Reassessment of 239Pu on planchets from human urine samples at ultra-trace levels using Aridus-ICP-SFMS and AMS

REASSESSMENT OF 239 PU ON PLANCHETS FROM HUMAN URINE SAMPLES AT ULTRA-TRACE LEVELS USING ARIDUS-ICP-SFMS AND AMS He ´ctor Herna ´ndez-Mendoza1, Elena Chamizo2,*, Antonio Delgado1, Manuel Garcı ´a-Leo ´n3 and Abel Yllera1 1 Centro de Investigaciones Energe ´ticas, Medioambientales y Tecnolo ´gicas (CIEMAT), Av. Complutense 22, 28040 Madrid, Spain 2 Centro Nacional de Aceleradores (CNA), Av. Thomas Alba Edison, 41092 Sevilla, Spain 3 Departamento de Fı ´sica Ato ´mica, Molecular y Nuclear, Universidad de Sevilla, 41012 Seville, Spain *Corresponding author: [email protected] Received October 17 2011, revised April 11 2012, accepted April 25 2012 New analytical methods developed at the facilities here, based on two ultra-sensitive mass spectrometry (MS) techniques, inductively coupled plasma sector field mass spectrometer with a desolvator system (Aridus-ICP-SFMS) and accelerator MS (AMS), have been applied in this work for the reassessment of 239 Pu in alpha spectrometry (AS) planchets corresponding to spiked human urine samples. The obtained 239 Pu minimum detectable activities (MDAs) values by Aridus-ICP-SFMS and AMS were 3 fg (∼6.92 mBq) and 0.4 fg (∼0.92 mBq), respectively, per sample, which are much better than those attainable by AS [50 fg (∼115.3 mBq) of 239 Pu per sample, approximately]. Therefore, it is demonstrated that the MS techniques employed in this work are very powerful tools for internal dosimetry studies in human urine samples, giving excellent results when the reassessment of AS planchets is needed (samples with a Pu concentration below or at the MDA levels measurable by AS). This work is the continuation of an article published in J. Anal. At. Spectrom. 25 (1410–1415) 2010. INTRODUCTION The successful measurement and assessment of internal exposure to radioactivity in dosimetry studies are highly dependent on the instrumental capabilities for measuring radionuclides in biological samples (urine, feces, blood or other secretions) (1,2) . Normally, during the Pu bioassay of human urine samples, it is mandatory to preconcentrate and purify the Pu fraction. In the case of mass spectrometry (MS) techniques, it is usually important to prevent the presence of 238 U in the sample, because it can interfere with the signal of the major Pu isotope 39 Pu (T 1/2 ¼24 110 y), and also to avoid matrix effects. For these purposes, ion-exchange or solid-phase extraction chromatography is extensively used in worldwide laboratories (3–8) . Alpha spectrometry (AS) is the most commonly used radioanalytical technique for the determination of Pu in routine bioassay studies, due to its simplicity and low cost. However, this method is very time-consuming and it cannot resolve the 239 Pu and 240 Pu peaks because of their very similar energies (5.15 and 5.16 MeV). Analytical results from AS are therefore expressed as the sum of the 239 Pu and 240 Pu activity concentration (i.e. 239þ240 Pu). Additionally, this technique shows some sensitivity shortcomings when the involved 239þ240 Pu activity levels are below 0.3 mBq per sample (1.3 pg of 239 Pu per sample), due to the noise associated with the detector and the poor statistics (9) . On the other hand, several parameters, such as counting efficiency, chemical yield, sample counting time, tracer activity used and background condition the precision and accuracy of the results for low-level samples and, therefore, the minimum detectable activity (MDA) of AS (10–12) . Because of its excellent detection sensitivity, MS techniques are useful tools for measuring 239 Pu at sub-fg levels, and also 240 Pu/ 239 Pu atomic ratios, which would be impossible to measure using conventional radioactive decay counting techniques. Due to the requirements imposed by the International Commission on Radiological Protection (ICRP), techniques capable of measuring 239/240/241 Pu isotopes at concentrations of 1fgl 21 in urine samples are becoming mandatory (13,14) . Different MS techniques, such as inductively coupled plasma (ICP) MS (3–6) , Thermal ionisation MS (TIMS) (15,16) and accelerator MS (AMS) (14,17,18) ,are good candidates for the dosimetric control of the population exposed to ionising radiation, opening up the possibility of identifying and quantifying small amounts of different radionuclides in a sample, and also offering isotopic composition unattainable by radioactive counting techniques (19 –22) . ICP-MS is one of the most suitable methods for the analysis of actinides at ultratrace levels, in particular 239 Pu, due to its high sensitivity, good #The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected] Radiation Protection Dosimetry (2012), Vol. 152, No. 4, pp. 296–303 doi:10.1093/rpd/ncs078 Advance Access publication 23 May 2012 accuracy and precision and generally simple sample preparation procedure versus AS (3) . However, the accurate determination of Pu isotopes by ICP-MS is hampered by both spectral and non-spectral interferences. For the analysis of Pu, the most common interferences are 238 UH þ and 238 UH 2 þ , which interfere with the 239 Pu and 240 Pu isotopes, respectively, due to the presence of U. To avoid these problems, two complementary solutions are usually applied: the preconcentration and purification of the Pu fraction using ion-chromatography resins (4,5) , and/or the reduction of uranium hydride formation using special sample introduction systems, such as micronebulizers with desolvator system (6) . On the other hand, AMS has recently been demonstrated to be a competitive technique for the precise and accurate determination of Pu at sub-fg levels in urine bioassay samples. It features a high sensitivity with virtually no matrix effects (high rejection of molecular isobaric interferences), allowing the identification of abnormal activities of Pu with no special chemical requirements (7) . The aim of this study was to develop and validate an analytical method for the reassessment of 239 Pu in AS planchets containing small amounts of 239 Pu (from 8 to 40 fg per sample), using the Aridus desolvator coupled with an ICP-SFMS, and a 1-MV compact AMS system. This is specially interesting for the authors’ research centre, CIEMAT (Research Center of Environment and Energy and Technology), where there is a big stock of Pu electrodeposits from human urine samples from the population of Palomares (Almeria, Spain), affected by a nuclear accident in 1966, and from professionally exposed people. MATERIALS AND REAGENTS Instrumentation Spectrophotometry and AS Creatinine determination was carried out by using a spectrophotometer Zuzi, Model 4211/20 (Auxilab S.L., Spain). AS measurements were performed with an Alpha Analyst Integrated Alpha Spectrometer (Canberra, TECNASA S.L., Spain) equipped with Passivated Implanted Planar Silicon detectors. These detectors have an active area of 450 mm 2 and their nominal alpha-peak energy resolution, expressed as full-width at half-maximum, is 18 keV. The planchets (Pu sources) were placed at 1.5 mm from the detector surface. For the evaluation of the data, the Genie 2000 v.2.2 software (Canberra) was used. HR-ICP-MS instrumentation Pu quantification was carried out with an Element XR Mass Spectrometer (Thermo Fisher Scientific, Bremen, Germany). The sample introduction system employed was an Aridus desolvator (CETAC Technologies, Inc., USA) with a Microflow nebulizer PFA-100 (Elemental Scientific Inc., USA). Aqueous samples were introduced to the desolvator in the continuous flow mode by using an auto sampler CETAC ASX-520 (CETAC Technologies, Inc., USA). The ICP torch was shielded with a grounded platinum electrode (GuardElectrodeTM, Thermo Scientific, Germany). Accelerator MS The measurements of 239 Pu were performed using the 1-MV compact AMS system set up at the Centro Nacional de Aceleradores (CNA, Seville, Spain). The system was designed and manufactured by High Voltage Engineering Europa (HVEE, Amersfoort, Holland). Materials and reagents 239 Pu and 242 Pu standards were obtained from National Physical Laboratory (NPL, UK). Standards were diluted with 2 M HNO 3 to obtain stock solutions with a concentration of 20.3 mBq ml 21 (140.4 ng l 21 )of 242 Pu and 7.9 mBq ml 21 (3.4 ng l 21 )of 239 Pu. 237 Np (chosen as external standard for ICP-SFMS measurements) was obtained from Amersham International plc (UK). This standard was diluted with 0.1 M HCl to obtain a stock solution with a concentration of 3.256 mBq ml 21 (123 ng l 21 ). For ICP-SFMS mass calibration, a certified multi-element solution XXIII (Ba, B, Co, Fe, Ga, In, K, Li, Lu, Na, Rh, Sc, Y, Tl and U) from Merck (Germany) was used. The rest of the salts and solutions were prepared using analytical grade reagents from Merck. High-purity water (.18 MVcm 21 ) was obtained from a Milli-Q Element A10 Century (Millipore Ibe ´rica, Spain). Nitric acid was purified by sub-boiling distillation (Duopur, Milestone S.r.l., Italy). Certified Ar gas (99.999 %) was supplied by Air Liquide (Spain). Certipur w iron ICP standard (Merck, Germany) and 500-mesh pure aluminium powder (Alfa-Aesar, UK) were employed for preparation of the AMS cathodes. AG1X2 anion-exchange chromatography resins (Bio-Rad Laboratories S.A., Spain) were used for Pu purification. The ICPSFMS measurements were carried out in a clean room (ISO 6 class) at 24+18C. Experimental The different features of the method for the extraction and purification of 239 Pu in human urine samples and the reassessment of AS planchets for ICP-SFMS and AMS determinations are shown in the diagram in Figure 1. REASSESSMENT OF 239 PU BY ARIDUS-ICP-SFMS AND AMS 297 Figure 1. Schematic diagram for the preparation procedure of urine samples and measurements of Pu by AS, Aridus-ICPSFMS and AMS: (1) collection of sample (time: 24 h); (2) co-precipitation (24 h); (3) Mineralisation (2 h); (4) radiochemistry (3 h); (5) electrodeposition (2 h); (6) recovery of Pu from planchets (24 h) and (7) preparation of the cathodes for AMS (3 h) and reassessment of 239 Pu by Aridus-ICP-SFMS and AMS. This figure appears in colour in the online version of Radiation Protection Dosimetry. H. HERNA ´NDEZ-MENDOZA ET AL. 298 Creatinine measurements Measurements of creatinine concentrations in urine have been commonly used in the laboratory to estimate 24 h excretion of radionuclides from urine samples collected in a day, then 14 samples of 24 h human urine (V sample ¼1725+114 ml) were collected from several healthy adult male volunteers. The analytical procedure was initiated the same day of the collection by the determination of creatinine content in the urine samples, in order to check whether those met the requirements needed previous to bioassay. The method is based on the Jaffe ´reaction: addition of sodium picrate into the urine and formation of a red-coluored complex (sodium picramate) (23) . The colour intensity is directly proportional to the concentration of creatinine at a specific wavelength (520 nm), which is measured with a spectrophotometer (24) . Co-precipitation The method carried out for co-precipitation is described in Kuwabara and Noguchi (8) and Robredo et al. (9) The 24 h urine samples were transferred into respective graduated cylinders, acidified with concentrated HNO 3 to pH¼1, and heated in a hot plate with magnetic stirring at 808C during 2 h. Then, the hot samples were spiked with known amounts of 242 Pu (8.4 pg) as internal standard, and 239 Pu (from 8 to 40 fg, see Table 1). Next, 1 ml of concentrated orthophosphoric acid was added, continuing the heating and the stirring for a further 1 h. Then, the Pu was co-precipitated by the addition of concentrated ammonium hydroxide until pH¼9, keeping the same conditions for 1 h. The precipitate was allowed to settle overnight and then separated by decanting and centrifugation. The supernatant was discarded, and the graduated cylinder was washed several times with 0.5 M HNO 3 . The washings were also transferred into the centrifuge tube, and the precipitate was dissolved with 5 ml 21 of 8 M HNO 3 plus slow addition 3 ml of concentrated H 2 O 2 . The procedural blanks (unspiked urine samples) were processed in the same way. Mineralisation, radiochemistry, electrodeposition and preparation of the cathodes for AMS A summary of the processes involved in this section is given in Figure 1. A detailed description has been provided in previous studies (25) . Recovery of Pu for planchets reassessment In order to determine the radiochemical yield during Pu purification, the samples were previously measured by AS. After that, the extraction of the electrodeposits was carried out by dipping the planchets into 20 ml of 5 % HNO 3 overnight. Next, the planchets were rinsed with 5 % HNO 3 and the resulting solutions were heated to dryness, diluted to 10 ml of the same acid in volumetric flasks, and divided in two twin aliquots for the Aridus-ICPSFMS and the AMS determinations. In the case of the ICP-SFMS aliquots, 4.4 fg of 237 Np were also added as an external standard. RESULTS AND DISCUSSION Creatinine test The Pu is a long-lived radionuclide that is excreted from the human body in very small amounts, which cannot always be easily detected by AS. The recommendations for good routine analysis of Pu in urine establish that the sample must contain the creatinine equivalent of, at least, of 24 h excretion (26) . This contributes to minimise the uncertainty of the results normalised to 24 h excretion, specially when analysing spot urine samples (7,13,27) . Therefore, the determination of creatinine in urine is essential for monitoring the excretion of long-lived radionuclides and it is also a decisive test for accepting or rejecting the sample (28) . In this work, the average creatinine concentration was 1.1+0.13 g l 21 (n¼15), which is in agreement with the ICRP-89 recommendations for accepting urine samples for dosimetry studies (10) . Treatment of sample and purification of Pu The co-precipitation of Pu with calcium phosphate in ammonia medium was performed to pre-concentrate it, because it is adequate to perform in human urine at a sub-pg/l concentration level of Pu. Normally, the routine bioassay of long-lived radionuclides requires mineralisation of the sample, which is usually carried out by wet ashing of the precipitate followed by muffle furnace incineration (9) . This process is very time-consuming and tedious. Therefore, in this work, the mineralisation was performed by microwave-assisted acidic digestion (8 M HNO 3 and concentrated H 2 O 2 ) of the calcium phosphate precipitate, which accelerates the preparation of the sample. When using MS and radiochemistry techniques for Pu quantification in low-level samples, the elimination of matrix elements contained in the sample is a must. In this work, this was achieved using anion-exchange resins (BioRad AG1X2). The purification of Pu using AG1X2 resin does not present significant drawbacks in contrast to other Pu purification processes which, in principle, are faster (5) .In case, the necessary time for processing 10 samples was 2–4 h. Moreover, as measured by AS, AG1X2 REASSESSMENT OF 239 PU BY ARIDUS-ICP-SFMS AND AMS 299 resin offers an excellent chemical recovery of the Pu fraction in urine samples (Table 1). Optimisation of Aridus-ICP-SFMS parameters The instrumental settings were optimised for the quantification of 237 Np 239 Pu and 242 Pu. Optimisation was performed in three steps: (1) Tuning the parameters of Aridus (argon and nitrogen flow) to obtain the maximum intensities for 115 In and 238 U isotopes, using a dilution (1:10) of the certified multielement solution in low resolution mode. The signal intensity for these isotopes was about 7.010 5 cps and 1.010 6 cps, respectively, and the 238 U oxides content in the plasma was 0.6 %. (2) Optimisation of maximum ion intensity for 242 Pu and minimum background at m/z¼239 using a 242 Pu standard solution of 0.4 pg l 21 . The background signal obtained during this step was 0.3+0.1 cps to m/z¼239. Additionally, a stability test was also performed for monitoring the signal of 237 Np, 238 U, 239 Pu, 240 Pu and 242 Pu isotopes (10, 0.1, 32, 2.7 and 42 pg l 21 , respectively) in standard samples of these isotopes during 15 min. (3) Optimisation of the method for the measurement of 237 Np, 238 U, 239 Pu, 240 Pu and 242 Pu isotopes. Instrumental operation conditions are summarised in Table 2. Reassessment of the Pu by MS techniques The main problem when measuring 239 Pu by ICPSFMS appears when U is present in the sample (molecular isobaric interference due to the formation of 238 U 1 H þ in the plasma), and also from the tailing effect of the 238 U peak, which leads to an increase in the background signal at m/z¼239. Some alternatives have been considered in the literature in order to avoid these problems: one option consists in using a low-flow micronebulizer coupled with a membrane desolvator as sample introduction system (Aridus) (6) , and the second option consists in using deuterated water as sample solvent (29) . In this work, the influence of 238 U on the background signal at m/z¼239 was investigated using the Aridus-ICP-SFMS. Thus, a certified standard solution of 0.1 ppb of 238 Uin HNO 3 5%[( 238 U)10 5 ( 239 Pu)samples] was measured, obtaining a background signal at m/z¼239 of 1.0+0.3 cps (n¼10). The background signal obtained with HNO 3 5 % blanks at m/z¼239 was 0.3+0.1 cps, indicating that the contribution of 238 U to the 239 Pu signal is negligible in these conditions. These results are in good agreement with other works for the analysis of Pu in urine samples (3) . External calibration was employed during the reassessment of 239 Pu and 242 Pu in AS planchets for Aridus-ICP-SFMS analysis (Figure 2). All the solutions (blanks, standards and samples) were spiked with 237 Np as an external standard, to monitor the instrument stability during the measurements (Figure 3). The results indicate the excellent chemical recovery achieved with the proposed extraction process for the reassessment of Pu from AS planchets Table 2. Instrumental conditions optimised for the measurement of Pu from planchets by Aridus-ICP-SFMS. Solution uptake rate 0.1 ml min 21 RF power 1300 W Cool gas flow rate 16.0 l min 21 Auxiliary gas flow rate 0.70 l min 21 Nebulizer gas flow rate 1.194 l min 21 Ion extraction lens potential 22000 V Mass resolution (m/Dm) 300 Isotope 237 Np; 238 U; 239 Pu; 240 Pu; 242 Pu Samples per peak 100 Settling time 10 ms Sample time 10 ms Points per width 10 Peak shift 1.0 Mass window 100 % Integration window 20 % Scan type E-Scan Detection mode Triple (ion counting, analogue and Faraday) Total analysis time per sample 3 min Sampler/skimmer cone Nickel Aridus desolvator Sweep gas (Ar): 4.57 ml min 21 Nitrogen flow: 6 ml min 21 Nebulizer Microflow nebulizer PFA-100 Table 1. Results of radiochemical yield for Pu (measured by AS). All the samples were initially spiked with 8.4 pg of 242 Pu. Sample 239 Pu added (fg) a 242 Pu measured (pg) Recovery yield of 242 Pu (%) 1 8 6.49+0.43 78 2 10 6.82+0.58 82 3 10 6.24+0.50 75 4 17 6.74+0.43 81 5 20 6.49+0.70 78 6 26 6.74+0.67 81 7 32 6.24+0.67 75 8 35 8.07+0.80 97 9 40 6.07+0.66 73 a239þ240 Pu measured is below the MDA. H. HERNA ´NDEZ-MENDOZA ET AL. 300 (Table 3). However, it must be pointed out that, in the case of AMS measurements, the presence of milligrams of iron coming from the planchet (due to corrosion) can affect the sensitivity of the analysis due to a dilution effect on the sample (7) . The calculation of the 239 Pu content in the initial urine samples was performed by the isotope dilution method, considering the 239 Pu/ 242 Pu isotopic ratio measured by Aridus-ICP-SF-MS in the Pu fraction leached from the planchets (90 %) and the amount of spike initially added to the samples; these results are shown in Table 3, column two and the results were confirmed by AMS. The levels of 239 Pu concentration by both methods no showed significant difference in results. Limit of detection (LOD) and MDA for 239 Pu using Aridus-ICP-SFMS were calculated from the analysis of a series of 10 procedural blanks, and compared with those reported in previous publications by AS, AMS and conventional ICP-SFMS (Table 4) (25) . The obtained results with the new set up are about one order of magnitude lower than those reported by conventional ICP-SFMS, and compares with those obtained by AMS. LOD for Pu by AS was calculated using the Currie criterion (30) and, then, MDA was calculated considering the LOD, the average chemical yield for Pu (80+7 %) and the total volume of urine (1725+114 ml). In the case of Aridus-ICP-SFMS and AMS, the LOD and AMD calculations were made as described in a previous work (7,25) . Accordingly with (8) , and assuming an acute intake by inhalation of a type S substance, an excretion of 2.3 10 –6 (Bq of Pu/Bq intake) is predicted at the first day of the intake (13) . Therefore, considering the MDA levels for Pu reported here (7.410 26 Bq per sample for Aridus-ICP-SFMS, and 1.010 26 Bq per sample for AMS), it can be concluded that the method developed in this paper provides enough sensitivity to perform the reassessment of planchets when AS is unable to give any information during internal dosimetric controls. Minimum detectable doses can be then calculated at given days after Figure 2. Calibration curves for 239 Pu and 242 Pu measured by Aridus-ICP-SFMS. Figure 3. Instrumental stability of the Aridus-ICP-SFMS, monitored during the analyses using 237 Np as external standard. Table 3. Reassessment of 239 Pu on planchets by AridusICP-SFMS and AMS. Sample Aridus-ICP-SFMS AMS 239 Pu (fg) a 239 Pu (fg) b 239 Pu (fg) b 1 5.4+0.9 8.1+1.7 8.2+0.8 2 6.5+0.8 10.1+1.7 9.8+0.6 3 7.2+0.5 10.0+1.2 10.3+0.8 4 13.7+0.7 17.1+1.7 17.4+0.9 5 13.9+0.8 20.0+2.1 19.5+0.9 6 19.3+1.2 27.1+3.0 25.8+1.4 7 22.4+1.2 32.3+3.4 31.6+1.6 8 30.6+1.4 36.1+3.4 34.8+1.3 9 27.6+1.4 40.1+4.0 38.3+1.8 a Result obtained of 239 Pu with external calibration by Aridus-ICP-SFMS. b Calculated mass of 239 Pu used method of isotope dilution method, considering the 239 Pu/ 242 Pu isotopic ratio measured. REASSESSMENT OF 239 PU BY ARIDUS-ICP-SFMS AND AMS 301 intake from the MDA of the method, urinary excretion rate (24 h urine samples) and dose coefficient. If the urine is collected on the first day of acute intake of type S 239 Pu after an accident, the minimum detectable dose of the method is 0.03 mSv for AridusICP-SFMS, and 0.004 mSv for AMS. One year after the intake, the minimum detectable dose of this method is lower than 0.5 mSv for both techniques. CONCLUSIONS Different analytical methods for quantifying 239 Pu in urine with MS techniques have been optimised and evaluated in this paper. The results indicate that they are suitable for dosimetric monitoring of the staff occupationally exposed to Pu. The analytical techniques employed (Aridus-ICP-SFMS and AMS) have shown success in these kinds of controls, exhibiting excellent sensitivity for Pu analysis, and a remarkable increase of productivity when compared with AS. This opens the possibility of giving support to the internal dosimetry service for routine analysis in those cases where the Pu content is below the MDA achieved by AS, both in urine samples and in the reassessment of planchets. FUNDING This work was supported by the Project Centro de Investigaciones Energe ´ticas, Medioambientales y Tecnolo ´gicas (CIEMAT) 2007/P1423 and with colaboration the Centro Nacional de Aceleradores de Sevilla. REFERENCES 1. 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