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Thorium determination by liquid scintillation counting using an extractive cocktail
Environment
International,
Pergamon
Vol. 22, Suppl. 1, pp. SlOl-S103, 1996 Copyright 01996 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/96$15.00+.00
PI1SOMO-4120(96)00095-5
THORIUM DETERMINATION SCINTILLATION COUNTING EXTRACTIVE COCKTAIL Gabriele
BY LIQUID USING AN
Wallner
Institute for Inorganic Chemistry, University of Vienna, WFahringerstraRe 42, A-1090 Vienna, Austria
.EI 951 O-301 M (Received I6 October 1995; accepted 23 June 1996)
A simple procedure was developed for the determination of thorium in excretion samples for evaluating Th intakes. Afier coprecipitation and purification of Th by extraction into an organic phase and backextraction with HCI, the solution was taken to dryness and the residue dissolved in dilute sulfuric acid. Then, Th was extracted into a scintillation cocktail and measured by liquid scintillation counting. Alpha/beta separation was achieved through pulse shape analysis. The betaemitting 23‘?h was used to determine the chemical yield. The lower limit of detection was about 90 ng or 0.3 mBq per sample, comparable to cl-spectrometry. CO&~&01996 H~ier SC;~~IC~ LM
INTRODUCTION
1220 low-level counter with a/P-separation capabilities. However, a/P-separation was found to be incomplete with acid samples and therefore it was impossible to measure samples also containing 234Th, as the highenergy betas of its short-lived daughter product 234Pa spill over into the a-spectrum. To overcome this problem, the separated thorium can be extracted into the scintillation cocktail Thorex (McDowell and McDowell 1991) and measured in small teflon vials.
To determine the intake of persons occupationally exposed to thorium, measurements of excretion samples must be performed. The annual limit of intake (ALI) for 232Th is 90 Bq for inhalation of class W compounds (ICRP 61 1991). The derived investigation level based ‘on excretion fractions for monitoring of daily urinary excretion during the first 4 d after an assumed intake of one tenth of the AL1 lies between 50 and 1 mBq (ICRP 54 1988). The proposed method permits the investigation of activities in this order of magnitude. Apart from the often-used alpha spectrometry and neutron activation analysis, liquid scintillation counting (LSC) has been shown to be a tool for thorium determination in urinalysis (Wallner and Ayromlou 1996). The method is the same for faecal analysis (Azeredo et al. 1991) after complete dissolution of the ashed and fused samples in nitric acid. For determination by LSC, the separated thorium was dissolved in 2M nitric acid, mixed with a cocktail (Optiphase HiSafe 3), and measured using a Quantulus
EXPERIMENTAL
The main part of the procedure is the same as described by Wallner and Ayromlou ( 1996) extended by a second extraction step. A mixture of 0.25 mL of a saturated solution of Ca(NO,),, 0.05 mL of half-concentrated H,PO, and a certain amount of the spike 234Th is added to 200 mL of urine. Then concentrated NH, is added. The precipitate is dissolved in 6 mL of 8M HNO,. Th is extracted into SlOl
s102
G. Wallner
Iii1 [aj
0.860CPWch 118.49 rin tJ.WXi CkWch118.49min
C:\IH_l.22W1453lH.081 SPtl.2 C:\lIl_l22W1458lN,881 SPUI
1 lee 283 388 488 588 690 788 8&l 988 llw INIEGR(l-1924) Ml 43.422CPll[El 119.346CPH liutKIi=8*3s* 1.816 ( 4.2 %I 3.911 ( 2.5 x1 Fig. 1. Spectra of a Thorex sample measured immediately after Th extraction, containing 20.0 f 1.2 dpm z32Th (channel 580-675), 20.8 f 1.3 dpm “‘Th (channel 675-760), 234Th and 234Pa (45.6 f 1.9 cpm in the P-spectrumbetween channel 550 and 900). Note that the two spectra have different energy scales.
a mixture of 5 mL trioctylamine (TOA) in 5 mL xylene and backextracted with 20 mL of concentrated HCl (both steps are carried out twice). Under these conditions, uranium, which can be present in the sample too, remains in the organic phase. This first separation step is important, as Thorex would also extract the uranium. The solution is repeatedly treated with H,O, and H,SO, to destroy any organic matter and slightly evaporated to dry in a teflon beaker. The white or yellow residue is dissolved in 10 mL of 0.36 M H,SO, and shaken with a measured volume of Thorex (1- 1.5 mL). After phase separation, a measured sample is transferred to a counting vial. The sample was not treated with dry argon saturated with toluene (McDowell and McDowell 1991), as this procedure would enhance the background. At first, plastic vials were used, but then it turned out that @-separation was unsatisfactory; moreover, within a few days part of the cocktail had diffused through the vials. Glass-vials avoided these problems, but showed poor resolution of the a-peaks, high blank values and a rather high background in the P-spectrum. Small teflon vials (with a volume of 4 mL, carried by ordinary 20 mL LSC-vials) handmade at the workshop of the department, showed both excellent resolution of the a-peaks and effective a/P-separation. To determine the chemical yield, the samples were spiked with 234Th (EDmaw = 190 keV, half-life = 24.1 d)
separated from uranium by ion exchange. The B-spectrum, separated from the a-spectrum electronically by pulse-shape analysis, consists of two peaks: 234Th and its daughter product 234Pa (Epmax= 2.33 MeV, half-life = 1.17 min). For the evaluation, the Pa-region is used, as it is undisturbed by any kind of chemiluminescence. The amount of spike is chosen so that it produces a count rate of about 0.8 s-l. If samples show shifting or fading P-spectra (which happened a few times for unknown reasons), it is still possible to evaluate the 234Thby measuring the 63.3 or 92.3 + 92.8 keV y-peaks with a high-purity Ge detector with a Be window (0.8 counts per s in the Pa-region correspond to 1850 234Th gamma counts in 1000 min). RESULTS Figure 1 shows the simultaneously-measured spectra of a sample containing the a-emitting isotopes 232Th (4.0 MeV) and 228Th (5.4 MeV) in secular equilibrium as well as the P-emitting 234Th (E,, = 190 keV) and 234Pa (E,, = 2.33 MeV). The alphas are detected at lOO%, the betas originating from 234Pa at about 60% counting efficiency. The third broad ‘peak’ in the aspectrum is generated by misclassified high-energy betas, which, however, do not interfere with the actual alphas. The spillover from beta to alpha spectrum is
Thorium determination by liquid scintillation
smaller than 1%. Radioactive equilibrium between ***Th and daughter products is reached after two weeks. As the 234Th spike used here also contains traces of the a-emitting Th isotopes, the blank value in the aspectrum, and therefore also the lower limit of detection (LLD), is dependent on the amount of spike added. For the above given amount of spike (50 counts per min (cpm) in the 234Pa region) the blank value is (0.2 f 0.05) cpm. With a counting time of 1000 min, the LLD for 232Th calculated according to the formula of Currie (1968) is 0.27 ug or 1.1 mBq per sample (200 mL urine here, but larger samples can be used). This LLD is comparable to that of the previously used LSC method, measuring the sample in dilute nitric acid mixed with a cocktail. A much smaller LLD of 0.09 ug or 0.3 mBq can be reached without spike. This is also possible, the yield of the chemical procedure being (77 f 5)% with good reproducibility (Wallner and Ayromlou 1996); the extraction step is quantitative. This smaller LLD is in the same order of magnitude as that of a-spectrometry (0.3-1.5 mBq for a day’s sampling (Dalheimer and Henrichs 1994)). If the sample contains 234Th and 234Pa (originating from uranium) and a direct determination of the chemical yield is desired, two samples have to be prepared: the first without spike to determine the count ratio R of 234Pa to 232Th, and the second with a certain amount of spike added. The portion of 234Pa originating from the spike in the second sample can then be calculated by subtraction of R times the count rate of 232Th from the total count rate of 234Pa.
s103
Acknowledgment-l thank S. Ayromlou for preparing the spike solutions and K. Irlweck for many valuable discussions.
REFERENCES Azeredo, A.M.G.F.; Melo, D.R.; Dantas, B.M.; Oliveira, C.A.N. An optimised method for simultaneous determination of uranium and thorium in urine and faeces samples. Radiat. Prot. Dosim. 37( 1): 51-54; 1991. Currie, L.A. Limits for qualitative detection and quantitative determination. Anal. Chem. 40(3): 586-593; 1968. Dalheimer, A.; Henrichs, K. Monitoring of workers occupationally exposed to thorium in Germany. Radiat. Prot. Dosim. 53( l-4): 207-209; 1994. ICRP (International Commission on Radiological Protection). Report of the task group on reference man. ICRP Publication 23. Oxford, UK: Pergamon Press; 1975. ICRP (International Commission on Radiological Protection). Limits for intakes of radionuclides by workers. ICRP Publication 30. Oxford, UK: Pergamon Press; 1979. ICRP (International Commission on Radiological Protection). Individual monitoring for intakes of radionuclides by workers: Design and interpretation. ICRP Publication 54. Oxford, UK: Pergamon Press; 1988. ICRP (International Commission on Radiological Protection). Annual limits on intake of radionuclides by workers based on the 1990 recommendations. ICRP Publication 61. Oxford, UK: Pergamon Press; 199 1. McDowell, W.J.; McDowell, B.L. Liquid scintillation alpha spectrometry: A method for today and tomorrow. In: Ross, H.; Noakes, J.E.; Spaulding, J.D., eds. Proc. int. conf. on new trends in liquid scintillation counting and organic scintillators, 1989. Chelsea, MI: Lewis Publ. Inc.; 1991. Wallner, G.; Ayromlou, S. Determination of thorium in urine by liquid scintillation counting. In: Proc. int. conf. on advances in liquid scintillation spectroietry 1994. Radiocarbon 1996 (in press). Tucson, AZ: Radiocarbon.
International,
Pergamon
Vol. 22, Suppl. 1, pp. SlOl-S103, 1996 Copyright 01996 Elsevier Science Ltd Printed in the USA. All rights reserved 0160-4120/96$15.00+.00
PI1SOMO-4120(96)00095-5
THORIUM DETERMINATION SCINTILLATION COUNTING EXTRACTIVE COCKTAIL Gabriele
BY LIQUID USING AN
Wallner
Institute for Inorganic Chemistry, University of Vienna, WFahringerstraRe 42, A-1090 Vienna, Austria
.EI 951 O-301 M (Received I6 October 1995; accepted 23 June 1996)
A simple procedure was developed for the determination of thorium in excretion samples for evaluating Th intakes. Afier coprecipitation and purification of Th by extraction into an organic phase and backextraction with HCI, the solution was taken to dryness and the residue dissolved in dilute sulfuric acid. Then, Th was extracted into a scintillation cocktail and measured by liquid scintillation counting. Alpha/beta separation was achieved through pulse shape analysis. The betaemitting 23‘?h was used to determine the chemical yield. The lower limit of detection was about 90 ng or 0.3 mBq per sample, comparable to cl-spectrometry. CO&~&01996 H~ier SC;~~IC~ LM
INTRODUCTION
1220 low-level counter with a/P-separation capabilities. However, a/P-separation was found to be incomplete with acid samples and therefore it was impossible to measure samples also containing 234Th, as the highenergy betas of its short-lived daughter product 234Pa spill over into the a-spectrum. To overcome this problem, the separated thorium can be extracted into the scintillation cocktail Thorex (McDowell and McDowell 1991) and measured in small teflon vials.
To determine the intake of persons occupationally exposed to thorium, measurements of excretion samples must be performed. The annual limit of intake (ALI) for 232Th is 90 Bq for inhalation of class W compounds (ICRP 61 1991). The derived investigation level based ‘on excretion fractions for monitoring of daily urinary excretion during the first 4 d after an assumed intake of one tenth of the AL1 lies between 50 and 1 mBq (ICRP 54 1988). The proposed method permits the investigation of activities in this order of magnitude. Apart from the often-used alpha spectrometry and neutron activation analysis, liquid scintillation counting (LSC) has been shown to be a tool for thorium determination in urinalysis (Wallner and Ayromlou 1996). The method is the same for faecal analysis (Azeredo et al. 1991) after complete dissolution of the ashed and fused samples in nitric acid. For determination by LSC, the separated thorium was dissolved in 2M nitric acid, mixed with a cocktail (Optiphase HiSafe 3), and measured using a Quantulus
EXPERIMENTAL
The main part of the procedure is the same as described by Wallner and Ayromlou ( 1996) extended by a second extraction step. A mixture of 0.25 mL of a saturated solution of Ca(NO,),, 0.05 mL of half-concentrated H,PO, and a certain amount of the spike 234Th is added to 200 mL of urine. Then concentrated NH, is added. The precipitate is dissolved in 6 mL of 8M HNO,. Th is extracted into SlOl
s102
G. Wallner
Iii1 [aj
0.860CPWch 118.49 rin tJ.WXi CkWch118.49min
C:\IH_l.22W1453lH.081 SPtl.2 C:\lIl_l22W1458lN,881 SPUI
1 lee 283 388 488 588 690 788 8&l 988 llw INIEGR(l-1924) Ml 43.422CPll[El 119.346CPH liutKIi=8*3s* 1.816 ( 4.2 %I 3.911 ( 2.5 x1 Fig. 1. Spectra of a Thorex sample measured immediately after Th extraction, containing 20.0 f 1.2 dpm z32Th (channel 580-675), 20.8 f 1.3 dpm “‘Th (channel 675-760), 234Th and 234Pa (45.6 f 1.9 cpm in the P-spectrumbetween channel 550 and 900). Note that the two spectra have different energy scales.
a mixture of 5 mL trioctylamine (TOA) in 5 mL xylene and backextracted with 20 mL of concentrated HCl (both steps are carried out twice). Under these conditions, uranium, which can be present in the sample too, remains in the organic phase. This first separation step is important, as Thorex would also extract the uranium. The solution is repeatedly treated with H,O, and H,SO, to destroy any organic matter and slightly evaporated to dry in a teflon beaker. The white or yellow residue is dissolved in 10 mL of 0.36 M H,SO, and shaken with a measured volume of Thorex (1- 1.5 mL). After phase separation, a measured sample is transferred to a counting vial. The sample was not treated with dry argon saturated with toluene (McDowell and McDowell 1991), as this procedure would enhance the background. At first, plastic vials were used, but then it turned out that @-separation was unsatisfactory; moreover, within a few days part of the cocktail had diffused through the vials. Glass-vials avoided these problems, but showed poor resolution of the a-peaks, high blank values and a rather high background in the P-spectrum. Small teflon vials (with a volume of 4 mL, carried by ordinary 20 mL LSC-vials) handmade at the workshop of the department, showed both excellent resolution of the a-peaks and effective a/P-separation. To determine the chemical yield, the samples were spiked with 234Th (EDmaw = 190 keV, half-life = 24.1 d)
separated from uranium by ion exchange. The B-spectrum, separated from the a-spectrum electronically by pulse-shape analysis, consists of two peaks: 234Th and its daughter product 234Pa (Epmax= 2.33 MeV, half-life = 1.17 min). For the evaluation, the Pa-region is used, as it is undisturbed by any kind of chemiluminescence. The amount of spike is chosen so that it produces a count rate of about 0.8 s-l. If samples show shifting or fading P-spectra (which happened a few times for unknown reasons), it is still possible to evaluate the 234Thby measuring the 63.3 or 92.3 + 92.8 keV y-peaks with a high-purity Ge detector with a Be window (0.8 counts per s in the Pa-region correspond to 1850 234Th gamma counts in 1000 min). RESULTS Figure 1 shows the simultaneously-measured spectra of a sample containing the a-emitting isotopes 232Th (4.0 MeV) and 228Th (5.4 MeV) in secular equilibrium as well as the P-emitting 234Th (E,, = 190 keV) and 234Pa (E,, = 2.33 MeV). The alphas are detected at lOO%, the betas originating from 234Pa at about 60% counting efficiency. The third broad ‘peak’ in the aspectrum is generated by misclassified high-energy betas, which, however, do not interfere with the actual alphas. The spillover from beta to alpha spectrum is
Thorium determination by liquid scintillation
smaller than 1%. Radioactive equilibrium between ***Th and daughter products is reached after two weeks. As the 234Th spike used here also contains traces of the a-emitting Th isotopes, the blank value in the aspectrum, and therefore also the lower limit of detection (LLD), is dependent on the amount of spike added. For the above given amount of spike (50 counts per min (cpm) in the 234Pa region) the blank value is (0.2 f 0.05) cpm. With a counting time of 1000 min, the LLD for 232Th calculated according to the formula of Currie (1968) is 0.27 ug or 1.1 mBq per sample (200 mL urine here, but larger samples can be used). This LLD is comparable to that of the previously used LSC method, measuring the sample in dilute nitric acid mixed with a cocktail. A much smaller LLD of 0.09 ug or 0.3 mBq can be reached without spike. This is also possible, the yield of the chemical procedure being (77 f 5)% with good reproducibility (Wallner and Ayromlou 1996); the extraction step is quantitative. This smaller LLD is in the same order of magnitude as that of a-spectrometry (0.3-1.5 mBq for a day’s sampling (Dalheimer and Henrichs 1994)). If the sample contains 234Th and 234Pa (originating from uranium) and a direct determination of the chemical yield is desired, two samples have to be prepared: the first without spike to determine the count ratio R of 234Pa to 232Th, and the second with a certain amount of spike added. The portion of 234Pa originating from the spike in the second sample can then be calculated by subtraction of R times the count rate of 232Th from the total count rate of 234Pa.
s103
Acknowledgment-l thank S. Ayromlou for preparing the spike solutions and K. Irlweck for many valuable discussions.
REFERENCES Azeredo, A.M.G.F.; Melo, D.R.; Dantas, B.M.; Oliveira, C.A.N. An optimised method for simultaneous determination of uranium and thorium in urine and faeces samples. Radiat. Prot. Dosim. 37( 1): 51-54; 1991. Currie, L.A. Limits for qualitative detection and quantitative determination. Anal. Chem. 40(3): 586-593; 1968. Dalheimer, A.; Henrichs, K. Monitoring of workers occupationally exposed to thorium in Germany. Radiat. Prot. Dosim. 53( l-4): 207-209; 1994. ICRP (International Commission on Radiological Protection). Report of the task group on reference man. ICRP Publication 23. Oxford, UK: Pergamon Press; 1975. ICRP (International Commission on Radiological Protection). Limits for intakes of radionuclides by workers. ICRP Publication 30. Oxford, UK: Pergamon Press; 1979. ICRP (International Commission on Radiological Protection). Individual monitoring for intakes of radionuclides by workers: Design and interpretation. ICRP Publication 54. Oxford, UK: Pergamon Press; 1988. ICRP (International Commission on Radiological Protection). Annual limits on intake of radionuclides by workers based on the 1990 recommendations. ICRP Publication 61. Oxford, UK: Pergamon Press; 199 1. McDowell, W.J.; McDowell, B.L. Liquid scintillation alpha spectrometry: A method for today and tomorrow. In: Ross, H.; Noakes, J.E.; Spaulding, J.D., eds. Proc. int. conf. on new trends in liquid scintillation counting and organic scintillators, 1989. Chelsea, MI: Lewis Publ. Inc.; 1991. Wallner, G.; Ayromlou, S. Determination of thorium in urine by liquid scintillation counting. In: Proc. int. conf. on advances in liquid scintillation spectroietry 1994. Radiocarbon 1996 (in press). Tucson, AZ: Radiocarbon.