Specmchimica Acta, Vol. 468. No. 2, pp. 171-181, 1993 0934-8547/93 $6.00 + .cm Printed in Gnat Britain. @ 1993 Pergamon Press Ltd

Trace metal determinations by total-reflection X-ray fluorescence analysis in the open Atlantic Ocean

DIETHER SCHMIDT, WOLFGANG GERWINSKI and INA RADKE Bundesamt ftir Seeschiffahrt und Hydrographie, Postfach 301220, D-2000 Hamburg 36, F.R.G.

(Received 7 September 1992; accepted 22 October 1992)

Abstract-The Intergovernmental Oceanographic Commission (IOC), as a major component of its programme “Global Investigation of Pollution in the Marine Environment” (GIPME), maintains a long-standing project on “Open Ocean Baseline Studies of Trace Contaminants”. Initially, the Atlantic Ocean and trace metals were selected. A first cruise with the RV Meteor to the eastern parts of the south and north Atlantic Ocean was successfully organized, in March and April 1990, from Cape Town (South Africa) to Funchal (Madeira, Portugal). Thirteen scientists from laboratories in Europe and North America participated with the first author as coordinator. Four deep-water stations in the Cape Basin, Angola Basin, Cape Verde Abyssal Plain and Seine Abyssal Plain were regularly sampled for at least 36 depths. Additional samples were taken between stations. Samples were distributed to participants and a similar number of additional laboratories. As a central part of our own contribution to the project, we determined the trace heavy metals manganese, nickel, copper, zinc and lead and the lighter selenium by total-reflection X-ray fluorescence analysis. Additional methods applied, inter ah, were anodic stripping voltammetry for lead and cadmium and graphite furnace atomic absorption spectrometry (GFAAS) for cadmium, using two different extraction procedures. For the TXRF, the pre-enrichment of the trace metals and the separation from the salt matrix were performed by complexation with sodium dibenxyldithiocarbamate and reverse-phase chromatography. Generally, very low levels of trace elements were found in filtered and unfiltered water samples from these remote areas of the open Atlantic Ocean. Typical examples of the distributions of trace metal concentrations on depth profiles from the four deep-water stations as well as intercomparisons between the stations are presented.

1. INTR~DU~~~~N

THE INTERGOVERNMENTAL Oceanographic Commission (IOC) in Paris has a Committee for the “Global Investigation of Pollution in the Marine Environment” (GIPME) that has developed a plan for baseline studies of trace contaminants in the world’s oceans [l]. The Atlantic Ocean, being the most accessible world ocean at this time, was selected for the first baseline survey.

The first international exercise for the Open Atlantic Ocean Baseline Study was performed on the first leg of the twelfth cruise of the RV Meteor that started in Cape Town, Republic of South Africa, on 13 March 1990. It ended in Funchal, Madeira, Portugal, on 15 April 1990. The trace metal chemistry team (led by D. SCHMIIYT) consisted of 13 participants from Germany (5) USA (4), Canada (2), Great Britain (1) and Sweden (1). They were responsible for the sampling, the distribution and pre- treatment of the samples, the ancillary determinations of physical oceanography and the first on-board analyses.

Four deep-water stations had been selected that were located close to the route of the RV MeteOr on its first leg from Cape Town to Madeira (Fig. 1).

The numbers and coordinates of the stations were:

(1) Station No. 9: 30%; 8”E Cape Basin (2) Station No. 7: 15%; OW Angola Basin (3) Station No. 5: 24”N; 23W Cape Verde Abyssal Plain (4) Station No. 4: 34”N; 13W Seine Abyssal Plain

A typical sequence of hydrocasts at the four deep water stations was:

171

51

capavde

--i

Flain a

Fig. I. Location of the four deep-water stations for the RV Meteor cruise.

(1) 1. CTD probe with rosette sampler: 24 samples deep (2) 1. Hydrocast series, 12 GoFlo samplers deep (3) 2. CTD probe with rosette sampler: 12 samples shallow (4) 2. Hydrocast series, 12 GoFlo samplers medium depth (5) 3. Hydrocast series, 12 GoFlo samplers shallow

During the hydrocast series, six 30 I and six 12 1 Niskin GoFlo sampimg bottles were attached to the wire in an a~te~at~g succession.

Occasionally, additional shorter hydrocast series and CID probes were run tu obtain special samples for individual participants, or to compensate for a few samplers that were suspected not to have closed completely as intended.

At the start of each analysis, the structure of the water column at the station was determined by taking temperature and salinity profiles with the CTD probe. From these data the exact depths for attaching samplers to the wire were determined. The rosette sampler was &ted with 24 Niskin bottles of volume I.5 1 each. The totat sampling duration at each station was appro~mate~y 24 h. A more detailed preread report of the cruise has been given by S~DT et al. f2].

One portion of the water samples was filtered and this was done directly from the 30 1 GoFlo bottles. Filtration was performed inside the clean room laboratory container. The entire complement of samplers was pressurized with nitrogen. A specially developed teflon filter holder equipped with a 142 mm diameter nuclepore filter was positioned below the GoFlo sampler; this system proved to be very efficient fur the filtration of large water volumes. Occasionally other participants conducted filtrations with their own equipment, part&zularIy for ~terc~~bration purposes,

From the 12 1 GoFlo samplers, only unfiltered samples were taken. The samples

Atlantic Ocean trace metals by TXRF 173

for the authors’ analyses were stored in 0.5 1 PTFE bottles, stabilized by addition of 1 ml 30% HCl and deep frozen at -20°C.

The largest part of the ultra-trace analytical determinations of the samples from the Meteor cruise was performed later in our land-based laboratories by total-reflection X-ray fluorescence spectroscopy (TXRF). Our first applications of TXRF to the monitoring of heavy metals in the German Bight of the North Sea proved very successful [3]. In recent years, the major load of metal determinations in our laboratory has gradually been taken over by TXRF from graphite furnace atomic absorption spectrometry (GFAAS) that previously maintained the leading position. At present, a total of approximately 40 000 individual trace element determinations per year are performed by our group using only TXRF.

2. EXPERIMENTAL

2.1. General As with most other analytical methods (e.g. AAS, ASV), a direct determination of trace

metals in sea-water is not possible using TXBF owing to interference by the salt matrix and generally very low trace metal concentrations. A chemical separation and prewncentration procedure developed by PRANCE et al. [4] has been modified and improved gradually by the authors, with the aim of lower detection limits. To avoid any contamination at the extremely low trace metal concentrations found in ocean water, all handling and chemical operations of the samples have to be done under clean room conditions.

2.2. Apparatus The extraction apparatus has been installed in a clean bench of class 10 (U.S. Federal

Standard 290 B). The apparatus [4] had several screw joints and showed leakages under routine operating conditions. In 1989, the authors developed a modified apparatus that has since proven its effectiveness (Fig. 2). The improvement consists mainly in combining the sample reservoir vessel, the chromatographic column and the eluant tube into a single piece of glass equipment. The smaller diameter of the glass tube fused to the sample reservoir acts as a support for the silica gel in the column, like the previous glass frit [4]. On this narrowing position, a silica wool layer acts as a permeable barrier for the gel that can be replaced after thorough use.

The column tube is fitted with a silicone rubber plug that connects and seals the intermediate piece and the collecting vessel. The joints have the German standard size NS 14/23. The collecting vessel can either be a 300 ml Erlenmeyer flask for the aqueous waste solutions or a 10 ml pointed flask for the eluate. The column material is a silanixed silica gel (80-100 mesh size, Chromosorb W).

The sample is sucked through the apparatus by applying a slight vacuum (c. 20 mbar pressure difference) to the horizontal joint of the connector piece.. The joint is connected by tubing to a low pressure vessel that has adjustable needle gauges for applying the exact low pressure. Preceding the elution, the whunn is dried by sucking air from the clean bench through the apparatus (c. 200 mbar pressure difference).

2.3. Extraction procedure (Fig. 3) A sea-water sample (200 g) is weighed into a silica glass vessel; 100 ~1 cobalt solution of

2 mgil is added as internal standard, resulting in 1 &l Co concentration in the sample. One millilitre of sodium acetate/acetic acid buffer solution is added and a pH of 4.7-4.9 is adjusted by stepwise addition of dilute NaOH. The exact pH value has to be controlled by repeated sub-sampling of c. 6 ml from the silica vessel measured by a pH electrode. After this, 1 ml of a 1% methanolic dithiocarbamate solution is added as the wmplexing agent. Qn average, approximately 175 g sea-water sample remain for the extraction. This mass is transferred into the reservoir vessel and sucked through the wlumn over 30 min. Rinsing three times with 15 ml ultrapure water (mill&Q) then follows. Suction by air is applied to dry the entire apparatus within 40 min. After exchanging the reception flasks, the metal complexes adsorbed to the column are eluted with approximately 4 ml of a mixture of 80% chloroform and 20% (by volume) methanol. The elements in the eluate are thus enriched by a concentration factor of about 45. All solvents have been purified by sub-boiling distillation.

174 D. SCHMIDT et al.

Glass tubing - i.d. 4 mm

(siianized ,80-100 mesh)

Glass tubing - i.d. 1 mm

Silkune rubber

- To vacuum pump

Flask

Fig. 2. Extraction apparatus.

2.4. Preparation of the TXRF sample carrier Circular silica glass disks (25 mm diameter, 3 mm thickness) are used as sample carriers for

TXRF measurement. They have a highly planar, polished surface to achieve the best possible total reflection of the incident X-ray beam. A special device (commercially available, Rich. Seifert) serves to put a deposit of 100 cl1 eluate onto the sample support carrier. This device is placed on a heating plate for 20 min at a temperature of 60°C in order to produce an evenly distributed, homogeneous thin circular sample deposit.

2.5. Measurement The TXFW spectrometer consists of an Extra II Module (Rontgen Seifert, Ahrensburg,

F.R.G.) with double beam excitation (molybdenum and tungsten tubes), two separate X-ray generators, a Si(Li) detector, an automatic sample changer and a computer controlled multichannel analyser system. The samples on their circular disks are processed by the autosampler automatically and irradiated for 70 min with the Mo-anode. The X-ray fluorescence induced is registered and counted in the energy dispersive mode. The spectrum is computed quantitatively using library programmes and specific so&are. The concentrations of the elements are calculated with reference to the internal standard added.

Atlantic Ocean trace metals by TXRF 175

Seawater sample

(acidified to pH 2)

F

a Internal standard (Se)

n Acetate buffer

Q Adjustment to pH 4.8 with NaOH

u Complexing agent

(Dithiocarbamate type)

LI

Metal chelates in 0 0

0. * seawater matrix

1

v

Separation by reverse phase

chromatography on silanized

silica gel column

1

ii

Elution with chloroform/methanol

mixture

n

Evaporation on a TXFW

- o sample holder

Fig. 3. Scheme of extraction, preconcentration and sample preparation procedure.

2.6. Elements determined The selection of elements to be determined by TXFW in a given sample series is dependent

on their abundance in sea-water, the chemical separation and the pre-enrichment procedure applied and the X-ray tube used for irradiation. In the present paper, only the Mo-anode was used making elements of atomic number 16-38 and 56-92 available for excitation and fluorescence. However, only a certain portion of these elements will be available for quantitative separation by the complexing agent applied for the reverse-phase chromatography. In routine analysis, Mn, Fe, Ni, Cu, Zn, Se and Pb have been determined. For some metals, complexing is dependent on the oxidation state in the sample: this is particularly relevant for V and U, for Cr(VI) and Cr(II1); Se has been measured only as Se(W). Fe was later omitted from the evaluations for this study due to apparent contamination problems at the extremely low oceanic levels encountered.

176 D. SCHMIDT et al.

3. REXJLTS AND DISCUSSION

Generally speaking, extremely low levels of trace heavy metals and selenium have been determined by TXRF (using the procedures described above) from the large number of samples taken during the first baseline study cruise. It has been shown that open ocean and deep-water masses from the eastern parts of the south and north Atlantic Ocean, remote from anthropogenic sources of many metals, exhibit such very low concentrations. In our contributions to the baseline study, TXRF has played the dominant role among trace analytical techniques. The purpose of this presentation is to demonstrate the capabilities and the advantages of TXRF for the analysis of open ocean samples. Including our determinations of cadmium and lead by anodic stripping voltammetry and of cadmium by GFAAS, but not counting our separate investigation on chromium speciation and associated TXRF determinations of a series of trace metals using another chemical separation technique, approximately 80 separate graphical representations of individual trace metal concentrations have been obtained for the four deep-water stations at up to 36 depths from filtered and unfiltered water samples. From these many depth profiles, a few typical examples have been selected for presentation here, mainly to demonstrate the potential of TXRF. The entire data and all graphical displays together with a detailed geochemical discussion of the results will be published in a separate paper. It is the intention of the participants in this international exercise to submit a series of individual or co-operative papers for possible publication in a special issue of Marine Chemistry.

At the southernmost “Station 9”, in the northern part of the Cape Basin south of the Walvis Ridge, a tongue of Antarctic Bottom Water can still be found in the deepest water layers, as shown by the typical high silicate concentrations below 4008 m and to a lesser degree also by the increase of phosphate and nitrate plus nitrite concentration at the same depth (Fig. 4). This bottom water tongue clearly shows up in the total (predominantly particulate) manganese concentrations (Fig. 5, unfiltered samples) and even more for “dissolved” zinc (Fig. 6, filtered samples) where the highest maxima occur in the deepest samplers. For manganese, in contrast to zinc, a high value is also found in the surface water: this may be an indicator for aeolian input from terrestrial sources, e.g. desert dust that is typically high in manganese (and iron). A high maximum is observed for zinc in water of depth around 1300 m where the characteristic intermediate maxima for the nutrients are found. Contrariwise, the non-metallic element selenium (Fig. 7) displays a different behaviour: very low concentration at the surface, which increases gradually to several smaller maxima between 1000 and 2500 m, then is maintained at an almost constant level down to the bottom, with a slight increase only.

Without going into detail about the structure of the deep-water masses at “Station 5” in the Cape Verde Abyssal Plain, the most characteristic feature shall be demonstrated here in the manganese profiles. Station 5 is located in the ocean area deeply influenced by the North East Trade Winds that almost constantly blow high quantities of Sahara desert dust across vast ranges of the Atlantic. Manganese as a typical element for terrestrial aeolian input shows an extreme maximum for the particulate metal in the sea surface-water (Fig. 8).

The northernmost “Station No. 4” northeast of Madeira Island is located close to the Straits of Gibraltar and displays the most prominent feature of that region: the plume of Mediterranean outflow water high in salinity at intermediate depths around 1300 m (Fig. 9). This Mediterranean plume turns up again in a high concentration of particulate lead at about the same depth typical for the Mediterranean water (Fig. 10). The heavy metal nickel (dissolved, Fig. 11) has a very uniform depth profile with a uniform increase from the surface down to about 1000 m and a slight increase below 4000 m and an even plateau at all depths in between. The marine chemistry of selenium in its tetravalent state (Fig. 12) can be expected to show a different behaviour than typical heavy metals. Its concentration is almost zero at the sea surface and increases with depth in a less uniform manner than other metallic elements. It should

Atlantic Ocean trace metals by TXRF

GoFlo Station 9

Salinity NO, + NO,

GoFlo Station 9

:a: : :

: : :

: 0: _+Tnnn :..... .:........:...................... .

I . . . I . . . I . . I . . , . . . I . . . I

0 20 40 60 80 100 120

Phosphate Silicate

Fig. 4. Salinity and some nutrients at Station 9, measured in samples from the GoFlo bottles.

t3oPlo Station 9

0 Station 9 Manwtese , unflltcred

. .

. .

. . . .

.

-5l....:....:....:....l . 0.0 OS 1.0 1.5 2.0

Concentration (nmol/l)

Fig. 5. Depth profile for par&date manganese c.oncentrations in unflkered samples at Station 9.

178 D. SCHMIDT et al.

Station 9 Zinc , filtered O-

. .

’ -1 ** l . .

2 5 -2 t c l

g3.. l

. -5v . : . : . : .

0 2 4 6 8 Concentration (nmol/l)

Fig. 6. Depth profiie for dissolved zinc concentrations in filtered samples at Station 9.

Station 9 Selenium , unfiltered

Concentration (nmol/l)

Fig. 7. Depth profile for particulate selenium concentrations in unfiltered samples at Station 9.

Station 5 Manganese , unWered . .

.

Concentration (nmol/l)

Fig. 8. Depth profile for particulate manganese inundations in unftltered samples at Station 5.

Atlantic Ocean trace metals by TXF@

GoFlo Station 4 GoFloStation 4

-5000 i .: . . . . . . . . . . .,., i . . . . . . . . . . . i.

,....I....,....,....,

34 35 36 37 38

Salinity

GoFlo Station 4

Phosphate

i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I....,....,....,....,

0 10 20 30 40

NO,+NO,

GoFlo Station 4

0 20 40 60 80 100 120

Silicate

Fig. 9. Salinity and nutrient concentrations at Station 4.

Station 4 Lead , ~~nfiltercd o-, .

-1 . . : .

2 5 -2 . . .

f ** 5 -3.. 9 n .*

-4.. :

-5. . : . : . : . : .

0 200 400 600 f?Oo loo0

Concentration (pmol/l)

Fig. 10. Depth profile for particulate lead concentrations in unfiltered samples at Station 4.

180 D. Scmwretal.

Station 4 Nickel , filtered 01 .

Fig. 11. Depth profile for dissolved nickel concentrations in filtered samples at Station 4.

-54 . : . : . : . : . 0.0 2.0 4.0 6.0 8.0 l(

Concentration (nmol/l)

I.( 1

Station 4 Schium , unfiltered

5 . & -3-m . n .

. -4 a. . .

-51 . : . : . : . 0.0 0.2 0.4 0.6

Concentration (nmol/l)

Fig. 12. Depth profile for particulate selenium(IV) conwntrations in unfiltered samples at Station 4.

be noted here that unfiltered and filtered samples did not originate from identical sampling bottles, but from different GoFlo samplers alternating at slightly different depths.

Accuracy and precision of the TXRF method as applied at present in our laboratory have been discussed extensively in an adjoining paper from our group [5]. An analytical intercomparison of all trace metal data obtained by the participants in the Baseline Survey using different analytical techniques as well as a discussion of the possible errors involved in sampling and sample handling will be presented by LANDING et al.

w

4. CONCLUSIONS

Trace metal determinations in deep-water samples from open ocean areas far remote from human activities are among the most difficult to perform. It can be demonstrated here that TXRF has proven its excellent performance to accomplish this goal of determining extremely low trace element concentrations with good accuracy and precision.

Atlantic Ocean trace metals by TXRF 181

REFERENCES

[l] Anonymus, A Comprehensive Plan for the Global Investigation of Pollution in the Marine Environment and Baseline Study Guidelines. Intergovernmental Oceanographic Commission technical series, No. 14, UNESCO, Paris (1976).

[2] D. Schmidt, W. M. Landing, P. A. Yeats, S. Westerlund, C. 1. Measures, P. J. Statham, W. Gerwinski, C.-D. Geisler, A. Jacobsen, J. A. Dalziel, R. S. Kluckhohn and C. J. Lath, Schwennetalle im Meerwasser. Probenahme und chemische Anaiysen ftir das Experiment IOC/GIPME Open Ocean Baseline Survey for Trace Metals in the Atlantic, in: Meteor-Berichte, Nr. 91-1, Ostatlantik 9OXxpedition, Reise Nr. 12, 13 Miirz-30 Juni 1990, Eds G. Wefer, W. Weigel and 0. Pfannkuche, p. 72, Universitat Hamburg, F.R.G. (1991).

[3] P. Freimann and D. Schmidt, Spectrochim. Acta 44B, 505 (1989). [4] A. Prange, A. Kniichel and W. Michaelis, Anal. Chim. Acta 172, 79 (1985). [5] P. Freimann, D. Schmidt and A. Neubauer-Ziebarth, Spectrochim. Acta 48B, 193 (1993). [6] W. M. Landing, G. A. Cutter, A. R. Fiegal, D. Schmidt, A. Shilier, P. Statham, S. Westerlund, P.

A. Yeats and J. Resing, Anulytical Intercomparison Resultsfrom the 1990 Intergovernmental Oceanographic Commission Open-Ocean Baseline Survey for Trace Metah: Atlantic Ocean (in press).