challenges in preparing soil samples and performing a reliable plutonium isotopic analysis by icp-ms

Challenges in preparing soil samples and performing a reliableplutonium isotopic analysis by ICP-MS

A. Puzas • P. Genys • V. Remeikis • R. Druteikien _e

Received: 18 May 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract Soil samples were prepared by performing ion

exchange and extraction chromatography separation tech-

niques and measured in the low resolution mode (m/

Dm = 300) using a standard nebulizer (1 mL/min, U-type,

‘‘Glass Expansion’’) and a cyclonic 50 mL spray chamber.

With this configuration if plutonium concentration is in a

sub-ppt range, 238U1H? must be taken into the account;

however, taking into the account 238U1H1H? is not nec-

essary. In all cases it is strongly recommended to avoid

using hydrochloric acid as a final stabilizing matrix and use

the ultrapure nitric acid as it creates the lowest background

at 239 and 240 m/z values.

Keywords Plutonium isotopes � ICP-MS � Uranium

hydride � Lead chloride � Polyatomic interferences

Introduction

Plutonium is a silvery white colour metal containing five

major isotopes: 238Pu (half-life T1/2 = 87.74 yr.), 239Pu

(T1/2 = 24,110 yr.), 240Pu (T1/2 = 6,563 yr.), 241Pu (T1/2 =

14.4 yr.) and 242Pu (T1/2 = 376,000 yr.), pointing out 239Pu

for its prevalence and radioecological awareness. It is

commonly considered an exclusively man-made radionu-

clide widely released to the environment during the

‘‘nuclear age’’ started in 1945 until the Partial Nuclear Test

Ban Treaty (PTBT) came into force in 1963, although, a

small amount of 239Pu is continuously formed in the

environment from 238U by spontaneous fission after neu-

tron capture. It is estimated that the concentration of nat-

urally produced 239Pu in the lithosphere crust is up to

1 9 10-19 g/g [1], whereas nuclear tests and accidents

distributed no less than a few thousand times higher con-

centration all around, thus, the plutonium origin is nowa-

days considered being anthropogenic.

Different plutonium sources have got different isotopic

patterns. An accurate determination of the plutonium isoto-

pic composition provides reliable information on plutonium

contamination sources and their pathways to the environ-

ment. The 240Pu/239Pu isotopic (mass) ratio, which is widely

used for plutonium origin ‘‘fingerprinting’’, is most common.

For example, produced nuclear weapon warheads typically

contain 0.03–0.07 ratios, whereas the stratospheric global

fallout after the tests in the northern hemisphere is well-

defined by the 0.18 ratio [2]. Other emission sources have got

their actual ratios [3]. Moreover, it is worth mentioning that

nuclear reactor fuel can be successfully ‘‘fingerprinted’’ by

this ratio as it strongly depends on the time of nuclear fuel

irradiation by neutrons and the neutron energy spectrum [3,

4]. For example, 240Pu/239Pu ratio of radioactive contami-

nation released during the Chernobyl accident was deter-

mined to be 0.40 [5].

The plutonium isotopic ratio is usually measured by

mass spectrometric techniques, commonly with inductively

coupled plasma mass spectrometers (ICP-MS) as it is a

relatively cheap, less time consuming method of analysis.

Samples of the environmental origin are usually measured

in a low resolution mode in order to get sufficient isotopic

signal intensities. In this mode polyatomic interferences

occur. They form in the spectrometer inductively coupled

plasma and arise from chemical reagents used for

A. Puzas (&) � V. Remeikis � R. Druteikien _eCenter for Physical Sciences and Technology, Savanoriu pr. 231,

LT-02300 Vilnius, Lithuania

e-mail: [email protected]

P. Genys

Vilnius University, Saul _etekio al. 9-3, LT-10222 Vilnius,

Lithuania

123

J Radioanal Nucl Chem

DOI 10.1007/s10967-014-3411-8

radiochemical treatment and traces of elements left after

radiochemical treatment in a sample. There are plenty of

polyatomic ions which overlap 240Pu and 239Pu isotopes in

mass spectra [6–14]. The aim of this study is to empirically

determine and assess interferences induced by uranium

hydrides, tellurium, lead, mercury chlorides and various

solutions of final chemical matrices.

Experimental

Preparation of samples

Selected soil samples (P1 and P2) were treated by a double-

step radiochemical separation technique that included ionic

exchange treatment followed by the extraction chromatog-

raphy technique. This separation was performed as follows.

Samples were taken from meadows which were undisturbed

for a few decades in the geographical territory of Lithuania.

First of all, all raw samples were dried at room temperature,

sieved out from small stones and plant roots and homoge-

nized. After that, 50 g of each sample was taken and put to a

muffle furnace, heated at 550 �C for 12 h followed by

mixing-up periodically and finally storing at 700 �C for 2 h

in order to burn out organic carbon. Afterwards, 242Pu tracer

was added and plutonium was extracted from soil matrix

using 80 mL aqua regia and evaporated to dryness. All

reagents used in this study were brought from ‘‘Merck

KGaA’’, Germany, and were of p.a. grade unless it was

stated differently. In order to be sure that all organic carbon

was burned out completely, 10 mL of 30 % (v/v) hydrogen

peroxide was also used. After that, samples were poured

twice with 5–10 mL conc. HNO3 and evaporated. Mineral

residues were washed out with 20–50 mL 8 M HNO3 and

with 200 mL of deionized H2O water (Millipore Milli-Q,

‘‘Barnstead’’), then filtrated with 0.45 lm filter paper. Fil-

trates were evaporated, and their residues were poured with

50 mL 1 M HNO3. It should be mentioned that plutonium

in solution is in various oxidation states (Pu3?, Pu4?, Pu5?,

Pu6?), although stable complexes form only Pu4?, thus it is

necessary all other valence forms transform to this one. In

that case Pu4?, Pu5? and Pu6? ions in the solution were

reduced to Pu3? with 1 g of Na2SO3 (stirring for 20 min),

thereafter it was oxidized to Pu4? with 1 g of NaNO2 in the

same way. Further, 50 mL of conc. HNO3 was poured in

each solution. Plutonium was pre-concentrated and purified

by using ion exchange resin AG 1X8 (Bio-Rad�) (100–200

mesh). The resin was conditioned with 50 mL 8 M HNO3

and 50 mL of freshly prepared 1 M HNO3 with 0.5 g of

NaNO2. Samples were passed through the columns at a flow

rate of 1–2 mL/min. Then columns were washed with 8 M

HNO3 to eliminate uranium, americium, curium, iron,

divalent metals and the bulk of rare earth elements. During

this step the major part of heavy elements, such as Pb, Hg,

Te, was removed, too. Thorium was eluted from the column

with conc. HCl. The elution of plutonium isotopes was done

with 4 M HCl with 0.6 g of Na2SO3. In order to clean

samples more accurately, ‘‘Eichrom’’ analytical procedure

ACW03 (Rev. 2.1) [15] was used additionally. Effluent was

evaporated to dryness, and then wet digestion was per-

formed using mineral acids (H2SO4, HClO4, HNO3, H2O2).

Further/deeper purification of Pu from the matrix ions was

performed by extraction chromatography. The separation of

Pu was performed using UTEVA� and TRU� resin col-

umns of 2 mL, commercially available from ‘‘Eichrom’’

(USA). A pre-packed UTEVA� resin column was con-

nected onto the top of a pre-packed TRU� resin and both

columns were preconditioned with 5 mL 3 M HNO3. The

dry residue was dissolved with 10 mL of 3MHNO3–1 M Al

(NO3)3, and 2 mL of 0.6 M ferrous sulphamate and 2 mL

(200 mg) of ascorbic acid are added. After this digestion the

sample was dissolved in 3 M HNO3–1.0 M Al(NO3)3, then

0.6 M iron sulphamate and 1 M ascorbic acid were added.

The solution was loaded onto preconditioned UTEVA�–

TRU� columns. Th was extracted onto the UTEVA� resin,

while the trivalent actinides, Pu and Am, were eluted with

3 M HNO3 and were extracted onto the TRU� resin. The

UTEVA� and TRU� columns were disconnected and the

procedure was continued with the TRU� column. The

TRU� resin was washed with 5 mL of 2 M HNO3 and 5 mL

of 2 M HNO3–0.1 M NaNO2 to oxidise Pu(III) to Pu(IV).

The TRU� resin was converted to the chloride form by

means of 3 mL of 9 M HCl, and Am was eluted with 20 mL

of 4 M HCl. Pu was eluted with 10 mL of 0.1 M NH4-

HC2O4. For electrolysis process, eluates were evaporated to

dryness, dissolved with 0.3 mL conc. H2SO4 and 1 mL

0.03 M Na2SO3 electrolyte solution. Finally, plutonium was

electrodeposited on stainless planchets. Electrolysis time

was 1 h, current density—0.6 A/cm2. Plutonium radio-

chemical output was 60–70 %. Alpha spectra are shown in

Fig. 1.

A further sample preparation step for ICP-MS analysis

and interferences assessment is described in the first par-

agraph of chapter 3.2.

Instrumental

Isotopic ratio measurements were performed with a double

focusing high resolution magnetic sector field inductively

coupled plasma mass spectrometer ‘‘Element 2’’ (Thermo

Fischer Scientific). Before measurements the instrument

was calibrated using a 2 ng/g natural uranium solution and

tuned for its best signal sensitivity. Operating conditions

and data acquisition parameters of the optimized ‘‘Element

2’’ spectrometer are summarized in Table 1.


123

Polyatomic interferences in plutonium mass spectrum

Plutonium concentration in soil samples was in a sub-ppt

range (\100 fg/g). Because of that isotopic analysis was

performed in a low resolution mode (m/Dm = 300) in order to

get a sufficient signal sensitivity. Whereas, in a low resolution

mode 239Pu? and 240Pu? ion signals start being overlapped

with uranium hydrides, tellurium, lead and mercury chlorides

ions. These background forming ionic compounds originate

from a sample solution. 239Pu? and 240Pu? masses are nearly

of the same magnitude. As plutonium isotopes could be

resolved from tellurium, lead and mercury chlorides in higher

resolution modes, it is not possible to resolve 238U1H? from239Pu? even at high resolution. Because of that, 239Pu? and240Pu? ions are practically indistinguishable from the major

part of polyatomic interferences. In Fig. 2 main polyatomic

interferences that occur in a low resolution mode are shown.

It was measured that tellurium was separated completely

in both samples while lead and mercury prevailed in minor

concentrations after a double step radiochemical treatment.

Because of that 203Tl36Ar? should not form and tellurium

was taken out from the consideration on interferences.

Uranium hydride and di-hydride, lead and mercury poly-

atomic ions were selected for the further study.

Uranium hydride and di-hydride formation

In order to determine the significance of 238U1H? and238U1H1H? ions in plutonium isotopic analysis, three ura-

nium standard solutions of 0.2, 2 and 20 ppb concentra-

tions were prepared from ‘‘Merck VI’’ standard (‘‘Merck

KGaA’’, Germany). In Figs. 3, 4, 5 their mass spectra

representing 238, 239 and 240 m/z values are shown.

In Figs. 3, 4, 5 the 238U1H? formation is clearly seen at

239 m/z value, whereas 238U1H1H? polyatomic ion does not

create any significant interference at the 240 m/z signal in

all solutions. In Table 2 238U1H?/238U? ratios are shown

(peaks were integrated using parameters as stated in

Table 1). It was assessed that 238U1H?/238U? ratio remained

constant in the 0.2–20 ppb concentration interval and it was

determined that 1 cps of 238U? signal increased the 239Pu?

signal by adding 6.5 9 10-5 cps. Thus, in order to influence

the 239Pu? signal by 1 cps more than 6.4 9 105 cps of

Fig. 1 Alpha spectra of P1 and P2 samples

Table 1 Optimized ‘‘Element 2’’ spectrometer operating conditions

and data acquisition parameters

Operating conditions

Forward power (W) 1,100

Cooling gas flow

rate (L min-1)

14.0

Auxiliary gas flow

rate (L min-1)

0.70

Nebulizer gas flow

rate (L min-1)

1.26

Solution uptake rate (mL min-1) 0.70 (pumped by a peristaltic

pump)

Nebulizer type Conical U-type 1 mL min-1

(‘‘Glass Expansion’’)

Spray chamber type Cyclonic of 50 mL volume

(‘‘Glass Expansion’’)

Data acquisition

Resolution (m/Dm) 300 (low)

Runs and passes, respectively 50, 1

Mass window (%) 100

Samples per peak 10

Search window (%) 100

Integration window (%) 30

Integration type Average

Sampling time (s) 0.100

Detector mode Counting

Scan type E-scan


123

238U? are needed; otherwise the uranium influence on the

plutonium isotopic signal could be neglected.

Lead chloride induced interferences

Two soil samples (P1 and P2) were treated using a dou-

ble-step radiochemical preparation procedure mentioned

previously. After alpha spectrometric measurements plu-

tonium isotopes from the stainless discs were washed

with 2–3 mL conc. p.a. grade hydrochloric acid, then

partly evaporated and diluted up to 0.36 % solution

choosing hydrochloric acid to stabilize the sample and to

leave it homogeneous. Important lead and mercury

polyatomic ions are 204Pb35Cl?, 204Hg35Cl?, 199Hg40Ar?

a b

238.5 239.5 239.5 240.5m/z m/z

Rel. Int.

Rel. Int.

c d

238.5 239.5 239.5 240.5m/z m/z

Rel. Int.

Rel. Int.

Fig. 2 239Pu?, 240Pu? (in

green), polyatomic interferences

(in black) and their relative

signal intensities in a mass

spectrum (concentrations each

species are considered of the

same magnitude): a 239Pu?

interference with 238U1H?;

b 240Pu? interference with238U1H1H?; c 239Pu?

interference with tellurium, lead

and mercury chlorides; d 240Pu?

signal interfering with

tellurium, lead and mercury

chlorides. (Color figure online)

Fig. 3 238U?, 238U1H? and238U1H1H? signals in a mass

spectrum in 0.2 ppb uranium

solution


123

and 200Hg40Ar? as they overlap 239Pu? and 240Pu? sig-

nals. Mercury concentration in p.a. grade hydrochloric

acid ‘‘blank’’ was found to be by a few orders of mag-

nitude lower than that of lead. As the first ionization

potential of mercury is 3 eV higher than that of lead, its

ionization is almost four times lower. Because of that,

mercury chlorides and argonides potential to interfere

with 239Pu? and 240Pu? signals was not evaluated and

neglected. 239Pu? and 240Pu? signal intensities in a mass

spectrum are shown in Fig. 6.

The 240Pu/239Pu atomic ratios were as follows: P1—

0.061 ± 0.005 (k = 2) and P2—0.057 ± 0.003 (k = 2).

The unusual signal intensity increase at the 239 m/z value

showed that the plutonium was of the unexploded nuclear

bomb origin as it was assessed by evaluating 240Pu/239Pu

atomic ratios [16], while 238Pu/239?240Pu activity ratios

measured by the complementary alpha spectrometric tech-

nique clearly stated the fallout origin. In order to clear it out,

both final samples were evaporated until dry sediments

appeared on a laboratory heating plate. After that the P1


spectrum in 2 ppb uranium

solution


spectrum in 20 ppb uranium

solution

Table 2 238U1H?/238U? ratio dependence on natural uranium

concentration

Uranium concentration, ppb 238U1H?/238U? ratio

0.2 6.4 9 10-5 ± 8.7 9 10-6 (k = 2)

2 6.5 9 10-5 ± 6.9 9 10-6 (k = 2)

20 6.7 9 10-5 ± 1.1 9 10-6 (k = 2)


123

sample was poured with 2 % ultrapure HNO3 acid (‘‘Merck

KGaA’’, Germany), whereas the P2 sample was poured with

conc. p.a. grade H2O2 (‘‘Stanchem’’, Poland) and conc.

ultrapure HNO3 acid (‘‘Merck KGaA’’, Germany). After-

wards it was evaporated again and filled with 2 % ultrapure

HNO3 acid. Figures 7 and 8 illustrate mass spectra before

and after these procedures of the matrix shift.

After the matrix shift the 240Pu/239Pu isotopic ratio in

the P1 sample became 0.084 ± 0.007 (k = 2), similarly

in the P2 sample—0.151 ± 0.009 (k = 2) (Figs. 7–8). In

other words, the signal in the P2 sample at 239 m/z

decreased more significantly than in a P1 sample. In

could be explained that part of chloride residues

remained in the sample after drying it out and pouring

2 % ultrapure HNO3 acid, while most of the chloride

compounds were broken and vanished from the sample

after leaching with conc. p.a. grade H2O2 and conc.

ultrapure HNO3 acid.

Fig. 6 Signal of 0.36 %

hydrochloric acid ‘‘blank’’ in239Pu and 240Pu mass spectra in

P1 and P2 samples

Fig. 7 P1 sample signal

intensities in mass spectra

before and after matrix shift


123

Chemical reagents matrix induced background

in samples

Various chemical reagents for final sample solution stabi-

lizing matrix could be used: diluted nitric and hydrochloric

acids, ammonium oxalate [8, 17]. Great concern is that

reagents are not purely clean from impurities. In order to

assess the chemical solution influence on the 239Pu? and240Pu? signal distortion, five different ‘‘blanks’’ were pre-

pared: 2 % ultrapure nitric acid (‘‘Merck KGaA’’, Ger-

many), 2 % p.a. grade nitric acid (‘‘Stanchem’’, Poland),

0.36 % p.a. grade hydrochloric acid (‘‘Stanchem’’, Poland),

1.25 % p.a. grade ammonium oxalate (‘‘Stanchem’’,

Poland) and 1.25 % p.a. grade ammonium oxalate (‘‘Merck

KGaA’’, Germany). Figures 9 and 10 illustrates signal

intensities in mass spectra of these ‘‘blanks’’.

As it is seen from Fig. 9, the formation of polyatomic

ions is the lowest in ultrapure 2 % nitric acid (‘‘Merck

KGaA’’, Germany) and the highest — in 1.25 % ammo-

nium oxalate solution (‘‘Stanchem’’, Poland). Actually,

interferences in high masses depend on the impurities

remained in chemicals from which polyatomic ions form in

Fig. 8 P2 sample signal

intensities in mass spectra

before and after matrix shift

Fig. 9 Mass spectra of all

‘‘blanks’’ of 239Pu? and 240Pu?


123

inductively coupled plasma sample inlet interface. If we

compare the same grade chemicals it is clearly seen that

interferences also vary according to the reagent manufac-

turer: for example, ammonium oxalate produced by

‘‘Merck KGaA’’ notably causes less interferences than the

same grade chemical produced by ‘‘Stanchem’’. Summing

up, it is recommended to use 2 % ultrapure nitric acid as a

final matrix medium of these five chemicals.

Conclusions

In this work the influence of 238U1H? polyatomic ions and

various chemicals on the 239Pu? ion mass signal was

assessed and determined. The most suitable reagent to use

as a final medium is 2 % ultrapure grade nitric acid,

whereas p.a. grade hydrochloric acid should be strongly

avoided due to the intensive formation of lead chlorides. It

has also been revealed that 238U1H? polyatomic ion for-

mation does not significantly distort the 239Pu? ion signal if

the 238U? signal intensity in the mass spectrum does not

exceed 6.4 9 105 cps (Table 2).

References

1. Varga Z (2007) Origin and release date assessment of environ-

mental plutonium by isotopic composition. Anal Bioanal Chem

389:725–732

2. Kelley JM, Bond LA, Beasley TM (1999) Global distribution of

Pu isotopes and 237Np. Sci Total Environ 237(238):483–500

3. Ketterer ME, Szechenyi SC (2008) Determination of plutonium

and other transuranic elements by inductively coupled plasma

mass spectrometry: a historical perspective and new frontiers in

the environmental sciences. Spectrochim Acta B 63:719–737

4. Wallenius M, Lutzenkirchen K et al (2007) Nuclear forensic

investigations with a focus on plutonium. J Alloy Compd

444–445:57–62

5. Muramatsu Y, Ruhm W et al (2000) Concentrations of 239Pu and240Pu and their isotopic ratios determined by ICP-MS in soils

collected from the Chernobyl 30-km zone. Environ Sci Technol

34:2913–2917

6. Kishimoto T, Sanada T et al (2002) Concentration of plutonium

in squids collected from Japanese inshore measured by HR-ICP-

MS, 1981–2000. J Radioanal Nucl Ch 252(2):395–398

7. Wyse EJ, Lee SH et al (2002) ICP-sector field mass spectrometry

analysis of plutonium isotopes: recognizing and resolving

potential interferences. J Anal Atom Spectrom 16:1107–1111

8. Kim CS, Kim CK et al (2007) Determination of Pu isotope

concentrations and isotope ratio by inductively coupled plasma

mass spectrometry: a review of analytical methodology. J Anal

Atom Spectrom 22:827–841

9. Roos P (2008) In: Baxter M (ed) Analysis of radionuclides using

ICP-MS. Elsevier, UK

10. Liao H, Zheng J et al (2008) Determination of plutonium isotopes

in freshwater lake sediments by sector-field ICP-MS after sepa-

ration using ion-exchange chromatography. Appl Radiat Isotopes

66:1138–1145

11. Qiao J, Hansen V et al (2012) Speciation analysis of 129I, 137Cs,232Th, 238U, 239Pu and 240Pu in environmental soil and sediment.

Appl Radiat Isotopes. doi:10.1016/j.apradiso.2012.04.006

12. Zheng J, Yamada M (2012) Determination of plutonium iso-

topes in seawater reference materials using isotope-dilution

ICP-MS. Appl Radiat Isotopes. doi:10.1016/j.apradiso.2012.02.

049

13. Bu K, Cizdziel JV et al (2013) Plutonium concentration and240Pu/239Pu atom ratio in biota collected from Amchitka Island,

Alaska: recent measurements using ICP-SFMS. J Environ

Radioactiv 124:29–36

14. Bu W, Zheng J et al (2014) Ultra-trace plutonium determination

in small volume seawater by sector field inductively coupled

plasma mass spectrometry with application to Fukushima sea-

water samples. J Chromatogr A 1337:171–178

Fig. 10 ‘‘Blanks’’ signals at

238 m/z


123

http://dx.doi.org/10.1016/j.apradiso.2012.04.006



15. Eichrom (2014—ACW03 method (rev. 2.1). http://www.triskem-

international.com/full_eichrom_methods.asp. Accessed 20 Feb

2014

16. Beasley T, Cooper L et al (1995) In: Strand P, Cooke A (eds)

Fuel reprocessing Pu in the Arctic Ocean basin: evidence from

mass-spectrometry measurements environmental radioactivity in

Arctic. Norway, Osteras

17. Qiao J, Hou X, Miro M et al (2009) Determination of plutonium

isotopes in waters and environmental solids: a review. Anal Chim

Acta 652:66–84


123

http://www.triskem-international.com/full_eichrom_methods.asp

http://www.triskem-international.com/full_eichrom_methods.asp

challenges in preparing soil samples and performing a reliable plutonium isotopic analysis by icp-ms

Documents