High-Performance Method for Determination of Pu Isotopes in Soil

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Technical Note

High performance method for rapid determination of Pu isotopes in soil and sediment samples by sector field inductively coupled plasma mass spectrometry Zhongtang Wang, Jian Zheng, Youyi Ni, Wu Men, Keiko Tagami, and Shigeo Uchida Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04975 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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Analytical Chemistry

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High performance method for rapid determination of Pu

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isotopes in soil and sediment samples by sector field

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inductively coupled plasma mass spectrometry

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Zhongtang Wang1, Jian Zheng1*, Youyi Ni1, 2, Wu Men1

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Keiko Tagami1, Shigeo Uchida1

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Biospheric Assessment for Waste Disposal Team,

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National Institute of Radiological Sciences,

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National Institutes for Quantum and Radiological Science and Technology

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4-9-1 Anagawa, Inage, Chiba 263-8555, Japan

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State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China

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______________________________________________________________________

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*Corresponding author. Tel.: +81 43 2064605; Fax: +81 43 2064601.

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E-mail address: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT

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Plutonium is extensively studied in radioecology (e.g. soil to plant transfer and

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radiological assessment) and geochemistry (e.g. sediment dating). Here, we reported a

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new chemical separation method for rapid determination of Pu in soil and sediment

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samples, based on the following investigations: extraction behaviors of interfering

28

elements (IEs, for ICPMS measurement) on TEVA resin; decontamination of U using

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TEVA, UTEVA and DGA resins and the impact of co-precipitation on Pu determination.

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The developed method consists of four steps: HNO3 leaching for Pu release; CaF2/LaF3

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co-precipitation

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TEVA+UTEVA+DGA procedure for the removal of U, Pb, Bi, Tl, Hg, Hf, Pt and Dy;

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and ICPMS measurement. The accuracy of this method in determining 239+240Pu activity

34

and 239Pu/240Pu and 241Pu/239Pu isotopic ratios was validated by analyzing five standard

35

reference materials (soil, fresh water sediment and ocean sediment). This method is

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characterized by its stable and high Pu recovery (90-97% for soil; 92-98% for sediment)

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and high decontamination factor of U (1.6 × 107) which is the highest reported for soil

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and sediment samples. In addition, the short analytical time of 12 h and the method

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detection limits, which are the lowest yet reported in literature, of 0.56 µBq g-1 (0.24 fg

40

g-1) for 239Pu, 1.2 µBq g-1 (0.14 fg g-1) for 240Pu, and 0.34 mBq g-1 (0.09 fg g-1) for 241Pu

for

the

removal

of

major

metals

2


and

U;

the

proposed

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(calculated on the basis of a 1 g soil sample) allow the rapid determination of ultratrace

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level Pu in soil and sediment samples.

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Introduction The globally distributed

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241

239

Pu (T1/2 = 2.4×104 years),

240

Pu (T1/2 = 6.5×103 years)

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and

Pu (T1/2 = 14.4 years) background resulted from nuclear weapon detonations in

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the last century. Meanwhile, regional Pu inputs are found at places adjacent to nuclear

49

weapon test sites and nuclear accident sites. Since its introduction into the environment,

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attention has been given to studying Pu not only for the purpose of radiological

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assessment due to its radiotoxicity,1,2 but also for applications using Pu as a tracer to

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study biological and geochemical processes, e.g. soil to plant transfer,3 soil erosion and

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sediment dating.4-6

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In the literature, both radiometric and mass spectrometric methods have been used

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in the determination of Pu for soil and sediment samples.7,8 In recent years, due to the

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great advantages for both quantitative and isotopic ratio measurements with respect to

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its easy sample preparation, relatively low cost and high sensitivity,9 inductively

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coupled plasma mass spectrometry (ICPMS) is replacing conventional radiometric

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methods for ultratrace Pu determination. However, ICPMS measurements are affected

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by polyatomic interferences which generate false signals at the same m/z ratio of Pu

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isotopes. Previous studies reported that elements such as U, Pb, Bi, Tl, Hg, Hf, Pt and

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Dy can cause possible polyatomic interferences for the ultra-trace determination of

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239

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should be removed during sample preparation.

Pu,

240

Pu,

241

Pu,

242

Pu by ICPMS,9-11 indicating that these interfering elements (IEs)

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In the sample preparation for ICPMS measurements, three separation strategies

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have been commonly used: solvent extraction, ion-exchange chromatography and

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extraction chromatography. Among them, extraction chromatography exhibits faster

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exchange kinetics, uses less acid and produces less hazardous waste, which explains its

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increasing popularity.12 Due to the high retention ability of Pu,13 TEVA resin is the

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mostly employed extraction resin in Pu studies. However, the reported methods mainly

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focused on the decontamination of U (for mass spectrometry) and Th (for alpha

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spectrometry), and the removal of other IEs has not been studied. In the reported

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methods using a single TEVA column for separation, decontamination factors of U [DFs

74

(U)] on the orders of 103 - 104 were achieved.14-17 However, the DFs (U) on the orders

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of 103 - 104 are not sufficient for ultratrace Pu analysis in high U content samples.18

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Furthermore, there was an inconsistency in the reported methods on the utilization of

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co-precipitation. For some researchers, co-precipitations were employed to remove the

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matrix in soil and sediment which might interfere with the subsequent separation of Pu

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on TEVA resin;15,19,20 while for others, no co-precipitation was applied.14,21,22 Further

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studies need to be carried out to demonstrate the impact of co-precipitation on Pu

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determination.

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In a recent study, Rosenberg et al. demonstrated the separation of interfering

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fission products, such as radiocesium, radioiodine and radiotellurium in gamma

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spectrometry analysis of

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extraction behaviors of IEs on TEVA resin, and quantified the decontamination abilities

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of sample matrix elements and U for CaF2/LaF3 co-precipitation and separation using

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TEVA, UTEVA and DGA resins. On the basis of these investigations, we proposed a

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new analytical method to rapidly determine Pu isotopes in soil and sediment samples by

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ICPMS for the preparedness of nuclear emergency response. Finally, we evaluated our

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method using five standard reference materials.

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Np using TEVA resin.23 In this study, we investigated the

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Experimental section

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Instrumentation. For the measurement of major matrix elements in soil samples,

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an ICP-AES instrument (Activa-M, Horiba, Kyoto, Japan) was employed. A

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SF-ICP-MS instrument (Element XR, Thermo Scientific, Bremen, Germany) equipped

96

with a Scott type spray chamber was utilized for the determination of IEs. To measure

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ultratrace Pu isotopes, a high efficiency sample introduction system Apex-Q was

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connected to the SF-ICPMS instrument. Detailed settings and evaluation of this system

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can be found in our previous work.24

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Elution experiment. An elution experiment was conducted to investigate the

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elution efficiency of HNO3 and HCl at various acidities for the removal of IEs from

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TEVA resin. The investigated IEs were U, Pb, Bi, Tl, Hg, Hf, Pt, Dy and Th. The elution

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experiment was carried out as follows: (1) preconditioning TEVA resin with 5 mL of

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tested acid solution; (2) loading 5 mL of spiked acid solution (each element at a

105

concentration of 2 ng mL-1, and at the same acidity) onto the TEVA resin cartridge; (3)

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rinsing with another 5 mL of the tested acid solution; and (4) collecting fractions (2) and

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(3) for ICPMS measurements. The tested molarities of HNO3 were 0.01 M, 0.05 M, 0.1

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M, 0.5 M, 1 M, 3 M, 5 M and 8 M. The tested molarities of HCl were 0.01 M, 0.05 M,

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0.1 M, 0.5 M, 1 M, 6 M, 9 M, and 9.5 M.

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HNO3 leaching and CaF2/LaF3 co-precipitation. The HNO3 leaching method

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was applied to leach Pu from soil and sediment samples. Specifically, 0.2 – 2.5 g soil or

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sediment sample was first ashed in a muffle furnace at 450 °C for 4 h to decompose

113

organic matter.25 Then the ashed sample was transferred to a 120 mL PTFE vessel, to

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which 10 mL conc. HNO3 and 0.57 pg 242Pu yield tracer were subsequently added. After

115

heating the vessel on a hotplate at 160 °C for 4 h, the leachate was filtered into a 50 mL

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plastic centrifuge tube using a filter paper (Ø150 mm). Milli-Q water was added to

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adjust the sample volume to 35 mL (ca. 3.8 M HNO3). Then 100 mg Ca (0.59 g

118

Ca(NO3)2•4H2O) and 100 mg La (0.76 g La(NO3)3•6H2O) were added, followed by the

119

addition of 2 mL of 20% TiCl3, which reduced Pu (IV) to Pu (III). After careful addition

120

of 7 mL of 46-48% HF, the suspension was mixed thoroughly and the precipitate

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allowed to settle for 15-20 min. After centrifugation at 3000 rpm for 15 min, the

122

supernatant was discarded and the precipitate was dissolved by 20 mL of 3 M HNO3

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with the addition of 0.5 g of H3BO3, upon which the solution was ready for plutonium

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valence adjustment and extraction chromatographic separation. To enhance the

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dissolution of CaF2/LaF3 precipitate in 3 M HNO3, H3BO3 was added because it reacts

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with fluoride precipitate and generate soluble BF4-. In addition, the addition of H3BO3

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can prevent the formation of Pu fluoride complex, thus improve the adsorption of Pu in

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TEVA resin.

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Pu separation using TEVA+UTEVA+DGA resins. After co-precipitation, the

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extraction chromatographic separation step followed (Figure 1). The valence state of

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plutonium was first adjusted to Pu (IV) by the addition of 0.3 g NaNO2 into the 50 mL

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centrifuge tube and heated at 40 °C for 0.5 h in a water bath. Then at a flow rate of 1 mL

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min-1, the sample solution was loaded onto a TEVA resin cartridge which had been

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preconditioned by 10 mL of 3 M HNO3 on a polycarbonate vacuum box (Eichrom, IL,

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USA). After sample loading, an additional 10 mL of 3 M HNO3 was used to remove Ca,

136

Fe and rare earth elements (REEs), followed by 40 mL of 1 M HNO3 to remove U, Pb,

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Tl and Pt, and 10 mL of 9 M HCl to remove Th, Bi and Hf (at a flow rate of 2 mL min-1).

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Before the elution of Pu, an UTEVA and a DGA resin cartridges both had been

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preconditioned by adding 10 mL of 3 M HNO3 were connected to the TEVA resin

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cartridge. Then 20 mL of 3 M HNO3 – 0.1 M ascorbic acid – 0.02 M Fe2+ (prepared

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from 25% iron(II) sulfamate) was employed to reduce Pu (IV) to Pu (III) and elute Pu

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(III) from TEVA resin (flow rate: 1 mL min-1). The eluted Pu (III) fraction passed

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through the UTEVA resin cartridge and was retained on the DGA resin. After elution,

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the TEVA and UTEVA resin cartridges were discarded, leaving the DGA resin cartridge

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which was then rinsed by 30 mL 0.1 M HNO3 to remove U, Tl, Pb, Pt, Hf and Fe (flow

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rate: 2 mL min-1). Finally, the plutonium on the DGA resin was eluted into a 50 mL

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PTFE vial by 20 mL of 0.5 M HCl – 0.1 M NH2OH·HCl (flow rate: 1 mL min-1). The

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eluted sample was evaporated to dryness at 250 °C and dissolved by 4 mL of aqua regia.

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After heating the dissolved sample solution to dryness at 200 ℃, 1 mL concentrated

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HNO3 was added and this was heated to near dryness at 250 ℃. Finally, the sample was

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dissolved in 0.7 mL of 4% HNO3 and ready for SF-ICPMS measurements.

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Results and discussion

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Results of elution experiment. The results of the elution experiment are

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shown in Figure S1, in which the y-axis indicates the eluted fractions of IEs during the

156

sample loading and subsequent washing steps. In HNO3 and HCl mediums, the eluted

157

fractions of U exhibited a decreasing trend as the acidities increased, in good agreement

158

with the distributions of the retention factors reported by Horwitz et al.13 Similar

159

consistency was found for Th: both datasets showed that Th was only retained on TEVA

160

resin in HNO3 for which molarity was higher than 0.5 M. These agreements

161

demonstrated the accuracy of our results and provided us with a valid base to discuss

162

the behaviors of other IEs. For Bi, it was adsorbed by TEVA resin in diluted HNO3 and

163

HCl and could only be stripped down by high concentration HNO3 (> 8 M) and HCl (>

164

9 M). In the case of Pb, no retention was shown in the whole HNO3 concentration range

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or in diluted HCl (< 0.1 M) and only weak retention could be found in 0.5-1 M HCl.

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Similar distribution patterns were observed for Tl, Pt and Dy; all these three IEs could

167

be eluted by HNO3 and diluted HCl (< 0.1 M). Differences appeared in 0.1- 9.5 M HCl:

168

the retention abilities of Tl and Pt increased with the acidity of HCl, while Dy was not

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retained on TEVA in the whole HCl concentration range. In the case of Hf, it could be

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eluted by 0.05 – 9 M HCl and 8 M HNO3. Finally, Hg could only be stripped down by

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HNO3 solutions having concentrations higher than 5 M.

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Decontamination of IEs using TEVA resin. In the literature, 1 M HNO3, 3 M

173

HNO3, and 8 M HNO3 are most frequently used for sample loading and matrix washing

174

on TEVA resin.26-31 Our results showed that 8 M HNO3 was unable to strip the most

175

important interfering element U, although it could elute Bi and Hg which 1 M and 3 M

176

HNO3 could not elute. 1 M and 3 M HNO3 did not show significant variations in the

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washing of IEs. Thus, we selected 3 M HNO3 (20 mL) for sample loading to take

178

advantage of the highest retention factor for Pu on TEVA resin.13 After sample loading,

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additional 10 mL of 3 M HNO3 and 40 mL of 1 M HNO3 solutions were used for the

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washing of IEs. Various researchers have employed 6 M HCl and 9 M HCl to remove

181

Th from TEVA resin.22,26,32,33 Our results showed that both acidities HCl had similar

182

elution efficiencies in removing Th, Pb, Dy and Hf, but 9 M HCl could additionally

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strip Bi (Figure S1). Thus, we chose 9 M HCl in our method. In summary, our approach

184

utilizing TEVA resin for the decontamination of IEs was established, and the detailed

185

description was given in the experimental section.

186

Improvement in the decontamination of U. Uranium is the key interfering

187

element in the Pu determination by ICPMS, and the decontamination of U is especially

188

essential for U-rich environmental soil and sediment samples with ultratrace level Pu.

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To find out the decontamination ability of U by our proposed TEVA separation

190

procedure, we used U spiked standard solution (10 µg of U) for an investigation, and

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our results showed that the DF (U) of the procedure was 2.4 × 104, which was in

192

accordance with other reported values using a single TEVA resin cartridge for Pu

193

separation.14-16 To thoroughly remove U interference on Pu measurement, the signal

194

intensities of U for soil and sediment samples in ICPMS measurement should be

195

controlled to an operational blank level (ca. 8 × 104 cps based on the sensitivity of 6 ×

196

107 cps per ng mL-1 U in our Apex-SF-ICPMS system). Considering the typical 238UH+ /

197

238

198

counting rate of

199

interference from

200

with a U concentration of 3µg g-1, we estimated a counting rate of 6.4 × 1011 cps when

201

no decontamination operation was applied, based on the sensitivity of our instrument

202

(final sample volume: 0.7 mL). Thus, a DF (U) of 8 × 106 was required to thoroughly

203

remove U interference. Obviously, passing through a single TEVA resin cartridge was

204

not sufficient.

U+ ratio in Apex-SF-ICPMS measurement (1-2 × 10-5),8 operational blank level 238

U+ could result in a UH+ signal of ca. 1 cps, indicating a negligible

238

U+ on ultratrace level Pu determination. For a 2.5-g soil sample

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Maxwell et al.34 reported an analytical procedure which combined TEVA resin with

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DGA resin for U removal. U and Pu (III) were stripped from TEVA resin to DGA resin

12


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by 20 mL of 3 M HNO3 - 0.1 M ascorbic acid - 0.02 M Fe2+. Then Pu (III) was oxidized

208

to Pu (IV) by 5 mL of 8 M HNO3, followed by 20 mL of 0.1 M HNO3 and 10 mL 0.05

209

M HNO3 for U removal. Finally, Pu (IV) was reduced to Pu (III) and eluted by 11 mL of

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0.02 M HCl – 0.005 M HF – 0.0001 M TiCl3. The Pu recovery of their analytical

211

method ranged from 70% to 87%. We made two improvements to simplify the

212

procedure and enhance U removal and Pu recovery. First, Pu (III) eluted from TEVA

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resin including U was directly loaded onto the DGA resin. Because our previous study

214

had shown that trivalent actinides were retained on DGA resin during the 0.1 M HNO3

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rinse to elute U,35 the Pu oxidation state adjustment was omitted. Second, to avoid

216

potential damage to the glassware in the sample introduction system by HF and

217

contamination of the ICPMS instrument by the highly abundant Ti, the elution reagent

218

was replaced by a novel reagent proposed in this study: 20 mL 0.5 M HCl – 0.1 M

219

NH2OH∙HCl. According to the elution curve shown in Figure S2, nearly 100% Pu was

220

recovered by 20 mL 0.5 M HCl – 0.1 M NH2OH∙HCl. After these improvements, the

221

separation step on DGA resin was simple: elution was conducted after introducing 30

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mL of 0.1 M HNO3 on the DGA resin cartridge to remove U, Tl, Pb, Pt, Hf, and Fe.35

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After combining the DGA separation with the TEVA procedure, a DF (U) of 2.3 × 105

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was achieved. When one UTEVA resin cartridge was added between the TEVA and

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DGA resins cartridges during stripping of Pu from TEVA resin to DGA resin, the DF

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(U) reached 6.7 × 106 due to the strong U adsorption of UTEVA resin in 3M HNO3

227

medium. DF (U) of 6.7 × 106 is generally sufficient for U removal for soil and sediment

228

samples. The whole chromatographic separation procedure using TEVA+UTEVA+DGA

229

resins is summarized in the experimental section.

230

Influence of CaF2/LaF3 coprecipitation. As we discussed in the introduction,

231

there was some inconsistence in the utilization of co-precipitation before Pu separation

232

using TEVA. To find out the effect of co-precipitation, we first evaluated the matrix

233

removal efficiency by CaF2/LaF3 co-precipitation for 5 g JSAC-0471 soil samples (n=3).

234

The co-precipitated fractions of matrix elements, Al, Fe, K, Mg, Na and U are shown in

235

Figure S3, which indicated that the majority of the matrix elements in soil were

236

removed after a co-precipitation was applied. Moreover, the CaF2/LaF3 co-precipitation

237

exhibited the ability of U decontamination; about 60% of U was removed.

238

To test the impact of co-precipitation on Pu recovery, we used Japanese soil

239

samples (sample information see Yang et al.36) for an investigation. Different amounts

240

of soil samples (0 g, 0.5 g, 1 g, 2 g, n=3) were applied for the analytical method shown

241

in Figure 1. A control group of samples was also prepared by the same method except

242

for the co-precipitation part. The results of Pu recovery for both groups are shown in

14


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Figure S4. For samples that were prepared employing the CaF2/LaF3 co-precipitation,

244

the Pu recoveries were steady and high (>90%, 0-2 g). On the other hand, for samples

245

without co-precipitation, the recovery of the operation blank was also higher than 90%,

246

but the recoveries for soil samples were relatively lower and unstable, ranging from

247

53% to 86%. Similar recoveries have been reported by other studies using TEVA resin

248

without co-precipitation to analyze soil and sediment samples, e.g. 46% - 80%

249

recoveries were reported by Muramatsu et al.14 and 44% - 83% recoveries were reported

250

by Nygen et al.22 High Pu recoveries for soil and sediment samples were only found in

251

those studies utilizing co-precipitations, e.g. 72% -92% reported by Varga et al.19 (CaF2

252

co-precipitation); 80% - 105% reported by Qiao et al.27 (Fe(OH)3 co-precipitation) and

253

104% ± 4.4% reported by Maxwell et al.20 (Fe(OH)3 + CeF3 co-precipitations). The

254

agreement between the reported results and our results confirmed the effect of

255

co-precipitation on the matrix removal and Pu recovery stabilization. Therefore, we

256

recommend the use of co-precipitation to obtain constantly high Pu recovery when

257

TEVA resin is employed for Pu separation for soil and sediment samples.

258

Evaluation of the analytical method. On the basis of above discussion, the

259

analytical method of ultratrace Pu determination for soil and sediment samples is

260

summarized in Figure 1. To demonstrate the accuracy of our method, 5 standard

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261

reference materials including IAEA-385 (ocean sediment), NIST-4354 (fresh water

262

sediment), IAEA-soil-6 (soil), NIST-4357 (ocean sediment) and JSAC-0471 (soil) were

263

analyzed. The Pu results (239+240Pu activity, 240Pu/239Pu isotopic ratio) of these materials

264

together with the certified/information/literature values are shown in Table S1. The

265

240

266

for all materials. Meanwhile, the determined 239+240Pu activities were consistent with the

267

reported ranges, illustrating the accuracy of our method in Pu determination for ocean

268

sediment, fresh water sediment and soil. In addition, 241Pu/239Pu isotopic ratio (0.0135 ±

269

0.0004, decay corrected to 1 January 2000, n=3) was also determined for NIST-4357,

270

and was in agreement with previously reported values: 0.0131 ± 0.0010 by Bu et al.;18

271

and 0.0132 ± 0.0007 by Zhang et al.37 As shown in Table S1, stable and high Pu

272

recoveries were achieved by our method: 92% - 98% for sediment and 90% - 97% for

273

sediment. These recoveries are among the highest values reported in Pu studies.

Pu/239Pu isotopic ratios were in good accordance with the certified/literature values

274

The decontamination ability for U of our method was also assessed by soil samples,

275

and the DF (U) of 1.6 ± 0.5 × 107 (n=3) was obtained. This value was close to the

276

theoretical value 1.7 × 107 which was calculated from the DF (U) of CaF2/LaF3

277

co-precipitation (100 % / 40 % = 2.5, initial fraction divided by co-precipitated fraction)

278

and DF (U) of the TEVA+UTEVA+DGA separation procedure (6.7 × 106). Compared to

16


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279

the previously reported DFs (U) of soil and sediment (Table 1),14,15,17,18,26,34,38 the DF

280

(U) in this study was higher than in previous studies using ion-exchange

281

chromatography or extraction chromatography. The signal intensities of ICPMS

282

measurement for other IEs were also controlled to an operational blank level, resulting

283

in high DFs for Hg (1.3×105), Tl (2.8×105), Dy (6.6×103), Pt (4.0×104), Pb (8.6×105), Bi

284

(6.0×104) and Hf (5.0×104). The instrument detection limits of

285

239

Pu,

240

Pu and

241

Pu were determined to be

286

0.06 fg mL-1, 0.05 fg mL-1 and 0.06 fg mL-1 respectively, based on the estimation of 3

287

times the standard deviation of a 4% HNO3 blank solution. The limit of detection

288

(LOD) of our analytical method was calculated in a similar way: 3 times the standard

289

deviation of the operation blanks. On the basis of analyzing 1 g soil or sediment with a

290

Pu recovery of 90%, the LODs of 239Pu, 240Pu and 241Pu were calculated to be 0.56 µBq

291

g-1 (0.24 fg g-1), 1.2 µBq g-1 (0.14 fg g-1) and 0.34 mBq g-1 (0.09 fg g-1). The LOD of

292

239

293

µBq g-1) reported for using α spectrometry.39 Compared to other reported LODs of 239Pu

294

using ICPMS for Pu determination, e.g. 1.7 fg g-1 reported by Truscott et al.,40 9 fg g-1

295

reported by Varga et al.19 and 6 fg g-1 reported by Kim et al.,41 our LOD of

296

fg g-1) was also lower, due to the higher sensitivity of our instrument and higher Pu

Pu obtained in this study (0.56 µBq g-1) was lower than the lowest LOD of 239Pu (1.2

17


239

Pu (0.24


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

241

Pu, our LOD (0.34 mBq g-1) was lower than the previously reported 2

297

recovery. For

298

mBq g-1 by ICPMS,42 and comparable to the lowest reported LOD of 0.1 mBq g-1 by a

299

liquid scintillation technique.43 Consequently, the lowest LOD of Pu for soil and

300

sediment samples was reported by this study.

301

The whole analytical method takes about 12 h (HNO3 leaching: 4h; filtration: 1 h;

302

coprecipitation: 1h; Pu separation on extraction resin: 2 h; sample preparation for

303

ICPMS measurements: 4 h) for 20 samples, which can be finished within two days.

304

Compared to conventional ion-exchange chromatography, which usually takes about 4-5

305

days for Pu separation,18 this method significantly shortens the analytical time. In

306

addition, this method produces a small amount of hazardous waste acid and requires

307

less evaporation of acid, greatly reducing the burden of radioactive laboratory

308

management.

309 310

Conclusions

311

In this study, we investigated the decontamination ability of IEs for the extraction

312

resin, TEVA, UTEVA and DGA. The results led to the establishment of a

313

TEVA+UTEVA+DGA separation procedure which is capable of removing IEs including

314

U, Pb, Bi, Tl, Hg, Hf, Pt, and Dy. We also assessed the effect of co-precipitation on Pu

18


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315

analysis, and found that CaF2/LaF3 co-precipitation removed the majority of matrix

316

elements and U in soil samples, leading to the stabilized Pu recovery in the subsequent

317

extraction chromatographic separation. On the basis of these fundamental investigations,

318

we proposed an analytical method for Pu analysis in soil and sediment samples. The

319

method consists of four steps: HNO3 leaching, CaF2/LaF3 co-precipitation, the

320

TEVA+UTEVA+DGA chromatographic separation and ICPMS measurement. We

321

evaluated this method by analyzing five standard reference materials. More importantly,

322

the highest DF (U) (1.6 × 107) and lowest LODs (0.56 µBq g-1 (0.24 fg g-1) for

323

1.2 µBq g-1 (0.14 fg g-1) for

324

achieved for soil and sediment samples. In addition, the stable and high Pu recoveries

325

(90-97% for soil; 92-98% for sediment) and short analytical time (12 h) demonstrated

326

the robustness and high sample throughput in the ultratrace determination of Pu isotopes

327

for soil and sediment samples.

240

Pu, and 0.34 mBq g-1 (0.09 fg g-1) for

241

239

Pu,

Pu) were

328 329 330 331

Acknowledgements This work was supported by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan.

332

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333

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Figure caption

Figure 1 Analytical method for Pu determination in soil and sediment samples by ICPMS

25



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Figure 1

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Table 1 Decontamination factors (DFs) of U for soil and sediment samples reported in literature and this study Separation methods

Co-precipitation

Sample type

DF (U)

References

Dowex 1X8

No co-precipitation

soil

1-10 × 104

Muramatsu et al.14

AG 1X8 + AG 1X8

No co-precipitation

sediment

1.4 × 104

Zheng et al.37

AG 1X8 + AG MP-1M

No co-precipitation

sediment

2 × 106

Bu et al.18

TEVA

No co-precipitation

soil and sediment

2.6 × 103

Godoy et al.17

TEVA

Fe(OH)3+ Fe(OH)2

soil

> 104

Qiao et al.15

TEVA+DGA

Fe(OH)3/Ti(OH)3 + LaF3

soil

> 106

Maxwell et al.33

AG 1X4 + TEVA

Fe(OH)3+ Fe(OH)2

soil and sediment

1-100 × 103

Xu et al.25

TEVA+UTEVA+DGA

CaF2/LaF3

soil and sediment

1.6 × 107

This study

27



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TOC

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High-Performance Method for Determination of Pu Isotopes in Soil

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