Use of ICP-MS with different analytical techniques to investigate uranium, thorium and plutonium in urine in a case of radiological emergency.

Transcription

1 Use of ICP-MS with different analytical techniques to investigate uranium, thorium and plutonium in urine in a case of radiological emergency. M.L. Cozzella, R. Pettirossi. Radiation Protection Institute ENEA, Casaccia, C.P. 2400, I Roma, Italy letizia.cozzella@casaccia.enea.it roberto.pettirossi@casaccia.enea.it Abstract. In case of a radiological emergency due to the likely high doses and to the time constraints of the operations, the possibility of using fast methods to determine the quantity of those radio-nuclides in human urine is crucial. Radionuclides like uranium, thorium and plutonium have been considered dangerous to mankind. They are traditionally measured by alpha spectrometry (U, Th, Pu) or gamma spectroscopy (Th, U) but those techniques are not ideally suited for rapid determination. The aim of this work is to test the use of mass spectrometry technique (ICP-MS) as a fast method to process a large number of samples in a short period of time with an expected uncertainty in 10-15% of the samples. Moreover the applicability of additional fast analytical methods (extraction chromatography, coprecipitation, ion exchange chromatography) to optimise the measurement according to each radionuclide are considered and tested, and special sample treatments to reduce the effect of the matrix (as signal depression) are investigated. 1. Introduction In the Safety Series No. 109 the International Atomic Energy Agency (IAEA) [1] provides the criteria for use in planning and preparedness for response to a radiological emergency, distinguishing among its different stages. Because of the rapidity of events, especially during the initial stage of the emergency, it is very difficult to carry out an accurate optimisation of the exposure, and an exceed of the dose limits is expected. Uranium, Thorium and Plutonium have traditionally been measured by radiometric techniques, such as gamma spectroscopy (Th, U) or alpha spectrometry (U, Th, Pu)[2, 3]. However, these techniques are not ideally suited for rapid determination due to the chemically complex and tedious radiochemical procedure to be followed for the preparation of the samples, especially for the alpha spectrometry, and the long duration of the measurements. Furthermore, the use of gamma spectroscopy is not always appropriate for a direct measurement of the radionuclides mentioned above. The aim of the work is to evaluate the use of mass spectrometry technique, ICP-MS, as a fast method to process a large number of samples in a short period of time with an uncertainty of the measurements approximately of 10%-15%. These values of uncertainty, acceptable in the case of accident dosimetry, are easily achievable with an ICP-MS technique. Moreover the applicability of additional fast analytical methods (extraction chromatography, coprecipitation, ion exchange chromatography) to optimise the measurement according to each radionuclide are considered and tested, and special sample treatments to reduce the effect of the matrix (as signal depression) are investigated. Since we want to investigate the potential of a standard routine analyses laboratory in case of radiological emergency, no particular system for improving instrumental sensitivity was used. To simulate a radiological emergency, a dose of 100 msv was used here. The Safety Series suggests not to exceed the limit dose of 100 msv per year and, on the other hand, the ICRP 60 [4] recommendation foresees the possibility of reaching a value of effective dose close to 500 msv. The corresponding values of activity in bioassay for monitoring purposes were calculated for radionuclides like uranium, thorium and plutonium in order to verify the applicability of the ICP-MS analytical techniques in terms of sensitivity and rapidity of execution in case of radiological emergency. 2. Experimental procedures Urine samples of non exposed people were used in the experiments. During the chemical treatment with TOPO for analyzing thorium no yield tracer was requested. We chose to use 242 Pu instead of 1

2 239 Pu to contaminate the urine samples to avoid the contamination of the instrumental components. The amount of 242 Pu was extrapolated by taking into account the activity of 239 Pu showed in table I. The Burgener Miramist nebulizer was used for uranium and thorium analysis. A PFA-100 nebulizer was used for the analysis of plutonium. Table I. 239 Pu in urine for an exposure of 100 msv. Time after uptake Conc. in urine (d) M M Bq*L-1 g*l E E E E E E E E E E E E E E E E E E E E E E E E Reagents and material Ultrapure nitric acid double distilled PPB/Teflon grade 70% (Sigma-Aldrich), Triton X-100 (Sigma- Aldrich), ultrapure water (18.2 MΏ cm -1 at 25 C) were obtained from Arium 611 UV system (Sartorius). The standard solutions for ICP-MS were provided by: Reagecon (Shannon Free Zone, Shannon, Co. Clare, Ireland), for uranium (1000 ± 5 µg ml -1 ) natural standard and thorium (1000 ± 10 µg ml -1 ) natural standard solution. Energy Department Environmental Measurements Laboratory N.Y. (U.S.A.) for plutonium (1.36 dpm ml -1 of solution 1 M in HNO 3 ) the activity of 242 Pu is % of total activity. TOPO (Tri-n-octylphosphine oxide) was obtained from Sigma-Aldrich. AG-X8 was obtained from BIO-RAD. The solid support for TOPO was Microtene 651 ( µm)(sigma- Aldrich). All solutions were prepared using analytical grade reagents (Carlo Erba, Fluka, Rudi Pont). The following analytical reagents were used: nitric acid 70%, sulfuric acid 96%, hydrochloric acid (70% ), ciclohexane, Ca(NO 3 ) 2, (NH 4 )HPO 4, HF 70% Sample preparation The urine were collected in urine containers. To analyze uranium, the urine samples (n=26) were prepared with 0.5 ml of urine diluted with 8.4 ml of ultrapure water (1:20). 0.1 ml of HNO 3 ultrapure and 1 ml of 238 U standard solution (1 ppb). 0.1% (p/v) of Triton X-100 were added to maintain a stable emulsion with the diluted samples. For 232 Th analysis (n=35) diluted urine samples were prepared with 1.0 ml of urine diluted with 8.3 ml of ultrapure water. 0.2 ml of HNO 3 ultrapure and 1 ml of the 232 Th standard solution (10 ppb). 0.1% (p/v) Triton X-100 were added to maintain a stable emulsion with the diluted sample. All samples have to be stored at 4 C. To investigate the possibility to use a fast chromatographic method of purification, samples of urine (n=3; 10 ml each) with 0.5, 1, 2 ppb of 232 Th standard solution were mixed with 1.5 g (for each sample) of MICROTENE-TOPO. The solution was shaken vigorously for 10 minutes. Then, the MICROTENE-TOPO was placed in a small plastic column, and washed with 10 ml HNO 3 4M. Thorium was eluted with 0.3 N H 2 SO 4. The eluate was dried and dissolved in 10 ml of bidistilled water and 100 µl of ultrapure nitric acid. 2

3 Urine samples (n=4; 100 ml) were acidified with 10 ml of HNO 3 conc., supplemented with respectively ( n=2) with 3.6E-10 mg ml -1 (5.2E-02 Bq L -1 ) of Pu standard solution and (n=2) with 3.611E-9 mg ml -1 ( 5.2 E-01 Bq L -1 ). They were heated to near boiling for 20 minutes. 1.25M Ca(NO 3 ) 2 solution and 50 µl of 3.2 M(NH 4 )HPO 4 solution were added to each samples while stirring. After a fast cooling (in ice), concentrated NH 4 OH was slowly added to reach a ph = 9. The supernatant was discard and the precipitate was centrifuged in a glass tube for 10 minutes at 3000 rpm. The precipitate was dissolved in HNO 3 7.2M. and 0.2g of NaNO 2 was added. The solution was heated to allow the nitrite to completely dissolve. The cooled down sample was transferred into a glass column filled with DOWEX 1x2. The resin was washed with 15 ml of HNO 3 (7.2M) and with 20 ml of HCl 10 N. Thereafter Pu was eluted with a mixture of 20 ml of 0.36 M HCl /0.014M HF. The eluate was collected in a Teflon beaker and evaporated to dryness. The residue was dissolved with 4.8 ml of ultrapure water and 0.2 ml of Ultrapure nitric acid double distilled and centrifuged to eliminate any other insoluble residue Analytical columns Plastic columns were prepared using TOPO supported by Microtene 651. The MICROTENE -TOPO was prepared immediately before use. 2 ml TOPO 0.3 M in cyclohexane was added to 3 g of MICROTENE. HNO 3 4 M was immediately used for conditioning of the solid phase. The solution was stirred for 30 min. Then, the MICROTENE-TOPO was filtered by a GOOCH G3. At this point, the MICROTENE-TOPO was ready to use. Glass columns were filled with 15ml of anionic resin DOWEX 1x2. The resin was first washed in large beaker with HNO 3 4 M overnight. Then the supernatant was discard and the conditioned resin was placed in a plastic bottle and kept in a cool, dark place and stored in HNO 3 7.2M. 3. Measurement procedure Optimization of experimental parameters of ICP-MS was performed with respect to the maximum ion intensity of a standard mix solution of 238 U(1 ppb) and a solution of 232 Th (1 ppb) in bidistilled water. All analyses were performed using polystyrene tubes and plastic tips. They were washed with a mixture of water/hno 3 ( 1:1) and rinsed with hot bidistilled water before use. We have checked the possibility to recycling the tubes ( not the tips). No particular contamination was found. Optimization of experimental parameters of ICP-MS was performed with respect to the maximum ion intensity of Pu standard solution (1.57E-10 mg*ml -1 ). At the end of the optimization procedure the system was washed with a solution of (3.0%HNO 3 v/v). This was due mainly to a very small level of 242 Pu contamination caused by the previous analysis. Before performing the analyses the dissolved residue that showed a precipitate had to be centrifuge. Only the supernatant was analyzed Instrumentation All ICP-MS uranium analyses were performed using a Thermo Elemental X Series ICP-MS. The operating conditions used are shown in table II. In square brackets are shown the operating conditions for thorium analyses. In table III are shown the operating conditions for plutonium analyses. The instrument was operated under hot screen conditions, without any particular device for improving the instrumental sensitivity. 3

4 Table II. Thermo Elemental X Series ICP-MS operating parameters for uranium and thorium determination. Table III. Thermo Elemental X Series ICP-MS operating parameters for plutonium determination. Forward power: 1250 W [1290 W] Plasma gas: 13.0 L/min [ 13.0 L/min] Auxiliary gas: 0.8 L/min [ Nebulizer gas: 0.98 L/min [ 0.97 L/min] Sample flow : 1 ml/min Torch: Single piece, quartz Nebulizer: Burgener Miramist Spray chamber: Quartz impact bead, peltier cooled to +2 C Sampler: Standard high sensitivity Ni sampler (1.0 mm i.d. orifice) Skimmer: Standard Ni skimmer cone (0.7 mm i.d. orifice) Data acquisition: Pulse counting, collected via embedded PC on instrument before transfer to user PC. Dead time: 35 ns Points per peak: 1 Forward power: 1420 W Plasma gas: 13.0 L/min Auxiliary gas: 0.8 L/min Nebulizer gas: 0.96 L/min Sample flow: 100 microlitres/min Torch: Single piece, quartz Nebulizer: PFA-100 Spray chamber: Quartz impact bead, peltier cooled to +2 C Sampler: Standard high sensitivity Ni sampler (1.0 mm i.d. orifice) Skimmer: Standard Ni skimmer cone (0.7 mm i.d. orifice) Data acquisition: Pulse counting, collected via embedded PC on instrument before transfer to user PC. Dead time: 35 ns Points per peak: 1 4.Results and discussion 4.1. Uranium and thorium The determination of uranium and thorium in urine samples by ICP-MS is widely used as analytical method [5, 6, 7] This technique provides an excellent tool for a rapid and straightforward measurement of uranium in urine. In this case no interference from the matrix was detected and no digestion process was used to decompose organic matter [8]. The salt content of urine is about 0.1 % (m/v) therefore in principle it could be aspirated directly into an ICP-MS, but most users dilute it 1:10 or 1:20 to reduce the analyte signal suppression problems that such a matrix induces. The concentration of uranium excreted during the emergency ( see tab n. 4) is so high that it is necessary to dilute the samples between 1E+05 and 1E+06 times. We supposed that the real problem with a great number of samples was the control of the long time stability and the loss of the instrumental signal. To determine the ICP-MS performance we have chosen to spike a sample set with 100 ppt of U natural standard solution. This concentration allows us to define the 235 U/ 238 U ratio with good precision (to test for a hypothetical contamination from depleted uranium). Our detection limit in urine aqueous solutions is: for 238 U 1 pg ml 1, for 232 Th 2.2 pg ml 1. 4

5 Table IV. 238 U in urine for an expos. of 100 msv. Time after uptake Conc. in urine (d) F F Bq*L-1 g*l-1 1 2,30E+04 1,86E ,90E+02 6,41E ,30E+02 5,11E ,70E+02 4,62E ,20E+02 4,22E ,70E+02 3,81E ,30E+02 3,49E ,90E+02 3,16E ,60E+02 2,92E ,30E+02 2,68E ,50E+02 1,22E ,40E+01 6,81E-03 Table V. 232 Th in urine for an expos. of 100 msv. Time after uptake Conc. in urine (d) M M Bq*L-1 g*l-1 1 3,30E-02 8,15E ,10E-03 2,00E ,80E-03 1,19E ,90E-03 9,64E ,40E-03 8,40E ,10E-03 7,66E ,80E-03 6,92E ,60E-03 6,43E ,40E-03 5,93E ,30E-03 5,68E ,70E-03 4,20E ,50E-03 3,71E-07 We have considered, in case of uranium contamination, two different situations: A) to execute many measures with a short time ( any possibility to execute a washout ) B) to execute many measure with a possibility to execute a washout The results are shown below. The washout time was 200 sec. It was a made by ultrapure water, ultrapure nitric acid (2.5 %v/v) and 0.1% Triton X cps time (minutes) FIG. 1. Continuing measurements of 238 U (100 ppt) in urine sample (1:20) without washout. 5

6 cps time (minutes) FIG. 2. Continuing measurements of 238 U (100 ppt) in urine sample with a washout of 200 sec. The most important events to avoid during the analysis are: the signal suppression and the lack of instrument stability during every measurement. We have notice that occasional some solid impurity can deposit in the pump tubes and the nebulizer. This may results in the loss of data. In the worst case, after a great number of analyses (not sure about this plural, could be analyses), we have found a mud inside the nebulizer tip. This situation can produce an artificial increase of the uranium and a contamination of all samples. Therefore, it is necessary to check carefully the irreplaceable parts of the instrument. The Burgener Miramist was used to avoid blocking during aspirations. As far as thorium determination in bioassay is concerned, faecal analysis had been classified by some authors as the only practicable way, due to the lack in sensitivity from urinary bioassay [9]. The use of ICP-MS has overcome this problem. Thorium has a natural tendency to adhere to internal components of the instrumentation. For this reason it could be necessary to wash it for a long period of time with 5% HNO3 (v/v) wash solution [10]. Since it can be easily used only with Pt cones ( not with Ni cones), we have decided to monitor the ICP-MS performance for thorium analyses with a diluted (1:10) urine samples added with 1 ppb of 232 Th standard solution, in continuing measurements. This concentration is very close to the excretion on the first day of exposure (see table V). cps time (minutes) 120 FIG. 3. Continuing measurements of 232 Th solution (1 ppb). We have also considered the possibility to apply a fast chromatography method to purify the samples before the ICP-MS analyses. For this reason we have modify the technique used in Radiotoxicology laboratories of E.N.E.A. Casaccia [11]. The results are shown below. 6

7 Table VI. 232 Th concentration in urine after MICROTENE TOPO column. Time after Spike of 232 Th Spike of Th in Th recovered RDS % uptake (day) in urine (Bq L -1 ) urine (µg L -1 ) % E E E Moreover we investigated the ICP-MS response for the analysis of diluted (1:20) urine samples complemented with thorium (1 ppb) natural standard solution and uranium (1 ppb) natural standard solution. Under these conditions the preliminary tests have shown a relevant interference for the determination of uranium, but not for the determination of thorium. This kind of interference among the radionuclides will be evaluated later Plutonium The most commonly used spectrometry in the [12] analytical routine to determinate Pu and other transuranic elements is alpha spectrometry. This method offers highly efficient counting of emitted α- particles but lacks the energy resolution necessary to discriminate all transuranic radionuclides. On the contrary, ICP-MS offers in a short time information about the isotopic composition of samples containing long-lived isotopic elements with low detection limit. The isotope 239 Pu is easily distinguished. To simulate a radiological emergency, a dose of 100 msv has been considered. Since we choose to use 242 Pu instead of 239 Pu we have spiked the urine samples (100 ml) with two different amount of 242 Pu. Those were extrapolated by taking into account the excretion of the first day and sixth day (see table I. The dissolved residues of the DOWEX column (see 2.1.2) were analyze by ICP-MS. The use of anionic resin is widely used for the determination of Pu in environmental samples [13], but in the present case it has been applied as a rapid method to purify and concentrate the Pu amount in the urine samples. Without any particularly system to magnify the instrumental sensitivity it is necessary to eliminate in the best possible way the uranium from the samples. We have found that the uranium hydride formation rate in standard operation was measured to be approximately between 0.002% %. Results for these conditions are shown in table VI. Blank sample counts per second at mass 242 was 2.0 cps (approximately corresponding to 2 ng L -1 ). Table VI. 242 Pu measured by ICP-MS after DOWEX column. Time after uptake (day) Spike of 242 Pu in urine (Bq L -1 ) 242 Pu recovered % RDS % 1 5.2E E E E

8 5. Conclusion In this work the ICP-MS has been successfully and easily applied for the determination in an radiological emergency of a radionuclides like U, Th and Pu. From the experiments made, taking into account the reference value of 100 msv for internal contamination, it can be deduced, for Uranium and Thorium, that: -the preparation of sample is a simple dilution. To reduce the analytical signal suppression is recommended a dilution 1:20; -a solution of 1 µg L -1 is a sufficient quantity to show an internal contamination corresponding to 100 msv after 30days from the incident; -reducing the washout time to 120 sec. it is possible to execute 10 analysis every hour and it is possible to involved many laboratories in the monitoring of radiological emergency using a very easy process. Analitycal determination of Pu is more critical and certain considerations must be taken into account. These can be summarized as follows: -even if the dilution of the samples is 1:10, the detectable amount 10 days after the event should be less then 1pg L -1. Under this condition only the laboratories equipped with high sensitivity instruments can be included in the list of the qualified laboratories involved in case of emergency situation; -the use of a well know fast method (extraction chromatography, coprecipitation, ion exchange chromatography ) of purification could involve many others laboratories. The progress of our work will be the drowning up of a method to define the specifications of the instruments request and to define a list of qualified laboratories for the monitoring of emergency situation. References 1. International Commission on Radiological Protection, Intervention Criteria in a Nuclear or Radiation Emergency, IAEA Safety Series n 109, IAEA, Vienna (1994). 2. De Regge, P., Boden, R., Review of chemical separation technique applicable to alpha spectrometry measurements, Nucl. Instrum. Meth. Phys. Res., 223: , (1984). 3. Wolf, S.F., Application of instrumental radioanalytical techniques to nuclear waste testing and characterization, Journal of Radioanalytical and Nuclear Chemistry, 235, No. 1-2: , (1998). 4. International Atomic Energy Agency, 1990 Recommendations of the International Commission on Radiological Protection. Publication 60. Annals ICRP, 21, No. 1-3, Pergamon Press, Oxford (1990). 5. Baglan, N., Cossonet, C., Trompier, F., Ritt, J., Berard, P., Implementation of ICP-MS protocols for uranium urinary measurements in worker monitoring Health Physics, 77, No. 4: , (1999). 6. Pappas, S.R., Ting, B.G., Jarrett, M.J., Paschal, D.C., Caudill, P.S., Miller, D. T., Determination of uranium-235, uranium-238 and thorium 232 in urine by magnetic sector inductively coupled plasma mass spectrometry, Jour. Anal. At. Spectrom., 17: , (2002). 7. Baglan, N., Cossonet, C., Ritt, J., Determination of 232 Th in urine by ICP-MS for individual monitoring purposes Health Physics, 81, No. 4: 76-81, (2001). 8. Kuwabara, J., Noguchi, H., Development of Rapid Urine Analysis Method for Uranium, Japan Atomic Energy research Institute, Tokai-mura, Naka-gun, Ibaraki-ken, 319:1195, 9. Lipzstein, J. L., Bertelli, L. N., Oliveira C. A. N., Azeredo, A. M. G., Melo, D. R., Laurenço, M. C., Grynspan, D., Dantas, B. M. Bioassay monitoring studies for thorium Radiat. Protect. Dosim. 26: 56-60, (1989). 10. Holmes,L., Pilvio, R., Determination of thorium in environmental and workplace materials by ICP-MS Applied Radiation and Isotope 53: 63-68, (2000). 11. Testa, C., Masi, G., Bazzarri, S., Marchionni, V., Santori, G., Tecniche radiotossicologiche in uso presso il C.N.E.N. RT/PROT(71), (1971). 8

9 12. Talvitie N. A., Electro deposition of actinides for α-spectrometric determination Anal. Chem., Vol 44, 280, (1972). 13. Rubio Montero, M. P., Martìn Sànchez, A., Crespo Vàzquez, M. T., Gascòn Murillo, J.L., Analysis of plutonium in soil samples, Applied Radiation and Isotopes 53: , (2000). 9