (Figure 8-3.7). The sample tube was inserted into the silica

glass reaction tube (9 mm i.d.; 90 mm long), was connected to the vacuum system (Figure

8-3.8) by a Cajon ultra-Torr union and was evacuated to about 10�4 Torr. Parts of the

system and the reaction tube were flamed quickly to remove adsorbed moisture (taking care

not to overheat). The sample was heated by a gas-oxygen cross-fire torch (see Figure 8-3.7)

from two sides, adjusted for softening the silica glass, in such a way that the end of the tube


started to collapse and the BaSO4 began to melt and decompose. Gas bubbles developed

from the liquefied mass, and heating had to be continued until the gas development stopped

(no new bubbles). Now the torch was moved 2–3 mm toward the quartz powder – if more

gas forms, completion of generation of the gas was delayed before another move forward

was made, again in a similar procedure, until the quartz powder was reached. The SO2 from

the decomposition was frozen in a liquid nitrogen cold trap during this procedure.

Complete trapping of SO2 took approximately 10 min after cessation of the sample heating,

reaching a pressure of 10�4 Torr again. After release and purification of the SO2 gas, the

yield was measured in the manometer and the sample was stored in the sample storage vessel

for MS measurement.

Yields obtained by this method with a range of 5–50 mg sample size were 100% – 1%

(1�). Precision of d34S measurements was –0.2‰ or better. CO2 production was low and can

FlameInner sample tube

Reaction tube

To vacuumsystem

BaSO4 and quartz powder

Cross-firetorch

Quartz wool

Figure 8-3.7 Reaction vessel for the sulfate thermal combustion method (after Holt &Engelkemeir, 1970).

Reactiontube

Heat shield

Storage vesselSO2 collection trap

Figure 8-3.8 Vacuum line for extraction and collection of SO2 from sulfates (after Holt &Engelkemeir, 1970).The reaction tube is shown in detail in Figure 8-3.7.

Sulfur 681

be decreased by treating the silica glass with a 1:1 HF solution; no SO3 was detected and only

traces of CS2 and COS. Heating with an induction furnace, where the sample and quartz

powder were in a Pt crucible, considering different set-ups and temperatures, was not

satisfactory (e.g. low yields; evaporation and deposition of the BaSO4 on the glass walls).

Bailey & Smith (1972) designed an improved method after Holt & Engelkemeir (1970)

(Figure 8-3.9). They tested sulfate with quartz powder in layers or mixed, or sulfate without

quartz powder, resulting in yields lower than expected in all experiments. Because 34S was

preferentially accumulated in the more oxidized species, with low yield, and/or with other

oxidized sulfur compounds present (e.g. SO3), isotopic fractionation was expected. Equa-

tions of reactions for the thermal decomposition of barite are

BaSO4 ! BaOþ SO2þ 1=2O2 [8-3.9]

and also of importance

BaSO4 $ BaOþ SO3 [8-3.10]

SO3 $ SO2þ 1=2O2 [8-3.11]

In these reactions, the fO2 is an important factor that is strongly dependent on temperature

changes. Temperature during the thermal decomposition process is not controlled, and

therefore also not fO2. Thus the SO3 production level is also not controlled.

For this reason, a furnace filled with Cu turnings (heated at 800�C) was introduced in the

system, to keep fO2 levels low, and thus avoiding significant SO3 production (also used by M.

Herman, unpublished; mentioned in Haur et al., 1973). Bailey & Smith (1972) placed a dry

ice/acetone slush trap in front of the SO2 collection trap to remove any water from the sample

gas. Further procedures were similar to the description given above for the method by Holt &

Engelkemeir (1970). Although good yields were obtained by Holt & Engelkemeir (1970)

with their method, good reproducibility was questioned by Bailey & Smith (1972).

A similar system as was used by Holt & Engelkemeir (1970) was used by Watanabe

(1979). BaSO4 (prepared from steel alloys, see Chapter 8-5) was placed in a silica glass tube

(8-mm bore) and kept in place by a plug of quartz wool. This tube was inserted into the

closed end of a wider silica glass tube, held in place by another plug of quartz wool and

connected to a vacuum device comparable with the one shown in Figure 8-3.9. The tube

To pumps

To pumps

Samplebreak seal

Furnace

Reaction tubeCu-t

Al foil heat shields

BaSO4 and sample tube

Figure 8-3.9 Device for S isotope analysis on sulfate (after Bailey & Smith, 1972).Cu-t= copper turnings.


with sample was evacuated to 10�6 mmHg (at least for 20 min, to remove pollutants) and

was heated to the weakening point of the glass to decompose the sulfate into SO2.

A similar system was used by F. Saupe (1995, personal communication), with amor-

phous silica (e.g. ground silica glass of high purity) instead of quartz powder.

Thermal decomposition (pyrolysis) was applied by Savarino et al. (2001) on AgSO4 samples

(other forms of sulfate were converted first). The procedure to prepare SO2 from the sulfate is

described in Chapter 6-4.5 and the system is schematically shown in Figure 6-4.6. If only d34S

values are desired, the SO2 can be measured directly. In case there is also interest for measure-

ment of 33S and 36S values, the SO2 was transferred cryogenically to a 30% H2O2 solution and

reacted by thawing to SO4. The SO4 was further processed by fluorination methods.

It is noted here that the ‘established’ methods as Savarino et al. (2001) refer to are

actually not describing fluorination of sulfate, but are referring in their turn to Gao &

Thiemens (1991) describing fluorination of sulfides. Sulfates must be reduced to H2S and

converted into Ag2S before fluorination is applied (Gao & Thiemens, 1991).

Conversion of small quantities of sulfate salts to silver sulfate is extensively described and

discussed by Savarino et al. (2001).

8-3.9. SAMPLES COMBUSTED WITH NaPO3

8-3.9.1. NaPO3

Halas & Wolacewicz (1981) presented a method based on the reaction of sulfate with

NaPO3 to release SO2 (Figure 8-3.10). This reaction is according to the equation

BaSO4þNaPO3 ! NaBaPO4þ SO2 [8-3.12]

NaPO3 was prepared from NaPO4 by thermal decomposition followed by melting and

heating for 2 h to purify it from organic and sulfur compounds. After cooling to room

temperature, the NaPO3 was pulverized and stored in a desiccator.

A mixture of 20 mg of sulfate (BaSO4) and 60 mg of NaPO3 was weighed accurately

and loaded on a porcelain boat. The boat was brought into the vacuum system, evacuated

and degassed. A Cu plug was inserted 5 cm from the boat, serving to reduce into SO2,

SS

SS-union

ts

f2f1

ips

vc

sv

Porcelain boat

qt

Cu net

Figure 8-3.10 Device for reducing sulfate by NaPO3 (after Halas & Wolacewicz, 1981). f1,f2= furnace 1, 2; ips= inert plastic seal; porc b=porcelain boat; qt=quartz glass; sv= samplevessel; ts=Teflon seal; vc= vacuum.

Sulfur 683

eventually formed SO3 during the reaction. Furnace 2 was heated at 850�C for reaction of

the sample with NaPO3. Furnace 1 was kept at 900�C. After 25 min the SO2 was measured

for yield and collected in a sample vessel for MS measurement.

After 6–10 runs, the Cu plug, coated with oxide, was reduced by H2 at �700�C. To

avoid contamination of the SO2 by H2O or CO2, samples should be treated with HCl and

roasted before extractions. The method had a precision better than –0.2‰. Alt et al. (1989)

used this method, with a furnace temperature at 950�C.

Halas et al. (1982) described an improved method for the NaPO3–sulfate reduction

technique (Figure 8-3.11). Instead of using porcelain boats, small Vycor glass tubes were

used. The sample tube (10 mm o.d., 20 cm long) with Cu turnings was placed inside a larger

silica glass tube (20 mm o.d., 30 cm long, 2-mm wall thickness), which can be used several

times. Cu turnings were prepared by heating to 800�C under vacuum. A sample of 100 mg

NaPO3 was loaded in the bottom of the sample tube, with 15–20 mg of the sample (BaSO4)

on top without mixing. The tube was plugged with a piece of quartz wool to keep the

sample and NaPO3 inside. The sample tube was inserted into the reaction tube and

evacuated in about 5 min, followed by outgassing by heating for a short time (do not pass

the melting temperature of the NaPO3 = 610�C). For reaction, the furnace was set at

1000�C, causing an internal heat of approximately 900�C. Reaction was continued for

20–25 minutes. The gas mixture (SO2, O2, CO2, H2O) from the reaction was collected in a

cold trap and SO2 was separated by the cryogenic method and, after collection, measured

on an MS for its isotopic composition. This method had a reproducibility of �= –0.04‰.

Halas & Szaran (1999) again simplified the procedure as described in this paragraph

(Figure 8-3.11). The former methods with NaPO3 were not producing sufficiently enough

high precision on small (1 mg) samples.

A low temperature method for reaction of sulfate with NaPO3 in Cu boats was described.

This method is simple, with cheap materials, and no need of complicated preparation

procedures. Boats of Cu can be produced from Cu foil (�0.1-mm thick) by the use of a

press, and which are preheated under vacuum at 750�C for cleaning. NaPO3 was prepared as

described above by Halas & Wolacewicz (1981). The volume of the vacuum system was as

small as possible and joints must be greaseless to prevent memory effects.

For reaction, 10 mg of sample and 60 mg of NaPO3 were loaded into a clean Cu boat.

The system was evacuated to about 0.13 Pa (1 mTorr), and the boat was heated for 10 min

tc

VacuumReactor

Furnace

Gas bottle

qwCu turnings NaPO3

Sulfate

Figure 8-3.11 Improved device for sulfate reduction by NaPO3 (after Halas et al., 1982).tc= thermocouple; qw=quartz wool.


at 200�C for outgassing. Reaction takes place at 650–700�C and was completed in

7–10 min. SO2 was trapped in a cold finger and after complete reaction immediately

admitted into the inlet of an MS. The advantage was that ‘clean’ SO2 was produced with

this method, and no further cryogenic separation was required.

Various amounts of between 1 and 20 mg were tested successfully with a reprodu-

cibility better than 0.03‰ in the case of pure BaSO4. No memory effect was found.

Halas & Szaran (2001) improved this method further by adding V2O5 to the NaPO3 in a

weight ratio of 5:3. This addition prevented overboiling of NaPO3 at reaction tempera-

tures above 800�C.

8-3.9.2. NaPO3 + Cu2O

Halas & Szaran (2004) used a mixture of Cu2O and NaPO3 in a 1:1 weight ratio for reaction

with sulfates. Sulfates were combined with this mixture in a 1:10 weight ratio and placed in

boats made of Cu foil (analytical grade). A similar vacuum device as is shown in Figure 8-3.12

was used. Mixtures were degassed at 300�C for 10 min. Reaction temperature was 750�C and

reaction time 15 min. The best reproducibility was obtained when the silica glass tube was

‘activated’ inside with several milligrams of V2O5 at �800�C. This thin layer of V2O5

probably protects the glass against extraction of spurious sulfates from the glass. The use of

pure Cu powder instead of Cu2O showed no catalytic properties. Graphs showing the result

of reaction time against d34S values and yield of SO2 are shown in Figure 8-3.13.

Y = 100.3 – 426.3exp(–t /2.05)

30

40

50

60Yie

ld (

%)

70

80

90

100

R 2

= 0.972

21

20

19

184 6 8

Reaction time (min)10 12 14 16

δ 34S

(pe

r m

il)

δ 34S = 21.17 – 15.78exp(–t /2.26)

R 2

= 0.981

Figure 8-3.13 Graphs showing the relationship between reaction time and �34S (lower graph)and SO2 yield (upper graph) found for the Cu2O^NaPO3 reaction me-thod with sulfates (afterHalas & Szaran, 2004).

Coldfinger

Vacuum

MS

qtz-tube

Cu-boat

Furnace

Figure 8-3.12 Vacuum device for low temperature sulfate oxidation by NaPO3 (after Halas &Szaran, 2004). qtz= quartz.

Sulfur 685

8-3.10. SAMPLES REACTED WITH V2O5

8-3.10.1. Samples reacted with V2O5 only

Larsen et al. (1959) used a similar procedure as that by Hagerman & Faust (1955) (see

Section 8-3.10.2 below), but they replaced the quartz packing to keep the sample in place

with a Cu turnings plug and used N2 gas instead of air as the ‘sweep gas’. Cu reduced and

eventually formed SO3 into SO2.

8-3.10.2. Samples combusted in a stream of air and with V2O5

Hagerman & Faust (1955) developed a method for sulfide and sulfate combustion by using a

stream of air and with V2O5 as an additional oxidant to release all sulfur from refractory

minerals. See Chapter 8-1.2.5 for further description of the method.

Mayer et al. (1995a) used an EA for combustion of BaSO4 (0.45 mg) under a stream of

He þO2 in the presence of V2O5 at 1100�C. SO2 resulting from the reaction was separated

from other gases by GC and carried with a He stream into an MS (see for method:

Giesemann et al., 1994).

8-3.11. SAMPLES MIXED WITH V2O5 OR CU2O (+SIO2)

Haur et al. (1973) developed a method for decomposition of sulfates by mixing with

V2O5 and silica in a ratio of 1:3:2. They reacted this mixture at 1000–1050�C (reaction

starts at 700�C) for 30–35 min in an evacuated, degassed (on vacuum line, at 400�C for 6 h:

vacuum of 10�5 Torr was obtained) and sealed ampoule (off-line). After reaction, the

ampoules were cooled immediately in cold water to prevent a reaction of SO2 into SO3

(silica glass must be used here to avoid cracking of the ampoule when ‘shocked’ in the cold

water). The ampoules were connected to an MS and broken to release the SO2 for isotopic

measurement.

Vanadium pentoxide binds the released basic components of the sulfate sample in

the form of nonvolatile vanadates and enables a more complete liberation of the acid

components: SO2 or SO3. Silica raises the melting point of the mixture and improves

thus the porosity during the reaction – a vitrified melt needs far longer time

for liberation of the gases of interest. Cu was added just above (but not touching)

the sample mixture; Cu reduces the PO2during reaction and prevents the formation

of SO3. Precision for this method, determined on a laboratory standard, was better

than –0.1‰; reproducibility of the method in general was better than 0.3‰. Accuracy

was <�0.4‰.

Yanagisawa & Sakai (1983) referred to methods by Holt & Engelkemeir (1970),

Bailey & Smith (1972), Haur et al. (1973), Coleman & Moore (1978) and Halas &

Wolacewicz (1981), where SO2 yields as low as 75% were obtained or where no

correction for the oxygen isotopic influence was applied. Yanagisawa & Sakai (1983)


presented a routine method for sulfate analysis resulting in SO2 yields of 95–99%. Their

extraction device is shown in Figure 8-3.14. Samples of sulfate (BaSO4 in this example;

5 mg) were mixed with V2O5 (50 mg) and SiO2 (50 mg). This mixture was loaded into a

silica glass reaction tube and covered by a plug of quartz wool. Another plug of quartz

wool was fixed 2 cm above the other plug. The reaction tube was heated in open air at

450�C for –30 min to remove organic contaminants. Cu wire (2 g) was placed on the

second quartz wool plug and the reaction tube was connected to the vacuum system and

evacuated. The Cu wire was heated by a hand torch to glowing red until degassed. The

reaction tube was heated by a furnace, placed just above the Cu wire. The furnace was

closed at the bottom end (= hot spot low in furnace). The furnace was heated up slowly

(important to avoid jumping of the mixture into the system, to ensure consistent

isotopic composition, and for complete reduction of SO3 on Cu wire). At 500�Ccollection of SO2 by a liquid nitrogen cold trap started; at 600�C the first SO2 was

evolved and heating was proceeded to 900–950�C to assure a complete sulfate decom-

position. After reaction the liquid nitrogen was replaced by an acetone slush bath to

remove CO2.

Difference in performance, by using a BaSO4–V2O5 mixture or a BaSO4–V2O5–SiO2

mixture, is shown in Figure 8-3.15.

An equation for the reaction with a mixture of barite and V2O5 (in presence of SiO2) is

BaSO4þ 6V2O5ðmeltÞ $ BaV12O30þ SO3þ 1=2O2 [8-3.13]

Without the SiO2 in it, the mixture is a viscous melt, creeping up the walls of the reaction

tube and reacting with the quartz wool and an irregular reaction (oxidation?) with the Cu

wire (Figure 8-3.15: middle tube in figure). With SiO2 present, a melt is formed in the

mixture, without reacting with the quartz wool and only forming an oxidized front in the

Cu wire (Figure 8-3.15: right tube in figure).

Yanagisawa & Sakai (1983) tested different mixtures and reaction temperatures; the

presented method showed the best results.

rt

s-t

Air

CT

Al-ref

Furnace

tc

vac

Vacuum

Figure 8-3.14 Vacuum preparation line for S isotopes on sulfates (afterYanagisawa & Sakai,1983). Al-ref=Al reflector, CT= cold trap; rt= reaction tube; s-t= sample tube,tc= thermocouple; vac= to vacuum.

Sulfur 687

Ueda & Krouse (1986) described a method in which a sulfate sample was mixed with

V2O5 and SiO2. A description is given in Chapter 8-1.2.3. Sulfate minerals started to

produce SO2 at 600�C and reached maximum production near 750�C.

Carmody et al. (1998), in their extended review of analyzing sulfur compounds from

waters, described their system for the S isotopic determination on BaSO4 (and Ag2S)

(Figure 8-3.16a). BaSO4 was mixed with reagent (Cu2Oþ SiO2; after Coleman &

Moore, 1978), placed in a silica glass tube closed at one end, and a Cu wire coil is

BVS

Cu wire

qw bl coldep.

rp

ReactedCu wire

Cu wireOxidizedCu wire

Figure 8-3.15 Reaction tubes (afterYanagisawa & Sakai, 1983 ^ see also Figure 8-3.14).Tubesfrom left to right: before decomposition, decomposition with a BaSO4^V2O5 mixture anddecomposition with a BaSO4^V2O5^SiO2 mixture. BVS =BaSO4^V2O5^SiO2; bl coldep.= black colored deposit; qw=quartz wool; reac= reacted; rp= reaction products.

Movable furnace

(a)

(b)

Sulfatesample

Cu wire

Silica glass tube

Double looptrap

To vacuumTo vacuumTo vacuum

Flow-throughtrap

Sample tube

Multiport

Nupro SS-4BG valve

Bellows

Micro-volume

Capillary

To wasteion pump

To sourceIRMS

Figure 8-3.16 (a) Schematic diagramof vacuumextraction device used for preparation of SO2from BaSO4 and Ag2S samples (after Carmody et al. 1998). (b) Multiport inlet system ofsample gases into the ion source of an IRMS, used in combinationwith the extraction device asshown in A (after Carmody et al. 1998).


placed in front of the sample mixture toward the open end of the tube (at 1.5 cm

distance). The tube was evacuated and then heated to 1150�C (chromel–alumel thermo-

couple was used). SO2 from the reaction was collected in the flow through trap, cooled

to halfway with liquid nitrogen. By slowly moving the furnace forward, the Cu wire was

heated finally to a range of 600–800�C. After 15 min heating the reaction was complete

and was stopped. The liquid nitrogen trap was cooled to the top now, and non-

condensable gas was pumped away. The trap was warmed to ��150�C, and eventually

present CO2 sublimated and was pumped away. In the U-shaped trap, SO2 was collected

between �135�C and �80�C. Yield was measured in a calibrated volume and the SO2

was measured on an MS (see inlet system in Figure 8-3.16b). An uncertainty of –0.15‰

(1�), associated with the weighing, the extraction and the MS methods, was reported for

repeated analyses.

Han et al. (2002) presented a method for analysis of sulfur isotopes of sulfide and sulfate

samples mixed with V2O5 and SiO2 in a stopcock reaction vessel device. Details of the

method are given in Chapter 8-1.2.3 and Figure 8-1.7.

O2 oxidation þ tungstic anhydride–CuO–Cu reactor in an elemental analyzer – Haystead

(1990) used O2 oxidation in a tungstic oxide–silica granules–Cu reactor (at 1000�C) under a

He flow (70 mL/min). Analysis of sulfate for S isotopic composition was tested, resulting in

incomplete conversion into SO2 without significant isotopic fractionation. A further

problem might be that two atoms of the SO4 may introduce an oxygen isotope effect on

the 66 ion beam in the MS, although this effect is expected to be very small.

Giesemann et al. (1992) described a system for on-line analysis of sulfates (or sulfides) in

an EA by O2 flush combustion followed by reaction in a reactor with tungstic anhydri-

de–CuO–Cu. A He carrier transported the gas through an anhydrone trap to remove water

and through a GC for SO2 purification. A similar system was used by Mayer et al. (1995b)

to analyze BaSO4 samples (0.45 mg) in an EA system with V2O5, at 1100�C, and with a

flush of O2 in a He carrier stream.

Samples mixed with V2O5 in an elemental analyzer – Giesemann et al. (1994) described a

system for on-line analyses of sulfates in an EA, with samples loaded in Zn capsules. See

Chapter 8-1.2.4 for description.

Bottcher et al. (1998) used this technique, with some modifications, for analysis of sulfur

isotopes in sulfate (BaSO4) samples precipitated from pore water samples. Bottcher et al.

(1998) packed 0.5 mg sample with 0.2 mg V2O5 in Sn capsules. Combustion occurred

under a He flow (80 mL/min) with a pulse of O2 in the combustion/reduction reactor (at

1100�C) of an EA, increasing temperature for a short period to �1800�C by flash

combustion (exothermic reaction of Sn to SnO).

Kester et al. (2001) mixed 0.7 mg BaSO4 with 1–2 mg of V2O5 in a Sn capsule crushed

into a small ball for analysis of S isotopes in an EA (with WO3–Cu wire reactor set-up).

8-3.12. SAMPLES MIXED WITH Cu2O + SiO2

Coleman & Moore (1978) presented a method (Figure 8-3.17), being a modifica-

tion of earlier published methods of combustion of sulfate to produce immediately SO2

for isotopic measurement. The method basically was following the procedure as

Sulfur 689

described by Robinson & Kusakabe (1975) in Chapter 8-1.2.6. An addition was the

furnace (600�C) with Cu turnings for reduction purpose. Cu2O is produced by heating

CuO in vacuum at 900�C. Silica used in the combustion was pure quartz ground to

–75 mm.

A sample (15 mg barite) was ground with 200 mg Cu2O and 600 mg SiO2 and loaded in

a small silica glass sample vessel. To retain the sample in the tube, a quartz wool plug was

added. This tube was inserted in the silica glass reaction tube, evacuated and degassed.

A magnet and magnetic pusher were used to push the sample tube in the furnace. Reaction

took place at 1120�C. H2O was trapped in a CO2–acetone trap, SO3 was reduced by the

Cu furnace into SO2 and CO2 was separated from the SO2 by a n-pentane trap (see Oana &

Ishikawa, 1966). Yield was measured and the SO2 sample was collected in a sample vessel

for measurement on an MS. The same vacuum device can be used for sulfide analyses; the

Cu furnace can then be left out.

Yields were consistently 99.8 – 1.3% and a precision of – 0.11‰ for the method was

given.

8-3.13. WO3 + Al2O3 OXIDATION IN AN EA-GC-MS SYSTEM

Grassineau et al. (2001) oxidized sulfate (and sulfide) samples (powder to coarse grain

size) in a reactor, packed with WO3 þ Al2O3 and pure Cu wires, at 1030�C and flushed

with He (80–120 mL/min). For details, see Chapter 8-1.2.7.

8-3.14. COMBINED O2 + WO3 OXIDATION–Cu REDUCTIONIN A EA/CF-GC-MS SYSTEM

Yun et al., (2004) analyzed BaSO4 in an ‘on-line’ system with combined combus-

tion–reduction reactor and GC analyte gas separation for SO2 purification. Description of

the system is given in Chapter 8-4.2.9.

Pusher

Magnet

Sample

Furnace Cu furnace

CO2–acetonetrap n -pentane

trap Sample vessels

Figure 8-3.17 Extraction device for S isotope determination on sulfates (after Coleman &Moore, 1978). Omitted in the figure are connections to the vacuum pumps and pressure gaugesfor all separate sections.


8-3.15. MAGNESIUM REDUCTION METHOD

Thode et al. (1954) described a method of sulfate reduction by Mg power. A sample (0.2–

0.5 g BaSO4 by Thode et al., 1954) was placed in an iron crucible with 1 g Mg powder below

and 2 g Mg powder on top of the sample. The crucible was closed and N2 was introduced

through a ceramic tube leading into the crucible. The crucible was heated at 400�C for 20 min

and then at 650�C for 5 min. In this way sulfate was reduced to sulfide and unreacted Mg to

magnesium nitride. After cooling, the contents of the crucible were reacted with concentrated

HCl and the H2S trapped with cadmium acetate as CdS, and converted into Ag2S by addition of

silver nitrate to the solution. The Ag2S was analyzed by methods as reviewed in Chapter 8-1.

8-3.16. LASER HEATING METHOD

Shanks et al. (1998) noted that in situ laser analysis of sulfate minerals, producing SO2,

is a potentially useful technique (analysis of an anhydrite was reported).

Alonso-Ascarate et al. (1999) and Macaulay et al. (2000) used a CO2 laser for de-

composition of anhydrite by heating in a vacuum system and release of SO2 for sulfur isotopic

measurement. Applied was a 4 W laser beam, resulting in a sampling spot of 100 mm size.

A CO2/acetone slush trap removed water and CO2 was removed by a standard n-pentane trap

separation procedure. A mean isotopic fractionation of �1.4‰ was introduced by the laser

beam, with a standard deviation of 0.2‰. This resulted in a correction equation:

�34Strue ¼ �34Slaser þ 1:4 ð– 0:2Þ [8-3.14]

If this method is applied on anhydrite included in, or directly surrounded by sulfides (pyrite

was given as example), a larger SO2 yield resulted than if produced only from the sulfate.

A reaction, following the equation

MSO4þ FeS2 ! M þ FeSþ 2SO2 [8-3.15a]

takes place. M stands for a metal (e.g. Ba or Ca). This reaction probably proceeds in parts:

FeS2 ! FeSþ S [8-3.15b]

MSO4þ S! M þ 2SO2 [8-3.15c]

8-3.17. FLUORINATION METHOD

8-3.17.1. Fluorination in Ni tubes

Sulfates have to be converted into sulfides before fluorination (Gao & Thiemens, 1991; see

Chapter 8-1.7.2 for description of method). Fluorination of sulfates, following this method,

was reported by Savarino et al. (2001).

Sulfur 691

8-3.17.2. Laser fluorination

Shanks et al. (1998; quoted is personal communication with R. Ilchik) mentioned the

testing of laser SF6 techniques on sulfate minerals. They concluded that it is unlikely that

this will become a valuable technique for S isotope analysis of sulfates because significant

SO2F2 and/or SOF2 production causes severe isotopic fractionation.

8-3.18. NEGATIVE-ION TIMS METHOD

Wachsmann & Heumann (1992) used a negative-ion TIMS method for measur-

ing S isotopes (for details, see Chapter 8-5.3). They reported that high S� ion

currents could be obtained by using BaSO4 as sample for measurement. BaSO4 is

formed by the addition of Ba(OH)2 to any sulfate compound at the loading procedure

stage (see Chapter 8-5.3.2). The ionization filament was heated up to 970–1020�C,

after which the current was elevated to 1.3–1.4 A with a heating rate of 0.1 A/min.

Using BaSO4, high ion currents of SO2�, with an intensity of about 5� 10�12 A,

were observed.

8-3.19. ICP-MS METHOD

Gregoire & Naka (1995) used an electrothermal vaporization (ETV) ICP-MS

analytical technique for quantitative determination of sulfur with a detection limit of

13 pg (corresponds to a relative limit of detection of 0.26 ng/mL using a 50 mL

sample). They studied the characteristics of (K, Fe(III), Mg)sulfates (and sulfuric acid

and thiourea). ETV was applied by using a pyrolytic graphite-coated tube and an

autosampler was used for sample introduction. Ar (–50 mL/min) was used as carrier

and plasma gas. During the dry and pyrolysis steps of the temperature program,

opposing flow of Ar (300 mL/min), originating from both ends of the graphite tube,

removed water (reduction of isobaric interfering polyatomic ions) and other vapors

through the dosing hole of the graphite tube. During the high temperature or

vaporization step, a pneumatically activated graphite probe sealed the dosing hole,

and the Ar flow (with evaporated analyte) was directed directly into the plasma (flow

rate of 900 mL/min). Chemical modifiers (KCl, KOH, KNO3), added to the analyte,

were tested, with KOH giving the most favorable results (behavior of chemical

modifiers was given in detail by Gregoire & Naka, 1995). Sulfates thermally decom-

pose with release of SO3 (highly hygroscopic, forming sulfuric acid even with traces of

water vapor).

8-3.20. SIMS METHOD

A general description of the SIMS technique is given in Volume I, Part 1, Chapter 30.

Sulfur isotopes of sulfates by the SIMS technique are reported in Chapter 8-1.10.



Section Method Analyte T�C Comments Status

8-3.0 Introduction

8-3.1 Reduction with carbon (graphite)

(‘off-line’)

Sulfates 800–1000; 1050–

1100

Sulfide or native sulfur formed Old and obsolete method

C reduction þ NaF Sulfates

(barite)

1100–1200 NaF ascertains complete reaction; BaS

from reaction

precipitated as Ag2S

Rare, obsolete method

Reduction with C in Muffle

furnace

Sulfates High temperature

(unspecified)

No details given Unsuccessful method

Reduction with glassy carbon Sulfates 1300 On-line EA-IRMS Carrier: H2þN2 or

He

Modern method

8-3.2 Johnson–Nishita reduction-

distillation

Sulfate n.a. H2S formed by reduction; special tool

required

Uncommon method

8-3.3 Reduction with Fe powder Sulfates

(Ba-

sulfate)

950–1050 Excess of Fe; off-line method Unsuccessful, rare method

8-3.4 Conversion to sulfide with Fe

carbonyl þ Zn

Sulfates Between

950 and 750

(depends

on sulfur

composition)

Zn powder accelerated reaction; H2S is

formed

Rare, obsolete method

8-3.5 HI–H3PO2–HCl reduction Sulfates ‘Boiling solution’ Reaction with reduction mixture; H2S

formed

Old, obsolete method

8-3.6 HI–H3PO2–formic acid reduction Sulfates ‘Boiling solution’ Johnson–Nishita reduction apparatus Very rare method

8-3.7 Kiba reagent (SnII-strong

phosphoric acid)

Sulfates Slowly heated to

280

H2S formed, converted to ZnS or Ag2S

eventually N2 flushed

Standard method

8-3.8 Thermal decomposition in

vacuum system þ silica glass

Sulfates �1400–1500 Hand torch heated reaction–glass is

weakening

Useful method needed for

experimenting glass weakness

8-3.9 Combustion with NaPO3

8-3.9.1 NaPO3 (– Cu) Sulfates

(BaSO4)

850–950 Vacuum device required Relatively modern method; less

common

8-3.9.2 NaPO3 þ Cu2O Sulfates 750 (�800) Glass walls first deactivated by reaction

with V2O5

Modern method; very

uncommon

(Continued )

Sulfur

69

3

Synopsis of methods reported in Chapter 8-3 (Continued )


8-3.10 Reaction with V2O5

8-3.10.1 V2O5 Sulfates 900–950 N2 flushing (‘sweep gas’) Older, rare method

8-3.10.2 Stream of air þ V2O5 Sulfates 900–950 Stream of air, ‘off-line’ Older method

1100 EA ‘on-line’ Modern method, less common

8-3.11 V2O5 or Cu2O þ SiO2 Sulfates 1000–1150 Off-line; vacuum system required Older standard method

O2 þ tungstic anhydride CuO–Cu

oxidation

Sulfates 1000 Incomplete reaction; oxyg. isot. effects

‘of-line’

Unsuccessful method

1100 þ flush

combustion

On-line EA system; He carrier Modern, rare method

V2O5 in EA Sulfates 1100 þ flush

combustion

He carrier; Sn capsules Modern method

8-3.12 Cu2O þ SiO2 Sulfates �1120 Cu turnings added for reduction Older standard method

8-3.13 WO3 þ Al2O3 oxidation (EA-

GC-IRMS)

Sulfates

(sulfides)

1030 On-line method, EA/CF-GC-IRMS Modern method

8-3.14 O2þWO3 oxidation þ Cu

reduction

BaSO4 1050 (flash �1700) On-line method EA/CF-GC-IRMS Modern method

8-3.15 Mg-reduction Sulfate 400, 650 Reduction in sealed tube; HCl reaction >

H2S, convented to CdS

Rare, old and obsolete method

8-3.16 Laser heating Sulfate

minerals

n.a. Spot analysis; side wall fractionation ‘Potential’ technique

8-3.17 Fluorination methods

8-3.17.1 Fluorination in Ni tubes Sulfates See Chapter 8-1.6.2 dangerous, toxic

chemicals

Older method

8-3.17.2 Laser fluorination Sulfates SF6 analyte gas Modern method

8-3.18 Negative-ion TIMS Sulfates n.a. Special equipment required Laborious and rare method

8-3.19 ICP-MS Sulfates n.a. Special equipment required rare method

8-3.20 SIMS Sulfates n.a. Special equipment required; spot analysis;

relative lower precision

Less common, modern method

69

4H

andb

ookof

Stab

leIsotop

eA

nalyticalTechniques

8-4. COMPLEX ORGANIC MATERIALS AND ORGANIC

COMPOUNDS

8-4.1. PREPARATION OF SAMPLES AND EXTRACTION METHODSFOR ORGANIC SULFUR

If organic sulfur is extracted from a matrix with a large amount of other components,

the sample material preferably is pre-treated with chromium (VI) in phosphoric acid to

remove inorganic sulfur impurities (Ohashi, 1955; Arikawa & Sasaki, 1987; van der Raaij

et al., 1992). This method is not suitable for samples with low sulfur contents (<2000 ppm)

where more than 1 g of sample needs to be processed (van der Raaij et al., 1992). Arikawa &

Sasaki (1987) found a reagent blank of 0.1–0.2 mg of sulfur in the Cr(VI)–H3PO4 acid

mixture.

An extra oxidative step, before treatment with chromium (VI)–phosphoric acid

(30 mL), by hydrogen peroxide (20 mL), and Fe (III) as a catalyst (5 mL) to oxidize excess

organic matter was introduced by van der Raaij et al. (1992). The reaction vessel with

sample and oxidant was placed in a water bath. If the reaction was too violent, distilled

water could be added to the reaction vessel. The same amounts of oxidation and catalyst

solutions were added after the first violent reaction was completed. The sample was

evaporated on a water bath to near dryness. Chromium (VI) strong phosphoric acid was

now added to the vessel to complete oxidation; the reaction vessel was placed in a water

bath again. When the residue has dissolved completely the vessel was heated to 180�C.

Heating was stopped when the solution had turned green and stopped fizzing. The solution

was cooled. The extra oxidative step reduced blank Ag2S from 0.5 to 0.2 mg.

Preliminary wet ashing of samples with low contents of sulfur was described by Johnson &

Nishita (1952). Wet ashing of dried and ground (sieved to pass 60 mesh) organic material

samples (with addition of ‘ashing aid solution’ = solution with 20% (m/v) of MgO and 2% (m/v)

Mg(NO3)2•6H2O prepared in 1.4 M HNO3) was carried out in a muffle furnace (initial

heating rate of 2�C/min to 150�C, held for 2 h and followed by heating at 4�C/min to

550�C, held for 8 h).

In order to remove inorganic sulfur in a first step treatment, simple dissolving of soluble

sulfate from samples with distilled water was applied by Lowe et al. (1971) and Wieder et al.

(1985; from peat samples). Mayer et al. (1995a) dissolved sulfate in hot double-distilled

water. In a next step, polysacharide sulfate was extracted by Lowe et al. (1971) by leaching

the residue of the first step with 1% NaCl, and humic acid fractions were obtained by

extraction with 0.1 N NaOH. All fractions were converted into sulfate, followed by

reduction into sulfide (Ag2S) for sulfur isotopic determination (see also Rees, undated).

Resano et al. (2001) noted the self-evidence that use of sulfuric acid in the extraction of

sulfur from organic (or other) compounds is excluded. In addition, the complex chemistry

of sulfur and the volatility of many sulfur compounds may result in significant losses of sulfur

during preparation.

Sulfur 695

Preparation of chromium (VI) strong phosphoric acid – A method was presented by van der

Raaij et al. (1992):

• 1 g of Sn (II) chloride dihydrate per 100 g of orthophosphoric acid was added to remove

sulfur impurities and the acid was dehydrated at 300�C in a stream of dry N2 gas.

• 12 g of potassium dichromate was dissolved in 100 g of the above solution by stirring

continuously while heating on a water bath.

Preparation of Fe (III) catalyst – A method was presented by van der Raaij et al. (1992):

10.1 g of Fe (III) nitrate (as Fe(NO3)3•9H2O) was dissolved in 250 mL water giving a

solution containing 0.1 mol/L Fe (III).

8-4.1.1. Plant materials

Case & Krouse (1980) collected lichens and tree needles (adhering substrate or litter was

removed by hand) in paper bags; they were air-dried immediately and sealed airtight in

plastic bags. Case & Krouse (1980) oxidized these samples by Parr bomb methods (see

Section 8-4.2.3 and 8-4.2.4).

Haystead (1990) gave an overview of methods for S isotope analysis on plant (or soil)

materials (Figure 8-4.1).

8-4.1.2. Petroleum and coal

A scheme for removal of sulfur of different phases in petroleum or coal was given by Westgate

& Anderson (1982) and Rees & Holt (1991) and is presented in Table 8-4.1. Extraction of

sulfur from coal is very similar to methods used for sediments (Rees & Holt, 1991).

Plant/soil BaSO4 Hydrogen sulfide Cadmium sulfide Silver sulfide

Sulfur dioxide

Sulfur dioxide

Sulfur dioxide DEA-MS

(see Section 8-4.2.7)

Figure 8-4.1 Diagram showing sample preparation methods for IRMS and DEA-MS (seeSection 8-4.2.7) (after Haystead,1990). DEA=Dumas elemental analysis.

Table 8-4.1 Scheme for sequential extraction ofdifferent forms of sulfur for isotopic analysis (afterWestgate & Anderson, 1982 and Rees & Holt, 1991)

A - massive pyrite removalB - extraction of acid-soluble sulfidesC - extraction of sulfate sulfurD - extraction of different disseminated pyriteE - extraction of organic sulfur


8-4.2. ANALYTICAL METHODS

8-4.2.1. Combustion in a stream of O2 – Pregl method

Beuerman & Meloan (1962) used the ‘Pregl’ method, in which an organic compound

was combusted (850�C) in a stream of O2 (6 mL/min) using a Pt catalyst (Pt on asbestos

and ‘Pt stars’) for quantitative sulfur determination (Figure 8-4.2). Water from the

combustion was removed with CaSO4, and SO2 and CO2, together with some O2,

were trapped with liquid nitrogen. The SO2 was separated from the CO2 and O2 by a

column (20 ft length; 92 – 3�C) with dinonylphtalate using He as carrier gas 45 mL/min.

A complete analysis took 20 min. After a combustion, the system was flushed with O2 for

3 min.

It was noted that combustion was conducted at lower temperature (650�Cþ Pt catalyst)

by other analysts.

SO3 was converted into sulfuric acid by reaction with H2O2. In case N2 or

halogens were present, the sulfate must be precipitated as BaSO4. Formation of SO3,

besides the SO2, at lower reaction temperatures causes isotopic fractionation and is

unwanted.

The only compound removing H2O without also removing SO2 is CaSO4 (50–80

mesh). Anhydrous Mg-perchlorate produced explosions if the organic compound was not

combusted correctly. P2O5 was not tested.

8-4.2.2. Carius reaction

Beuerman & Meloan (1962) described the Carius method for measuring sulfur in organic

compounds (presented as quantitative method). An organic compound was reacted in a

sealed tube in the presence of nitric acid and an alkali salt other than a sulfate (Figure 8-4.2).

A sulfate was formed and was measured gravimetrically or titrimetrically for quantity and

Movable furnace

Cork

Sample boat (Pt or porcelain)

Pt ‘contact stars’

Platinized (10%) asbestos

O2Bubble counterwith ascarite,anhydrone andmineral oil

Glass wool

CaSO4

Liquid nitrogen trap

Vacuum

Figure 8-4.2 Pt catalyzed combustion device for analysis of the sulfur isotopic compositionsof organic compounds (after Beuerman &Meloan,1962).

Sulfur 697

can be analyzed following general methods for S isotope ratios composition in sulfates (see

Chapter 8-3).

8-4.2.3. Parr bomb oxidation

Siegfried et al. (1951), after giving a summation of existing methods for oxidation of

organic matter, gave a description of an off-line ‘Parr-bomb’ oxidation for organic

material to determine sulfur quantitatively (by titration of the sulfate formed by the

oxidation).

Case & Krouse (1980) used ‘Parr-bomb’ oxidation (no details given) to convert all plant

sulfur into sulfate. They added H2O2 to all washings to assure complete conversion. Ba2þ

was added as BaCl2 for BaSO4 precipitation. BaSO4 was converted to Ag2S following the

method by Thode et al. (1961). Ag2S was analyzed by Cu2O combustion and SO2

measurement on an MS. Precision of the method was –0.2‰.

8-4.2.4. Parr-bomb oxidation (+starch additive) followed bySn(II)–H3PO4 reduction

Arikawa & Sasaki (1987) described a method in which organic material was oxidized in a

Parr bomb (1 g of sample þ �1 g starch, 1 mL H2O at bottom of the bomb, filled with 30

atm O2) under pressure. The bomb was placed in a water bath and was ignited by electrode

discharge (takes �1/2 s). SO3 was formed this way. The contents of the bomb were

recovered by water washing. The washings were filtered, and 1 M HCl and 5 mL of 10%

BaCl2 were added to the filtrate to precipitate BaSO4. The BaSO4 (20 mg) was heated with

Sn(II)–H3PO4 (‘strong phosphoric acid’; see also Chapter 8-1) at 280�C under a flow of N2

for 20 min. The H2S formed in the reaction and taken away by the N2 flow was trapped as

ZnS in a 30 mL 4% Zn-acetate solution. After recovery of H2S as ZnS was completed, ZnS

was converted into Ag2S by adding 0.1 M Ag-nitrate to the solution. Ag2S was converted

into SO2 by methods such as reported in Chapter 8-1. This procedure was tested on pure

samples and samples with 0.1 mL 1-butanol or 1 g of starch as additive for the recovery of

sulfur. For cystine as testing compound, recoveries of 45%, 70% and �100%, respectively,

were found.

8-4.2.5. Kiba reagent method

Sasaki et al. (1979) reported the use of Kiba reagent for organic sulfur measurement. See

also Kiba et al. (1957b) for organic matter reaction.

Van der Raaij et al. (1992) used, after pre-treatment of the sample to remove

interfering components and excess organic matter by oxidation using hydrogen peroxide

and Fe (III) catalyst, the Kiba reagent method followed by chromium (VI) strong

phosphoric acid (see above). About 50–60 mL of Sn (II) strong phosphoric acid was

added to the reaction vessel containing the sample solution left after the pre-treatment.

The vessel was connected to a system with a dry N2 flow, and the flow was passed

through a trap containing 25–50 mL AgNO3 solution. The reaction vessel was heated to

280�C and the H2S was deposited as Ag2S in the trap. A second trap with AgNO3


solution may be used to ascertain complete H2S recovery. Ag2S was collected on a

millipore filter and dried in air before analyses for S isotopes following techniques as

described in Chapter 8-1.

8-4.2.6. Eschka reduction

Smith et al. (1964) extracted organic sulfur from oil shales by reaction with Eschka mixture

calcinating the sample. The calcined sample was acidified and bromine water was added;

BaSO4 was precipitated from the solution after adding BaCl2 (see Chapter 13-1.5).

Eschka mixture10 (two parts MgO þ one part Na2CO3 by weight; Smith et al., 1964;

Giesemann, personal communication 1999; Kester et al., 2001) is available in prepared form

commercially. It is used for standard tests of sulfur contents in coal and coke (Giesemann,

personal communication 1999).

Kester et al. (2001) mixed 0.5–3.0 g of sample with three times the weight of Eschka’s

mixture in a ceramic crucible and covered this with an additional 1–3 g Eschka’s mixture.

The crucible with sample mixture was placed in an oven slowly heated to 800�C and kept

at this temperature for several hours. The crucible was allowed to cool overnight, and the

mixture was then dissolved in distilled water (10 mL/0.1 g sample) and the solution heated for

30 min. The solid residue was filtered off and discarded. The filtrate was adjusted to pH< 4.0

(6 N HCl) followed by addition of 10 mL Br-saturated distilled water solution. This

solution was boiled (in fume hood) until all Br was expelled (clear solution). Then 10 mL

of 10% BaCl2 solution was added and boiling continued for 15 min. Heating was stopped

and the covered solution digested overnight. The BaSO4 was filtered, rinsed and dried for

further analysis (see Chapter 8-3 for methods).

8-4.2.7. Oxidizing fusion to sulfate

Rees (undated) reported about oxidizing fusion of vegetation samples into sulfate (BaSO4)

for sulfur isotope analysis and referred to Mekhtiyeva et al. (1976) for this method.

Wet combustion to sulfate of marine plants and animals was shortly described by Kaplan

et al. (1963). Br2, nitric acid, NaCl and a mixture of nitric and perchloric acids (1:1) were

used respectively for extraction of sulfur from organic materials as sulfate.

8-4.2.8. Sodium hypobromite oxidation

Reduced sulfur species (in sediments, see also Chapter 13-2) were oxidized by hypobromite

(freshly made before analysis: 3% Br2 in 2 M NaOH) (Amaral et al., 1989). A mixture of

2–3 g sediment with 5 mL NaOBr solution was placed in a three-port boiling flask. The

contents in the flask were swirled, allowed to sit for 5 min and heated at 250–260�C until

dry. Another batch of 2–3 mL NaOBr solution was added and was again heated to dryness.

Heating continued for 30 min to ensure complete oxidation of the organic sulfur into

sulfate. After cooling, 3 mL of 90% formic acid was added, the contents swirled and allowed

to stand for 25 min. The formic acid destroyed any remaining traces of hypobromite and

acidified the sample in preparation for reduction of the sulfate to H2S.

10 Commercially available Eschka mixtures have a significant S blank (note by B. Mayer).

Sulfur 699

8-4.2.9. Flash combustion with O2 in He atmosphere

Dugan & Aluise (1969) presented a system for simultaneous quantitative determination of S

with H, N, C and O in organic compounds by combustion with O2 in a He atmosphere.

Effluent gases were collected and partly separated in carbowax and molecular sieve 5 A

columns. Total separation can be obtained by cryogenic distillation. See Chapter 1-3.2.2 for

more details.

8-4.2.10. Elemental analyzer combustion–oxidation by on-linemethods

Pichlmayer & Blochberger (1988) described an elemental analyzer (EA) flash combustion

system for determination of S isotopes in organic compounds. Two systems were men-

tioned: one for S isotopes only and another for a combined determination of C, N and S

isotopes. In both systems combined combustion–oxidation occurs, with a sample in a Sn

capsule, dropped on WO3 on Al2O3 and with a quantity of O2 added. Precision, for

samples between 50 and 1000 mg S, was better than –0.5‰.

Haystead (1990) presented a Dumas elemental analyzer–MS method (DEA-MS) for S

and C isotope determination on plant (10–30 mg) or soil (50–150 mg) materials (Figure

8-4.3). Sample size was chosen to be equivalent to �20 mg S (lower limit 10–15 mg S

equivalent). Samples were packed in Sn foil capsules and oxidized at 1000�C in 10 mL pure

O2 with a WO3 catalyst. Combustion gas was swept over heated reduced Cu (reduces traces

of SO3 into SO2 and NOx into N2) by a He carrier (70 mL/min). Water was removed in a

desiccant trap and SO2 was concentrated in a Hayesep column [column filled with a porous

polymer = polydivinylbenzene (P. Eby, 2001, ID)]. The Heþ SO2 gas stream was lead into

a T-valve (1/16 in.), from where part of the gas was entering the ion source of an MS. Both

d34S and %S data can be obtained simultaneously (as well as d13C and %C). Nearly

coincident peaks of N2 and CO2 were separated from the SO2 peak by GC. This procedure

took 9 min for one sample. A precision of –0.5‰ (at natural abundance levels) was reached

50 place autosampler

H-u

Outletto user

GC

wt

Pneumatic sample slider

Silica wool

Tungsten oxide

Silica wool

Silica wool

Silica wool

Silica granules

Reduced copper

Combustiontube (1000°C)

o-MS

Figure 8-4.3 Elemental analyzer device with Dumas type combustion for determination of Sisotope composition of plant or soil materials (after Haystead, 1990). H-u=Hayesep unit;o-MS=outlet toMS; wt=water traps.


with this method. Memory effects for consequent samples (SO2 is a chemical reactive

compound) with large difference in isotopic values can be solved by passing a number of

dummy samples before measuring the next sample.

Measurement on masses 48, 49 and 50 (SO varieties) instead on 64, 65 and 66 (SO2

varieties) prevent impossible resolution of combined peaks (e.g. 34S16O and 32S18O). Ion

currents of ‘SO’ are small and increasing the energy level of the electron beam was needed.

Giesemann et al. (1994) also used an EA combustion–oxidation preparation/extraction

method on soil or plant materials, resulting in Ba-sulfate or Ag2S (see Chapters 8-3 and 8-1,

respectively, for further procedures). On-line methods with EA techniques are described in

Chapter 8-1.1.4.

Kester et al. (2001) used a WO3–Cu wire (quartz wool on top, between and below

layers) reactor setup for direct analysis of organic samples (2–10 mg) mixed with V2O5

(1–2 mg) in an EA (Figure 8-4.4). Further, the method is similar to that of Giesemann et al.

(1994) (reaction at 1020�C and with 5 mL pulse of O2; flash combustion with Sn at

1800�C).

Fry et al. (2002) discussed the impact on the measured d34S values by the oxygen

isotope variation in SO2 analyte gas in on-line EA-IRMS systems. They presented a

simplified combustion reactor (Figure 8-4.5), similar to standard EA sulfur reactors, but

without tungstic oxide and only containing ultra-high-purity Cu metal and quartz chips.

The furnace was set at 1020�C, and temperatures were found to exceed 1084�C, the

melting point of copper, during combustion. Therefore, copper wire was placed at

the lower parts in the reactor, well below the hotspot to prevent melting. Removal

of the sample ash buildup by vacuuming out the ash after every 10–30 samples was

routine procedure. Samples were weighed in Sn boats (0.1–8 mg, depending on S

content of sample) and mixed with �5 mg V2O5 and optionally for refractive materials

Autosampler

Quartz wool

Quartz wool

WO 3

Cu wire

GCcolumn

(removal ofN2 + CO2)

cm: 0.5 6.0 2.0

9.0

4.0

Mg perchlorate water trap

MS

Con Flo II Gas Mixing

Box

Silica glass He flow

Quartz wool

Figure 8-4.4 EA-MS system withWO3^Cu wire reactor for analysis of organic samples þV2O5 in Sn capsules (or BaSO4 samplesþV2O5) (after Kester et al., 2001).

Sulfur 701

2 to 10 mg of dextrose was also added. The system was prepared in such a way that the

SO2 analyte gas was measured sequentially for its SO2 (mass 66 and 64 ion beams) and

SO daughter fragments (mass 50 and 48 ion beams) in order to obtain the oxygen

isotope composition for correction of the sulfur isotope composition. Precision of

–0.13‰ (1 SD; n = 39) for replicate combustion was obtained. Analysis time needed

for a sample was 4 min.

Yun et al. (2004) developed an on-line EA-CF-IRMS system for S isotope analysis of

lichen (organic) samples (Figure 8-4.6). Dry lichen powder (15 mg = equivalent to 9 mg S)

was combined with 0.2 mg V2O5 in an ultra-light (40 mg) Sn capsule. The capsule with

sample was combusted at 1050�C (flash combustion reaches �1700�C) with addition of

10 mL O2. All gases were carried by He (80 mL/min) through the oxidation–reduction

reactor, consisting of WO3 and Cu and separated by quartz wool. Water vapor was

removed from the analyte gases by a water trap [75% Mg(ClO4)2 þ 25% quartz chips].

The remaining gases were passed through a GC column (1.2 m length; Poropac QS,

Teflon) at 75�C to separate CO2 and N2 from SO2. SO2 was carried to the inlet of an

MS. Performance of combustion can be monitored by the color of the WO3 – if combus-

tion was not complete oxygen from the WO3 will be extracted for reaction and the WO3

will show a progressive color shift from yellow, through green, through blue-green and

finally to black. Time required for a single sample analysis is 15 min. About 150 samples can

be analyzed before the reactor packing needs to be renewed. Yun et al. (2004) reported

similar precision with this method as obtained with off-line methods (also described in this

chapter).

Sample To MS

Water trap

Quartzwool

Cu wire

Quartzchips

Quartzchips

Quartzwool

Quartzwool

Figure 8-4.5 On-line reactors for S isotope analyses on organic compounds (after Fry et al.,2002). The second quartz chip reactor is for O isotope buffering of SO2.Water trap containsMg-perchlorate (– P2O5).


8-4.2.11. Chromatography – chemical reaction interface (CRIMS)method

Chace & Abramson (1989, 1990) described a chemical reaction interface–MS method in

which organic compounds were converted into small molecules in a microwave-powered

reaction interface (Figure 8-4.7).

Abramson & Markey (1986) tested successfully the determination of S/C ratios and S

quantities by this method for a variety of amino acids.

For more details of the CRIMS method, see below and Chapter 1-3.2.11.

Moini et al. (1991) gave a more extended description of the CRIMS method. Addition

of a reaction gas to a low temperature microwave-induced plasma creates a reaction

interface in which complex organic molecules are converted into small polyatomic neutral

species. HCl (anhydrous) was found to be the most effective reactant gas for selective

detection of sulfur-containing compounds. Mixtures of compounds first were separated in a

capillary gas chromatograph before chemical reaction. A detection limit as low as 30 pg was

achieved. A 2450-MHz microwave (operated at 70 W forward power and �5 W reflected

power) was placed inside a GC column (30 m, 0.25 mm i.d., 0.25-mm film thickness

capillary; fitted with split/splitless capillary adapter) oven. The column was connected to

a 1/16 in. T-piece, into which also the reactant gas flowed (adjusted by a variable leak).

This was connected to a ceramic tube (1/32 in. i.d.), using SS hardware and tubing and 5%

vespel/graphite ferrules. The ceramic tube was connected to the MS using a deactivated,

Autosampler

O2 He

Elemental analyzer

Water trap

Ash tube (quartz)

Combustion reactor

TCD

Interface

IRMS

GC (with furnace)

WO3

Cu

qw

qw

qw

SO2 monitoring gas

Figure 8-4.6 ‘On-line’ EA/CF-IRMS system for sulfur isotope analysis of organic samples(lichen) (after Yun et al., 2004). The same system can be used for sulfur isotope analysis ofBaSO4 (see Chapter 8-3). qw=quartz wool.

Sulfur 703

uncoated widebore (0.53 mm i.d.) fused silica tubing housed in a heated (250�C) SS tube.

He was used as carrier gas (flow: 1–2 mL/min) in the GC and as bulk gas in the microwave.

Reaction of the organic sulfur compound with HCl, available in excess (xs), is expressed as

Organic sulfur compoundsþHCl! SClþHCNþ [8-4.1a]

Other important reactions with oxygen present in the organic compounds are

ðxsÞHClþO2 ! H2Oþ Cl2þ H2þHCl [8-4.1b]

HClþH2O! complex [8-4.1c]

Sulfur is predominantly (>95%) converted into SCl (SCl2 is minor component), having

m/z of 67 and 69. The method can be used, besides for 34S determination, for selective

detection of D, 13C, 15N and 14C.

8-4.2.12. Conversion into BaSO4 by HNO3 + Br2HNO3þBr2 mixture digestion of organic matter was reported by Krouse & Tabatabai

(1986) and Ding et al. (2000). More details can be found in Chapter 8-2.7.

8-4.2.13. Fluorination techniques

Johnson & Lovelock (1988) introduced a method of fluorination of sulfur-containing com-

pounds by using a mixture of N2 with a low volume of F2 (200 ppm F2 in N2) (Figure 8-4.8a).

The N2–F2 mixture was prepared in a permeation source (Figure 8-4.8c), where a gas mixture

of 1% F2 and 99% N2 was flushed in a closed box (3 L volume) containing a coiled TFE11

To MS

To integrator

Microwave cavity

Silica glass reaction tube

Microwave

Generator

Ball valve Roughing vacuum

Depositedsample onprobe tip

Silica glass rod

Solenoidmagnet

Magnetizable rod

He and reactant gas in

Figure 8-4.7 Schematic presentation of the direct insertion system and the reaction interfaceof a CRIMS probe (afterAbramson &Markey,1986).

11 TFE = tetrafluoroethylene and FEP = fluorinated ethylene–propylene; both products are varieties of Teflon.

Teflon is a registered trademark by DuPont company.


tubing (0.066 in. o.d., 0.042 in. i.d.; 30 m length) permeating the F2 in the flow of N2

through the TFE tubing. This results in a gas mixture of 200 ppm F2 in N2. Samples were

introduced through a GC [2 m FEP11 Teflon tube column, 1/8 in. o.d.; packed with 60/

80 carbopack B/1% XE-60/1.5% H3PO4 (Supel-co)]. Reaction took place on AgF2 (Ag

wool is converted into AgF2 by reaction with the F2) in the reactor (Figure 8-4.8b). H2

was flushed through the Pd reactor, where excess of F2 was removed by conversion into

HF on the heated Pd catalyst. Johnson & Lovelock (1988) measured quantitatively the

SF6 by electron capture detector (ECD). Alternatively, the SF6 can be collected for

isotopic measurement. Very small amounts (femtomole level) of sulfur in compounds

were detected with this method.

Velinsky et al. (1990) analyzed S isotopes in organic compounds after the preparation

and fluorination technique as is described in Chapter 8-2.8. Precision of their method is

given on a cysteine compound at –1.4‰. Other sulfur-comprising compounds analyzed

were glutathione and thiosalicylic acid.

8-4.2.14. ICP-MS method

Menegario et al. (1998) analyzed S isotope ratios (34S/32S) in plant material by ICP-MS

method. The resultant ash (white ash for soya flour) from wet ashing (see Section 8-4.1)

was dissolved in HNO3 (warming on a hot plate) and diluted. Nitric acid concentration

up to 0.028 M in the digest solution allowed the effective retention of sulfate by the resin

(AG1-X8 resin; 200–400 mesh; Cl� form) in the column fitted in the injection port of

the ICP-MS nebulizer (Figure 8-4.9). Constant quantities of sample solution (2.8 mL)

were introduced in the injector sample loop, and, with changed positions of the central

Sample

GC carrier

(N2)

GC oven Vent

Make-up N2

Fluorinatingcolumn Pd reduction

column

N2

200 ppm F2

F2 permeationsource

H2 (1 mL/min)

ECD

FromGC column

(40 mL/min)

200 ppm F2 in N2(20 mL/min) 30 m

TFEtubing

3 L volume

To hood

Monel tube

Heat tape

Ag wool

200 ppm F2in N2

N2

1% F2, 99% N2

4-wayball valve

(a)

(b)

(c)

B

C

Figure 8-4.8 (a) Schematic diagram of device for fluorination of sulfur compounds. Includedare a GC for sample purification, a fluorination reactor, a permeation source and a Pd reactor.(b) Fluorination reactor in detail. (c) Permeation section for producing N2 with a low F2content for reactionwith the sample (after Johnson & Lovelock, 1988). ECD= electron capturedetector.

Sulfur 705

block in the injector, the solution is flushed through the column in which the sulfur (in

the form of sulfate) is stripped from the solution. After returning the position of the

central block in the injector, the sulfur component was released from the column by an

eluent solution (0.25 M HNO3; 1.9 mL/min) and introduced into the nebulizer of the

ICP-MS. For details on the operation conditions of the ICP-MS system, see Menegario

et al. (1998).

The sulfur isotopes were measured on the 32S16Oþ and 34S16Oþ signals (m/z at 48 and

50). Interference with ions with similar masses (see Table 8-4.2) was prevented by column

purification. Drift during measurement was corrected for.

Prohaska et al. (1999) measured sulfur isotopes (34S/32S ratios) in various matrices such

as soil, biological matrices, oil and sludge. Samples were prepared in a class 10,000 clean-

laboratory, and only ultra-clean chemicals or solvents were used for preparation. Nitric

acid was used for extraction of sulfur from the sulfur-containing compounds in the

samples.

An ICP-double focusing sector field MS, with a microconcentric nebulizer equipped

with a heated PTFE spray chamber and a membrane desolvation system or a conventional

T-type microconcentric nebulizer with a double-pass Scott type spray chamber cooled at

4�C, were used at a medium mass resolution of m/Dm = 4000 and at a fast electrostatic

scanning mode. Specific machine settings for best operation were discussed by Prohaska

et al. (1999). Limits of detection could be reduced to �0.01 ng/g (only limited by the blank

level), and the signal intensities for sulfur were high enough for the accurate determination

of sulfur isotopic ratios on 1 ng/g sulfur level. A precision of better than 1‰ (relative

standard deviation) for 10 successive measurements was achieved (at S concentration down

to 1 ng/g). A total analysis time (excluding preparation of samples and extraction of sulfur

into solution) for a single sample was 2 min. For higher sulfur concentration levels,

precisions up to 0.4‰ could be reached.

Gregoire & Naka (1995) and Resano et al. (2001) used ICP-MS for quantitative

determinations and also measured sulfur isotopes in organic compounds (studies on

thiourea and bisphenol A, respectively). Because pneumatic nebulization is plagued by

interferences from oxygen and needs high mass resolution, solid sampling electrothermal

ICP-MSnebulizer

Waste

Eluent solution

H2O

Sample solution

Sample loop

Resin column

Waste

Figure 8-4.9 Diagram of sample injector for S isotope determination on organic materials byICP-MS (after Menega¤ rio et al., 1998).


vaporizer ICP-MS was applied. Samples were weighed into cups (pyrolytic graphite-

coated tubes = cup-in-tube technique) (Resano et al., 2001). The cup was placed in the

furnace of the ETV, and by autosampler, 10 mL Pd solution (matrix modifier; reduces

matrix interference) and 10 mL HNO3 (65%; acts as in situ micro digestion agent) were

added. Blank value was monitored; best signal-to-blank ratio was obtained for a vapor-

ization temperature of 1000–1200�C. Standard solutions were used for calibration

purpose. Because ETV produces a dry plasma, interferences from polyatomic ions

containing oxygen should be significantly reduced. N2 also can be applied as oxygen

scavenger, giving the possibility to measure 32S (Lam & Horlick, 1990; Resano et al.,

2001; Yu et al., 2001). Small amounts of oxygen, in the form of water, metal oxide,

carbon monoxide or salts of oxy-acids, are unavoidably introduced with the analyte into

the plasma. Unfortunately nitrogen lowers the stability of the plasma and thus is not very

suitable for solid sampling. Detection limits for solid sampling-ETV-ICP-MS are even

lower than for high-resolution ICP-MS.

Yu et al. (2001) noted that irresolvable interference of 36Ar, a predominant species in

the plasma, with 36S makes it impossible to measure 36S isotopes by this method.

Multi-collector ICP-MS was applied by Martınez-Sierra et al. (2007) for sulfur isotopic

measurement in 34S-enriched yeast.

Table 8-4.2 Spectral interferences with sulfur isotopicmeasurement by ICP-MS

Isotope compound Interference32S 16O2

14N18O15N16OH14N16OH2

33S 16O2H16O17O15N18O

34S 33SH32SH216O18O17O216O17OH16O2H215N18OH

36S 36Ar32S16O 48Ca

36Ar12C31P17O31P16OH

34S16O 50Cr50Ti50V38Ar12C

Data compiled from Reed et al. (1994), Menegario et al. (1998), Prohaska

et al. (1999) and Yu et al. (2001).

Sulfur 707

High-accuracy isotope dilution MS (IDMS), with ICP-MS techniques using a Ar–N2

plasma, for measuring S in fossil fuels was used by Yu et al. (2001).

Common spectral interferences with sulfur isotopes, measured on ‘Sþ’ or ‘SOþ’ ions,

are given in Table 8-4.2.

8-4.2.15. Secondary ionization mass spectrometry

A new development of a specially designed SIMS (NanoSIMS 50 by Cameca; see Chapter

12-0.2.4.1 for details) introduced the possibility to measure stable isotopes in organic and

biological materials of nanometer sample size. Atomic 32S� ions can be measured in organic

compounds by this technique (Marxer et al., 2005).

Audinot et al. (2004) analyzed 32S� ions by NanoSIMS imaging in a study to localize As

traces in human hair.

8-4.3. SULFUR IN OIL AND COAL

H2S produced by cracking of oil and by destructive distillation of coal is precipitated

as PbS for S isotopic measurement (Thode et al., 1949; see also Chapter 8-1 for

procedures).

Nakai & Jensen (1967) burned coal samples in a device as is shown in Figure

8-4.10. Air, purified in NaOH and distilled water traps, was streaming over a heated

coal sample to burn it. SO2 from the sample was oxidized in a iodine solution to

SO42�, precipitated as BaSO4 and analyzed for its isotopic composition (see Chapter 8-3

for methods).

Hicks et al. (1974) used a Parr-bomb combustion method for S isotope analysis on coal

samples. Coal was dried at 105�C for 1 h. Accurately weighed coal (1 – 0.1 mg) was placed

in the Parr bomb with 1 mL of distilled water. The bomb was pressurized to 25 atm with

O2. The sample was burned and the gases were released within 1-min time period. All

contents of the bomb were washed (50 mL water) into a 100-mL beaker, and the S content

Air

Coal sample

NaOHsolution

Distilledwater

Iodine solution

Topump

Furnace

Figure 8-4.10 Apparatus for coal burning for S isotope analysis (after Nakai & Jensen,1967).


was measured by titration (sulfate content). For isotopic determination, the sulfate should

be extracted with omission of the titration; the sulfate was measured according to methods

such as presented in Chapter 8-3.

Smith & Batts (1974) converted all sulfur, regardless of the form in which it was initially

present, to soluble sulfate, which was precipitated as BaSO4. Nitric acid (if used) was

removed by boiling and the pH adjusted to 3 by HCl. After filtration, the solution was

boiled and 10% BaCl2 solution was slowly added. Precipitated BaSO4 was, after standing for

12 h and baking at 800�C for 1 h, recovered by filtration. 10 g portions of the ground

sample were used.

Native-S was extracted by boiling the sample three times with 50 mL benzene, evapora-

tion to dryness (80�C) and boiling with bromine and aqua regia until a clear solution was

obtained. Sulfate in the solution was recovered. Solvent was removed from the benzene-

insoluble coal under vacuum (80�C), and the dry residue was extracted twice with 100 mL

of boiling 5 N HCl. The soluble sulfate was recovered. A few drops of bromine and 50 mL

(10%) nitric acid were added to the HCl-insoluble residue. After boiling for 30 min, both

soluble sulfate and iron (pyrite fraction!) were determined. Organic sulfur in the remaining

solid sample was extracted either with Eschka mixture reaction or with an alkaline (40%)

NaOH solution followed by aqua regia. Total sulfur is the summation of all forms of

extracted sulfur.

Rees & Holt (1991) stated that extraction of sulfur from coal is similar to the extraction

of sulfur from sediments.

Kelly et al. (1994) measured sulfur contents of oil (0.2–0.3 g; added by plastic syringe

fitted with a plastic cap) and coal (0.15 g; added by glass funnel) samples by IDMS methods.

Samples were dried at 105�C for 2 h and weight corrections were applied. Samples were

spiked with enriched 34S (mixture 32S/34S ratio of �2–3) and placed in Carius tubes with

nitric and hydrochloric acids (oil samples: 10 g HNO3, 2 g HCl; coal samples 8 g HNO3).

The Carius tubes were frozen in solid CO2–CHCl3–CCl4 mixture and sealed by O2–

natural gas torch. All samples were heated at 50�C increments to 240�C, which temperature

was maintained for 12–15 h (pressure in tube is 10 MPa at this temperature). The Carius

tube ‘combustion’ is a truly closed system capable of complete wet oxidation of organic

matter. The oxidized sulfur species were converted into chloride form, were consequently

reduced to H2S and precipitated (by dropwise addition of concentrated HCl) as As2S3 in a

solution of As2O3 (concentration of 1000 mg/mL). The precipitate was washed with

distilled water and dissolved in aqueous ammonia containing As such that the final

concentration of S was 100 mg/mL and the As/S ratio was 2. A portion of this solution

(equivalent to 1.5 mg of S) was added to a single Re filament coated with silica gel (as

emitter). The 32S/34S ratio was determined on 75As32Sþ/75As34Sþ (As has only one stable

isotope). Corrections were made for blanks. Total uncertainty (32S/34S ratios) for oils

(homogeneous material) is about 0.5% and for coals (less homogeneous material) is 1–4%

(95% confidence interval).

Kelly et al. (1994) mentioned the use of neutron activation to measure the 36S isotope

on fossil fuel samples, without giving details on the technique.

Yu et al. (2001) used IDMS/ICP-MS methods for determination of sulfur levels

(quantitative) in fossil fuels.

Literature references presenting studies including sulfur stable isotope analysis on oil or

coal are Hahn-Weinheimer (1965) and Bailey et al. (1973).

Sulfur 709



8-4.1 Preparation of samples and extraction methods8-4.1.1 Plant materials8-4.1.2 Petroleum and coal

8-4.2 Analytical methods

8-4.2.1 Combustion instream of O2

(‘Pregl method’)

Organic compound 850 650 þPt-cat.

‘on-line’ method; SO2

purified by liquidnitrogen separation

Old obsolete method

8-4.2.2 Carius reaction Organic compound Sealed tubed reaction;‘off-line’ method

Old obsolete method

8-4.2.3 Parr bomb oxidation Organic compound Electricdischarge

‘Off-line’ method; Sconverted intoZnS > Ag2S

Former standard

8-4.2.4 Parr bomb oxidation(þ starch);reduction bySn(II)-H3PO4

Organic material Electricdischarge

‘Off-line’ method;laborious method

Former standard

8-4.2.5 Kiba reagent Organic matter 280 ‘Off-line’ method;‘on-line’ flushingsystem, H2S recoveredþ converted into Ag2S

Older method, rarelyused for Organicmaterials; laborious

8-4.2.6 Eschka reduction Organic compound (oilshales)

800 (slowheating)

‘Off-line’ method; BaSO4

precipitationOld but common

method8-4.2.7 Oxidizing fusion (Marine) plants. animal

samples‘Wet combustion’ Rare, obsolete method

8-4.2.8 Na-hypobromiteoxidation

Reduced sulfur spec.(sedim.)

250–260 Reaction in diffusionsteps, laborious

Obsolete method

8-4.2.9 Organic compound Basis for modern method

710H

andb

ookof

Stab

leIsotop

eA

nalyticalTechniques

O2 þ He flashcombustion

Flashcombustion

Effluent gases separated;He carrier

8-4.2.10 EA combust/oxidation(‘on-line’)

Organic compound 1000–1050(flashcombustion�1700)

EA-CF-IRMS, ‘on-line’;O2 þ oxygen donors;He carrier

Modern method,standard

8-4.2.11 CRIMS method Organic compound n.a. Plasma reaction; SClanalyte for measurementin MS

Very rare method

8-4.2.12 Conversion byHNO3 þ Br2

Organic matter See Chapter 8-2.7 Conversion method only

8-4.2.13 Fluorinationtechniques

Compound withOrganic sulfur

n.a. Small quantities of Sanalyzed; SF6 analyte gas

Very rare method

8-4.2.14 ICP-MS Organic matter (plant,soil, oil, sludge)

n.a. Preparation of samplerequired;

Rare method, modern

8-4.2.15 SIMS Organic þ biologicalmaterials

n.a. Nano-scale spot analysis;very expensive tool

Modern, very raremethod

8-4.3 Sulfur in oil and coal

Sulfur

711

8-5. SULFUR IN METALS

8-5.1. QUANTITATIVE METHODS

Quantitative methods for sulfur determination in metals can be used for isotopic

determination by slightly changing the procedures. Basic procedures for extraction of

total sulfur content from the metal alloys can be a valuable starting point for the isotopic

analysis.

Bagshawe & Pill (1955) reported a BaSO4 extraction method, meant for quantitative

measurement of S in a carbon steel. The steel was dissolved in an acid (e.g. nitric acid) and

the sulfur was precipitated in oxidized form as BaSO4 (addition of Ba chloride to the

solution). For quantitative measurement, gravimetric methods were used. For isotopic

measurement, BaSO4 can be analyzed by methods as given in Chapter 8-3.

It was reported by Bagshawe & Pill (1955) that high Cr–steel alloys do not dissolve

completely and therefore sulfur yields were incomplete. Addition of hydroxylamine hydro-

chloride after nitric acid attack was solving this yield problem by complete dissolution of the

steel alloy.

It was mentioned that another standard method for the determination of sulfur quantity

is by high-temperature combustion of steel (no references given).

8-5.2. ACID DIGESTION – SULFATE PRECIPITATION

Watanabe (1975, 1979, 1983) analyzed metals/alloys for S isotopic compositions by

acid digestion followed by sulfate precipitation and reduction of sulfate into sulfide

(see Chapter 8-3). Sulfide was combusted into SO2 (see Chapter 8-1.1.1).

Metal/alloy samples (1 g; chips or turnings) were digested by an acid mixture (15 mL of

12 M hydrochloric acid, 16 M nitric acid, water in the ratio 1:1:1, plus 3 drops of hydro-

fluoric acid). After complete dissolution, 1 mL of 16 M nitric acid was added and boiled for

several min. Eventually, spike solution (1 g of 100 mg S/g) was added, e.g. for samples with

low sulfur contents. Solutions were evaporated to dryness and nitrate was removed by

repeated evaporation with 2 mL portions of 12 M hydrochloric acid. The residue was

dissolved in 5 mL of 12 M hydrochloric acid while heating. Alloys resistive against this

treatment can be handled differently:

Inconel 713: 1 g dissolved in 50 mL of 12 M hydrochloric acid–16 M nitric acid mixture

(3:2) with heating at �150�C for 2–3 h.

Hastelloy X: 1 g dissolved in 40 mL of 12 M hydrochloric acid–16 M nitric acid mixture

(3:1) with heating at �150�C for 2–3 h.

Watanabe (1979) dissolved carbon steels (0.2–2 g samples) in a conical beaker (500 mL) in a

mixture of 10–20 mL of 16 M nitric acid and 1 mL of bromine. Small portions of 6 M HCl

were added, eventually with heating if needed. Either adding or not adding bromine to the


dissolution process made no difference for Ni–Cr-steel, mild steel, Cr–Mo–steel, high-

speed steel, 13% Cr SS and heat-resisting alloys.

Isotope dilution – It was noted by Watanabe (1979) that only the dissolution process

needed to be quantitative, and after spiking no quantitative separation was required. This is

a primary advantage of the isotope dilution method (see Volume I, Part 1, Chapter 37 for a

discussion on the isotope dilution method).

Watanabe (1979, 1983) used similar techniques as described above for quantitative

sulfur determination, but used isotope dilution techniques for samples with small amounts

of sulfur. After dissolution was completed, Watanabe (1979, see above) added a weighed

aliquot of spike solution. The spiked solution was boiled for several minutes and evaporated

to dryness (at 250�C). HCl was added and evaporated to remove nitrate. The residue was

dissolved in 10–20 mL HCl while heating and was diluted with water to 50 mL and filtered

(medium texture paper), and sulfur was precipitated as BaSO4 (with 10% BaCl2 solution),

washed and dried.

Watanabe (1983) used spike solutions of elementary sulfur enriched in 34S (45% and

94%). The methods used for metals and alloys can also be used for rock samples (dissolution

in HF).

8-5.3. THERMAL IONIZATION MASS SPECTROMETRY

8-5.3.1. Positive ion TIMS34S-spiked As2S3 – Sulfur is a trace constituent in many metals. In some applications for

steel alloys or copper only a very small S content is allowed. A procedure to measure

microgram quantities of sulfur in metals was presented by Paulsen & Kelly (1984). Although

their method was meant for quantitative determinations, the 32S/34S12 ratio of a

spiked sample was measured, and thus the isotopic ratio of the sulfur component was

obtained too.

Following the procedure by Paulsen & Kelly (1984) and Kelly et al. (1990), a 1 g metal

sample was spiked with 34S-enriched spike. In a sealed tube, the sulfur in the sample was

oxidized to sulfate, which was reduced to H2S (by a mixture of analytical grade H3PO2, HI

and HCl) and trapped in an As3þ–NH3 solution and precipitated as As2S3. As2S3 was

dissolved in NH3 and 1.5 mg of S was loaded (performed in a plastic box under N2

atmosphere) onto a Re-flat filament (0.0012 � 0.030 � 0.70 in., outgassed at 4 A =�1900�C at 10�7 Torr pressure for 30 min: Kelly et al., 1990) with silica gel. The32S/34S ratio was measured at 950�C as 75As32Sþ and 75As34Sþ with a precision of 0.1%.

Arsenic has only one isotope, and therefore the ion currents of mass 107 and 109 are

proportional to the 32S and 34S abundances.

Figure 8-5.1 shows the reduction apparatus for the generation and trapping of H2S from

a metal sample.

Preparation of spike – The 34S-enriched spike (elemental S) was converted to sulfate by

oxidation with 10 g of 16 M NHO3 and 4 g of 11 M HCl in a sealed Carius tube at 240�C.

12 Although the convention is to use isotope ratios with the heavy isotope as numerator and the light isotope as

denominator (e.g. 34S/32S), in Paulsen & Kelly (1984) and Kelly et al. (1990) 32S/34S ratios were reported.

Sulfur 713

After dissolution, the tube was opened and the solution transferred to a 50 mL beaker.

Na2CO3 (high-purity) was added to a Na/S atom ratio of 4. The solution was evaporated to

dryness and nitrates were destroyed by repeated addition of HCl and evaporation. The

dried spike was dissolved in 2 M HCl to a volume of 200 mL and transferred to a storage

flask (250 mL capacity). Two different S spike solutions are prepared: one of 100 mg of S/g

and one of 300 mg of S/g.

Kelly et al. (1990) gave a detailed description of calibration of the spike. Flat Re

filaments were outgassed at 1500�C at 10�7 Torr for 0.5 h. Sample loading was performed

in a plastic box with N2 atmosphere. The silica gel–H3PO4 solution was shaken vigorously

to disperse the silica gel and 5 mL was placed in the central part of a filament. A current of

0.9 A was passed through the filament until the silica gel was dry and was heated for an

additional 15 s. As2S3 sample solution (15 mL = 1.5 mg of S) was loaded in single drops on

the filament; after the first drop the current was adjusted to 1.6 A. The silica gel breaks up

and disperses into the sample drop. The rest of the solution was added without evaporating

the sample to dryness between additions. After the sample was dry, it was heated for 15 s

longer. The current was increased and the sample was dried at �700�C for 5 s, without

production of fumes. The filament with sample was loaded into the MS, exposing it for less

then 2 min to room atmosphere. The pressure in the source of the MS was reduced to

2 � 10�7 Torr with the aid of a liquid nitrogen cold finger. The filament was initially

heated to 800�C, commonly giving an AsSþ beam of 2–5 V with 1011 � feedback resistor.

At this temperature the signal was decaying rapidly. The filament temperature was increased

to 950�C in 50�C increments at 5-min intervals. The source was focused for maximum

intensity after each temperature adjustment. The signal increased and then decayed after

each temperature increase.

The ion signal at 950�C was stable and commonly in the 10 V range. Larger ion currents

could be obtained above 950�C, but they were unstable. A data set consisted of 6 integra-

tions of mass 107 and 5 integrations of mass 109 at an integration time of 15 s. Background

signals and electronic offsets were read before each data set. During 21 min for a data set

collection, decrease in 107/109 ratio was less than 0.1% as a result of isotopic fractionation.

H3PO2

HI + SO4 =

HCl

N2

H2O

H2O

As+3–NH3

H2O

H2S

Figure 8-5.1 Apparatus for generating and trapping H2S (after Paulsen &Kelly,1984).


8-5.3.2. Negative ion TIMS

Wachsmann & Heumann (1992) described a method for S isotope measurement by

negative TIMS. Re filaments in a double filament ion source were used in a single focusing

magnetic sector MS equipped with a Faraday cup detector. Filaments were cleaned by

treatment with diluted HNO3 and subsequently degassing under high vacuum conditions

(at a 5 A current for 0.5 h) prior to their use for measurements. The material used for the

filament is of importance, because the electron work function (WF) of the filament material

(Re has a WF value of �5.0 eV) influences the intensity of the ion currents. Ba (30 mg), as a

Ba(OH)2 solution, was deposited on the ionization filament. Sulfur was loaded (1–5 mg) in

the form of (NH4)2SO4, H2SO4 or Na2S (all have comparable ion current intensities).

Solutions were evaporated to dryness, and subsequently 10 mL of silica gel suspension was

added dropwise on the evaporation filament and was also heated to dryness. Filaments were

loaded into the source of the MS. The ionization filament was heated at a current rate of

0.15 A/min to about 1.4 A, then decreased to 0.04 A/min. The first S� ion current was

monitored at �850�C. The filament current was increased very slowly (0.01 A/min) until

the signal remained stable with respect to time (variation <3� 10�14 A/min; at �1050–

1100�C). Higher temperatures led to a strongly decreasing S� signal and also SO2� or SO�

ions could not be detected then. All sulfur compounds ((NH4)2SO4, H2SO4, Na2S) gave

similar results.

If SO2� or SO� ions were measured, it was necessary to correct the obtained isotope

ratio of S for 17O and 18O. When measuring S� ions, a (so far) unidentified interference

(mass number 33) had been observed, excluding 33S values from the measurement. The 36S

isotope could not be determined because of a too low signal on natural isotopic levels.

Using high-resolution TIMS, a double peak was found on mass number 32, identified to be16O2

� and 32S�, having a mass difference of 0.01776 u. This interference could be

suppressed by using BaSO4 as sample material and by heating the evaporation filament by

a current of 1.4 A.

The advantage of TIMS compared with electron impact MS (on SO2 or SF6 gases) is

the possibility that measurement can be applied on solid samples (e.g. sulfates, sulfides),

without conversion into a gas, although the latter method shows a better relative standard

deviation (�–0.2‰). Another option to improve relative standard deviations with TIMS is

the use of isotope dilution MS (IDMS) techniques (see Section 8-5.2 above and Volume I,

Part 1, Chapter 37).

8-5.4. ICP-MS METHOD

Sulfur in steel was quantitatively measured by Naka & Gregoire (1996) by the ETV

ICP-MS technique. Because they used 34S-enriched spikes (isotope dilution) in their

procedure, the method is reported here. Sulfur was extracted from steel by dissolution (in

aqua regia) and precipitation as As2S3. Iron was removed by solvent extraction with

4-methylpentan-2-one. The sulfur-containing solvent was measured. Thermal vaporization

of the analyte was applied at 2500�C, and a chemical modifier in the form of KOH was

added to enhance the sulfur signal (S compounds were converted into K2SO4). Settings of

the ICP-MS (Elan 5000) can be found in Naka & Gregoire (1996). Isobaric interferences,

Sulfur 715

for example, of 16O2þ, 16O2

1Hþ and 16O18Oþ with 32Sþ, 33Sþ and 34Sþ, respectively, and36Arþ with 36Sþ were reduced by use of ETV sample introduction; water was removed

from the analyte by the ETV technique and thus the background level of interfering

polyatomic ions was reduced. The preparation of the spike was similar to the procedure

as is described in Section 8-5.3.1.

8-5.5. MASS BALANCE RELATIONS TO CALCULATE SULFURVOLUMES BY IDMS

Kelly et al. (1990) presented an equation to calculate the amount of sulfur by using

the 32S/34S ratios obtained by the IDMS (named ID-TIMS for the method used by Kelly

et al., 1990). An equation for the measured ratio in the mixture may be written as follows:

ð32S=34

SÞm ¼ ð32

SÞs þ ð32

SÞtð34

SÞ s þ ð34SÞt

[8-5.1]

where subscripts m, s and t stand for measured mixture, natural sulfur and tracer,

respectively.


8-6. D33S AND D36S: MASS INDEPENDENT

FRACTIONATION

Mass-independent isotopic fractionation of 33S and 36S is found in extra-terrestrial

samples (meteorites) and on the Martian surface (Farquhar et al., 2000a) and is also found

on Earth, where it is considered as a sensitive proxy of the atmospheric redox state

(Farquhar et al., 2000a, 2001; Pavlov & Kasting, 2002; Ono et al., 2003, 2006a; Papineau

et al., 2005). Deviations from a mass-dependent fractionation are expressed by D33S 6¼ 0

and D36S 6¼ 0, defined in a similar way as D17O (see Chapter 6-9 for definitions). Mass-

dependent thermodynamic, kinetic and biotic fractionation processes, considering mass

differences, produce highly correlated and constant values for (Farquhar et al., 2000a; Rai

et al., 2005):

D33S ¼ �33S� 1000 ½ð1þ �34S=1000Þ0:515� 1� [8-6.1]

D36S ¼ �36S� 1000 ½ð1þ �34S=1000Þ1:90� 1� [8-6.2]

or, defined by a ‘logarithmic approach’ (Ono et al., 2006a; see also discussions on MIF in

Chapter 6-9):

D33S ¼ ½lnð1þ �33S=1000Þ � 0:515 � lnð1þ �34S=1000Þ� � 1000 [8-6.3]

D36S ¼ ½lnð1þ �36S=1000Þ � 1:90� lnð1þ �34S=1000Þ� � 1000 [8-6.4]

Low isotopic abundance and small sample size give rise to large uncertainties for D36S,

preventing solid interpretations on base of 36S.

Older definitions for D33S and D36S were given in Gao & Thiemens (1991):

D33S ¼ �33Smeasured� 0:50 �34Smeasured [8-6.5]

D36S ¼ �36Smeasured� 1:97 �34Smeasured [8-6.6]

Mechanisms for mass-independent fractionation as reported in literature are as follows:

Cosmic ray reactions (spallation): 33S and 36S are produced by cosmic ray spallation and can

therefore show enriched values compared with mass-dependent fractionations (Gao &

Thiemens, 1991; Farquhar et al., 2000a; Rai et al., 2005). Such enrichments were detected

in iron meteorites (Hulston & Thode, 1965a, b; Thiemens, 2006), but were excluded to be

caused by cosmic ray spallation in achondrites by Rai et al. (2005) because it should produce

a nearly constant ration of 36S/33S of �6 (or D36S/D33S of �8) in the metallic phase of iron

meteorites. Major sulfide minerals in iron meteorites are troilite (FeS) and daubreelite

(FeCr2S4) (Gao & Thiemens, 1991).

UV photolysis of SO2 – Mass-independent fractionation by UV photolysis (specifically in

the spectral region of 187–250 nm) is suggested to occur during massive volcanic eruption

(e.g. Pinatubo and an unnamed eruption in 1259 AD; hundreds of Tg of SO2 emission)

when the volcanic plume reaches stratospheric levels (Savarino et al., 2003a, b; Thiemens,

Sulfur 717

2006), or in solar nebula (Farquhar et al., 2000c; Rai et al., 2005), or in early planetary

atmospheres (Rai et al, 2005; Ono et al., 2006b). Mass-independent isotopic fractionations

were found in ancient volcanic debris, in Archean basins (Hamersley and Transvaal), in

Archean greenstone belts (Barberton) and in the Pilbara district, Isua and Akilia rocks, and

Archean sediments (e.g. Farquhar et al., 2000a; Pavlov & Kasting, 2002; Hu et al., 2003;

Mojzsis et al., 2003; Ono et al., 2003, 2006a, b; Pavlov et al., 2005).

Laboratory observations confirm anomalous sulfur isotopic signatures resulting from

UV photolysis processes of SO2 (Farquhar et al., 2000a; Farquhar et al., 2001; Savarino

et al., 2003a, b; Thiemens, 2006), SO2 and SO (Lyons, 2007), and H2S (Farquhar et al.,

2000a). Pavlov et al. (2005) found sulfur MIF recorded in sulfates in Antarctic ice cores of

the Pinatubo 1991 eruption and related this to atmospheric SO3 photolysis processes. The

sulfur MIF-signal was delivered to the ground as sulfates and stored in the Antarctic ice.

Sulfur aerosols have a small highly variable anomaly that derives from a small strato-

spheric cycled component (Romero & Thiemens, 2003). If short wavelength UV could

reach the Earth’s surface, photolysis would be a major process, causing sulfur isotopic

anomalies (Farquhar et al., 2000b; Thiemens, 2006). Multiple sulfur isotopic studies can

reveal information on ancient atmospheric conditions (if UV could reach the Earth’s surface

and caused photolysis or was absorbed and processes are mass dependent) (Thiemens, 2006).

Sulfur in Precambrian rocks was analyzed. Water-soluble oxidized sulfur (sulfate) had

negative D33S values and reduced sulfides had positive D33S, as expected for material mass

balance. It was concluded that little or no ozone existed in the Archean atmosphere

(Farquhar et al., 2000b). Mass-independent sulfur isotope fractionation (D33S) in sulfides

of banded iron-formations (2.47 GA) against mass-dependent sulfur isotope fractionation in

black shales (2.32 GA) was used to explain the Paleoproterozoic anoxic–oxic transition of

the Earth’s atmosphere (Papineau et al., 2007).

Rai & Thiemens (2007) found significant excess of 33S in a chondritic chondule,

interpreted by photochemical irradiation of sulfur gaseous species in the early solar nebula.

Photopolymerization of CS2 – Large mass-independent sulfur isotope fractionations were

found by photopolymerization of CS2 (Colman et al., 1996; Farquhar et al., 2000a; Rai

et al., 2005).

Stellar nucleosynthesis – Sulfur isotopes are synthesized in different stellar environments.32S and 34S are made from hydrostatic and explosive oxygen burning, whereas 33S is

produced from explosive oxygen and neon burning. 36S is produced in the convective

shell in C-burning massive stars under hydrostatic conditions before a supernova II explo-

sion (Howard et al., 1972; Woosley et al., 1973; Hartmann et al., 1985; Gao & Thiemens,

1991; Rai et al., 2005 and references in there).

Analytical considerations – Common procedure to include 33S and 36S to sulfur isotope

measurements is to convert samples by fluorination methods into SF6 (see Chapters 8-1-4

for fluorination methods). If the more common SO2 gas is used for measurement, 33S and36S cannot be obtained with any reasonable certainty because of mass interference of the

oxygen isotopes in the SO2.

Greenwood et al. (2000) used ‘high spatial ion probe’ isotope measurements on Marsian

samples to develop further understanding of the Martian sulfur cycle.

A protocol for laser fluorination (CO2 laser) has been developed at the Geophysical

Laboratory of the Carnegie Institution of Washington (see: Hu et al., 2003; Ono,

et al., 2006b, c). Typically �1.5 mg Ag2S (= 6 mmol sulfur; obtained by extraction and


conversion from sample materials; for extraction and preparation procedures of different

sulfur compounds into Ag2S, see Chapters 8-1-4) is heated by a laser in a F2 atmosphere

(�30 Torr =�40 mbar) to produce SF6, which is purified by a newly developed, two-

dimensional GC system. Isotopic ratios are measured by a dual-inlet MS. Precision and

accuracy of the method are reported to be –0.01‰ and –0.2‰ for D33S and D36S,

respectively (e.g. Ono et al., 2006c).

Sulfur 719

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