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Liquid scintillation counting of 45Ca in geochemical studies
Chemical Geology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
LIQUID SCINTILLATION COUNTING OF 45 Ca IN GEOCHEMICAL STUDIES G.D. SHARMA and R.J. BARSDATE Institute o f Marine Science, University o f Alaska, College, Alaska (U.S.A.) (Received January 11, 1971) (Resubmitted May 17, 1971)
ABSTRACT Sharma, G.D. and Barsdate, R.J., 1971. Liquid scintillation counting of 45Ca in geochemical studies. Chem. GeoL, 8: 33-36. A simplified method for the use of radioactive calcium in experimental geochemistry is described. The technique accomodates aqueous samples from 1.0 to 3.5 ml with high and constant efficiency (50%). Solutions containing sea water, HC1, and organic complexing agent have been assayed for 45Ca with a precision of -+3%for one standard deviation. The use of Triton X-100 in the two-phase emulsion liquid-scintillationmethod noted here reduced serf absorption and sample-preparation problems usually associated with the counting of aqueous samples of beta emitters such as 45Ca. With this technique the application of radioisotope methodology to the study of calcium is facilitated.
INTRODUCTION There are many potential geological applications for radioisotope tracer techniques using radioactive calcium, as in studies o f replacement or selective solution phenomenon in both low- and high-temperature experimental geology and in laboratory studies o f diagenesis. These techniques should prove very useful as well in the investigation of selective exchange, absorption or adsorption, and crystallization studies of clay minerals. Rates of shell growth and algal uptake of calcium also could be determined readily by use of radioactive calcium. The use o f radioactive calcium in experimental geochemistry has been severely hindered due to the tedious and time-consuming procedures required prior to counting. Geiger or proportional counting o f solid samples containing a s Ca can be accomplished with high precision, but the techniques are laborious and require a skilled analyst. Because of selfadsorption, the amount of material in a sample which can be assayed by a gas-pulse counter must be k e p t quite small, and for reproducibility it usually is necessary to precipitate the calcium in a well-defined matrix (carbonate or oxalate). The application o f liquid scintillation methods to the counting of 4s Ca has been delayed by the problems associated with the low solubility o f calcium salts in liquid scintillators. Lutwak (1959) and Carr and Parsons (1962) developed techniques for counting calcium in solution in calcium oxalate - hydrochloric acid - ethyl alcohol - scintillator and in a Chem. Geol., 8 (1971) 33-36
34
G.D. SHARMA ANDI~.I
BARSDA1E
similar mixture with calcium chloride. The latter authors achieved excellent efficiency, but the preparation of purified calcium chloride still was rather laborious. A parallel path of investigation involved the counting of materials held in suspension by ,q thixotmpic gelling agent (c.f., Helf et al. 1960). Self-adsorption and other problems were qu~te severe Meade and Stiglitz (1962) found the geUing agent Triton X-100 (Rohm and Haas, lnc ) t,, be useful fi~r the suspension of biological materials for liquid scintitlaticm c'ounting. Patterson and Greene (1965) and Benson (1966) expanded the work with Triton Y 100 to ItS use in the counting of aqueous emulsions of ~4 C, 3 s S, and ~lt.
EXPERIMENTAI
In investigations of the source of cements in marine carbonate rocks and of complexa tion of organic substances in natural waters, we employed Triton X-10{/for the assay ,~f 4~ Ca in fresh and sea waters and sediments Test samples were prepared by adding 0.005 g('i ,ff4~('a to scintillation vials containing, from 0 g Io 3.5 ml of artifi('ial sea water (Rila) and one vial containing () g ml of distilled water Twelve ml ~f a gelled scin tillation mixture was added to each vial, which then was capped and mixed thoroughly by shaking. The scintillation mixture contained 0.1 g POPOP (0-bis-12-(5-Phenyloxazolyl }lBenzene) and 4 g PPO (2,5-Diphenyloxazole) per liter ,ff t) to 4 mixture ,~f toluene and Triton X-100. Counting was done with a conventi(mal refrigerated liquid scintillation spectrometer (Nuclear Chicago Mark, I, Model 6860). The results (Table I) indicate that, with the exception ,+f the 0 ~ ml samples, ct,,mting efficiency was quite satisfactory (approximately g0%), that the ~ample v,)lume had little effect, and that the counts taken after six days showed only a slight loss of activity and precision. The Triton X-100 scintillator mixture is in part miscible with water, and the additions of small quantities of an aqueous sample volume improved the sensitivity of the technique and the stability of the emulsion. After dissolution in HCl, carbonate precipitates also can be counted by this pr~,cedure The data in Table II indicate that the counting efficiency is quite satisfactory in the presence of HC1 or Chelon 80 (Cowles Chemical Co.), an organic complexing agent. TABLE I Activity of samples containing 0.005 uCi 4SCa * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b :VlDt'." I, IFI|IJ"
: 2 : 4 (~
~ tlllp].O- VO[II"~ (l;1J)
,~nitlal a(,rwlry (~ ,plll[%,]T~lln }
(,'{~tll]¢/t~iH
'-',.5 0 5 i '~' 1.5 2.5 3.5
,1.10(; 4'%,; 5430 543{) 5440 5360
2850 4450 5"~20 5120 5320 5200
• % , m i n d I ,, ,tl~tllled w a l c r ; ~arnplc~ 2 ~nrotGh 6 are artificial ~ea w a t e r
ALt,Vlt; .dr~'r (, ,t,~,,., }
LIQUID SCINTILLATION COUNTING OF 4SCa
35
TABLE lI 4SCa activity of 3.0 ml synthetic seawater samples after the addition of Chelon 80 and 6 N hydrochloric acid. Addend
Initial activity (counts/min)
Activity after 6 days (counts/min)
None 0.1 ml 0.2 ml 0.3 ml 0.2 ml
7043 6924 7075 7219 7028
7188 7137 7198 7354 6974
Chelon 80 Chelon 80 Chelon 80 HCI
With the conditions we employed (1-minute counts of 5 nanocurie activities), the standard deviation for initial counts of identical samples was -+ 150 counts/min or -+ 3%. Since this is twice the uncertainty associated with the random nature of the radioactive decay process, only a small improvement in precision would be expected from increasing the sample activity or the counting time. If greater precision is required, investigation should be made of the variable associated with sample agitation and distribution in the vial and, if a refrigerated scintillation counter is used, the time and extent o f cooling. Benson's (1966) work on emulsion formation in tritium-distilled water systems is a useful model for optimizing precision and sensitivity in a specific system.
DISCUSSION The performance of the method noted here is similar to that of Carr and Parsons (1962) in that the counting efficiency is high and self-absorption problems are negligible, efficiency being nearly constant over a high range of sample size. However, the sample preparation stages have been vastly simplified in comparison with the techniques of Carr and Parsons (1962), which involve two precipitations of the samples as calcium oxalate, conversion to carbonate by ashing, and the slow extraction of calcium chloride into ethyl alcohol on an automatic shaking device. The tolerance of our method for varying amounts of HCI in some circumstances further simplifies the initial sample preparation as carbonate sediments can simply be dissolved in HC1 without exact calculation of the excess acid being necessary. This simple and rapid liqmd scintillation technique is suitable for the countmg o[ 4s Ca in a complex salt mixture (sea water), a strong mineral acid (HCI), and an organic complexing agent (Chelon 80). In view of the counting mixture employed, a two phase emulsion, we suggest that the method will permit the counting of 4s Ca and other beta emitters in the presence of a wide variety of reagents in addition to the ones investigated above.
Chem. Geol., 8 (1971) 33-36
36
G.D. SHARMA AND R.J. BARSDATE
ACKNOWLEDGEMENT This work was supported in part by the Petroleum Research Fund of the American Chemical Society and the Atomic Energy Commission, Contract No. AT(04-3)-310-PA4. This is Publication No. 93 of the Institute of Marine Science of the University of Alaska.
REFERENCES Benson, R.H., 1966. Limitation of tritium measurements by liquid scintillation counting emulsions. Anal Chem., 38: 1353-1356. Carr, T.E.F. and Parsons, B.J., 1962. A method for the assay of Calcium-45 by liquid scintillation counting. Intern. J. Appl. Radiation Isotopes, 13: 57--62. Hell, S., White, C.G. and Schelly, R.N., 1960. Radioassay of finely divided solids by suspension in a gel scintillator.AnaL Chem., 32: 238--241. Lutwak, L., 1959. Estimation of radioactive calcium-45 by liquid scintillation counting. Anal. Chem., 31: 340-343. Meade, R.C. and Stiglitz, R., 1962. Improved solvent systems for liquid scintillation counting of body fluids and tissues. Intern. J. AppL Radiation Isotopes, 13:11--14. Patterson, M.S. and Greene, R.C., 1965. Measurement of low energy beta emitters in aqueous solution by liquid scintillation counting of emulsions. Anal. Chem., 37: 854-857.
LIQUID SCINTILLATION COUNTING OF 45 Ca IN GEOCHEMICAL STUDIES G.D. SHARMA and R.J. BARSDATE Institute o f Marine Science, University o f Alaska, College, Alaska (U.S.A.) (Received January 11, 1971) (Resubmitted May 17, 1971)
ABSTRACT Sharma, G.D. and Barsdate, R.J., 1971. Liquid scintillation counting of 45Ca in geochemical studies. Chem. GeoL, 8: 33-36. A simplified method for the use of radioactive calcium in experimental geochemistry is described. The technique accomodates aqueous samples from 1.0 to 3.5 ml with high and constant efficiency (50%). Solutions containing sea water, HC1, and organic complexing agent have been assayed for 45Ca with a precision of -+3%for one standard deviation. The use of Triton X-100 in the two-phase emulsion liquid-scintillationmethod noted here reduced serf absorption and sample-preparation problems usually associated with the counting of aqueous samples of beta emitters such as 45Ca. With this technique the application of radioisotope methodology to the study of calcium is facilitated.
INTRODUCTION There are many potential geological applications for radioisotope tracer techniques using radioactive calcium, as in studies o f replacement or selective solution phenomenon in both low- and high-temperature experimental geology and in laboratory studies o f diagenesis. These techniques should prove very useful as well in the investigation of selective exchange, absorption or adsorption, and crystallization studies of clay minerals. Rates of shell growth and algal uptake of calcium also could be determined readily by use of radioactive calcium. The use o f radioactive calcium in experimental geochemistry has been severely hindered due to the tedious and time-consuming procedures required prior to counting. Geiger or proportional counting o f solid samples containing a s Ca can be accomplished with high precision, but the techniques are laborious and require a skilled analyst. Because of selfadsorption, the amount of material in a sample which can be assayed by a gas-pulse counter must be k e p t quite small, and for reproducibility it usually is necessary to precipitate the calcium in a well-defined matrix (carbonate or oxalate). The application o f liquid scintillation methods to the counting of 4s Ca has been delayed by the problems associated with the low solubility o f calcium salts in liquid scintillators. Lutwak (1959) and Carr and Parsons (1962) developed techniques for counting calcium in solution in calcium oxalate - hydrochloric acid - ethyl alcohol - scintillator and in a Chem. Geol., 8 (1971) 33-36
34
G.D. SHARMA ANDI~.I
BARSDA1E
similar mixture with calcium chloride. The latter authors achieved excellent efficiency, but the preparation of purified calcium chloride still was rather laborious. A parallel path of investigation involved the counting of materials held in suspension by ,q thixotmpic gelling agent (c.f., Helf et al. 1960). Self-adsorption and other problems were qu~te severe Meade and Stiglitz (1962) found the geUing agent Triton X-100 (Rohm and Haas, lnc ) t,, be useful fi~r the suspension of biological materials for liquid scintitlaticm c'ounting. Patterson and Greene (1965) and Benson (1966) expanded the work with Triton Y 100 to ItS use in the counting of aqueous emulsions of ~4 C, 3 s S, and ~lt.
EXPERIMENTAI
In investigations of the source of cements in marine carbonate rocks and of complexa tion of organic substances in natural waters, we employed Triton X-10{/for the assay ,~f 4~ Ca in fresh and sea waters and sediments Test samples were prepared by adding 0.005 g('i ,ff4~('a to scintillation vials containing, from 0 g Io 3.5 ml of artifi('ial sea water (Rila) and one vial containing () g ml of distilled water Twelve ml ~f a gelled scin tillation mixture was added to each vial, which then was capped and mixed thoroughly by shaking. The scintillation mixture contained 0.1 g POPOP (0-bis-12-(5-Phenyloxazolyl }lBenzene) and 4 g PPO (2,5-Diphenyloxazole) per liter ,ff t) to 4 mixture ,~f toluene and Triton X-100. Counting was done with a conventi(mal refrigerated liquid scintillation spectrometer (Nuclear Chicago Mark, I, Model 6860). The results (Table I) indicate that, with the exception ,+f the 0 ~ ml samples, ct,,mting efficiency was quite satisfactory (approximately g0%), that the ~ample v,)lume had little effect, and that the counts taken after six days showed only a slight loss of activity and precision. The Triton X-100 scintillator mixture is in part miscible with water, and the additions of small quantities of an aqueous sample volume improved the sensitivity of the technique and the stability of the emulsion. After dissolution in HCl, carbonate precipitates also can be counted by this pr~,cedure The data in Table II indicate that the counting efficiency is quite satisfactory in the presence of HC1 or Chelon 80 (Cowles Chemical Co.), an organic complexing agent. TABLE I Activity of samples containing 0.005 uCi 4SCa * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
b :VlDt'." I, IFI|IJ"
: 2 : 4 (~
~ tlllp].O- VO[II"~ (l;1J)
,~nitlal a(,rwlry (~ ,plll[%,]T~lln }
(,'{~tll]¢/t~iH
'-',.5 0 5 i '~' 1.5 2.5 3.5
,1.10(; 4'%,; 5430 543{) 5440 5360
2850 4450 5"~20 5120 5320 5200
• % , m i n d I ,, ,tl~tllled w a l c r ; ~arnplc~ 2 ~nrotGh 6 are artificial ~ea w a t e r
ALt,Vlt; .dr~'r (, ,t,~,,., }
LIQUID SCINTILLATION COUNTING OF 4SCa
35
TABLE lI 4SCa activity of 3.0 ml synthetic seawater samples after the addition of Chelon 80 and 6 N hydrochloric acid. Addend
Initial activity (counts/min)
Activity after 6 days (counts/min)
None 0.1 ml 0.2 ml 0.3 ml 0.2 ml
7043 6924 7075 7219 7028
7188 7137 7198 7354 6974
Chelon 80 Chelon 80 Chelon 80 HCI
With the conditions we employed (1-minute counts of 5 nanocurie activities), the standard deviation for initial counts of identical samples was -+ 150 counts/min or -+ 3%. Since this is twice the uncertainty associated with the random nature of the radioactive decay process, only a small improvement in precision would be expected from increasing the sample activity or the counting time. If greater precision is required, investigation should be made of the variable associated with sample agitation and distribution in the vial and, if a refrigerated scintillation counter is used, the time and extent o f cooling. Benson's (1966) work on emulsion formation in tritium-distilled water systems is a useful model for optimizing precision and sensitivity in a specific system.
DISCUSSION The performance of the method noted here is similar to that of Carr and Parsons (1962) in that the counting efficiency is high and self-absorption problems are negligible, efficiency being nearly constant over a high range of sample size. However, the sample preparation stages have been vastly simplified in comparison with the techniques of Carr and Parsons (1962), which involve two precipitations of the samples as calcium oxalate, conversion to carbonate by ashing, and the slow extraction of calcium chloride into ethyl alcohol on an automatic shaking device. The tolerance of our method for varying amounts of HCI in some circumstances further simplifies the initial sample preparation as carbonate sediments can simply be dissolved in HC1 without exact calculation of the excess acid being necessary. This simple and rapid liqmd scintillation technique is suitable for the countmg o[ 4s Ca in a complex salt mixture (sea water), a strong mineral acid (HCI), and an organic complexing agent (Chelon 80). In view of the counting mixture employed, a two phase emulsion, we suggest that the method will permit the counting of 4s Ca and other beta emitters in the presence of a wide variety of reagents in addition to the ones investigated above.
Chem. Geol., 8 (1971) 33-36
36
G.D. SHARMA AND R.J. BARSDATE
ACKNOWLEDGEMENT This work was supported in part by the Petroleum Research Fund of the American Chemical Society and the Atomic Energy Commission, Contract No. AT(04-3)-310-PA4. This is Publication No. 93 of the Institute of Marine Science of the University of Alaska.
REFERENCES Benson, R.H., 1966. Limitation of tritium measurements by liquid scintillation counting emulsions. Anal Chem., 38: 1353-1356. Carr, T.E.F. and Parsons, B.J., 1962. A method for the assay of Calcium-45 by liquid scintillation counting. Intern. J. Appl. Radiation Isotopes, 13: 57--62. Hell, S., White, C.G. and Schelly, R.N., 1960. Radioassay of finely divided solids by suspension in a gel scintillator.AnaL Chem., 32: 238--241. Lutwak, L., 1959. Estimation of radioactive calcium-45 by liquid scintillation counting. Anal. Chem., 31: 340-343. Meade, R.C. and Stiglitz, R., 1962. Improved solvent systems for liquid scintillation counting of body fluids and tissues. Intern. J. AppL Radiation Isotopes, 13:11--14. Patterson, M.S. and Greene, R.C., 1965. Measurement of low energy beta emitters in aqueous solution by liquid scintillation counting of emulsions. Anal. Chem., 37: 854-857.