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Triton X-100 as a complete liquid scintillation cocktail for counting aqueous solutions and ionic nutrient salts
Triton
X-100 as a Complete
Liquid
Scintillation Cocktail for Counting Aqueous Solutions and Ionic Nutrient Salts* D.A\-ID Department of Hsrticultumi
WM.
REED
Sciences. Texts A&M Uni\erGty. Colizge Station. TX 77843, U.5.4.
Triton X-100+, used alone. was found to act as a complete liquid scintillation cocktail. Triton X-100 acted as a scintillator and the eiTect was not due to Cerenkov radiation. A variety of other commercially available surfactants also acted as scintillators. but with different levels of efficiency. Triton X-100 water combinations were suitable for counting aqueous solutions of “P and ‘“Rb and the count rate was stable over extended periods of time. Triton X-100 toluene combinations also yielded high counting efftcien&. Triton N-100 was more sensitive to quenching than standard cocktails containing Huors.
Introduction Current liquid scintillation cocktails are organic based. which results in instable mixtures when combined with aqueous solutions of ionic salts. The mixtures may be stabilized by emulsion formation”’ or the use of solid supports or suspensions.“’ For strong p-emitters, aqueous solutions of ionic salts can be counted effectively by Cerenkov radiation in u.ater.‘:“ Of these techniques the use of surfactants, such as Triton X-100, as an emulsifier is the most widely utilized. Triton X-100 was first added to toluene or xylene based cocktails to increase micibility with aqueous solutions.“.‘“’ Others have eliminated toluene and utilized Triton X-100 as the only solvent, demonstrating that it can act as an energy transferring agent in conjunction with PPO and POPOP.‘s,6’ Recent studies have shovvn that Triton X-100 can act as a primary scintillator in a Triton X-100,water system.“.” Other surfactants also have been shown to act as scintillators.‘” These studies report on the use of Triton X-100 alone as a compete liquid scintillation cocktail for counting aqueous solutions of ionic nutrient salts.
Materials
and Methods
A standard cocktail of 7: 1 (v,v) toluene: Triton X- 100 containing 5 g,‘L PPO and 0.1 g L POPOP was used for comparison of efficiencies with al1 other * Texas Agricultural Experiment Station Article So. TA I7?55. + Triton X-100 is a regstered trademark of Rohm and Haas Corporation. 367
cocktails. All samples were counted to at least 0.5”, preset error in 20-mL volume low-background glass vials containing IO-mL cocktail. Samples of Triton S-100 (Sigma Chemical Co.. RPI. Rohm and Haas) were purified according to the procedures of Patterson and Greene”“’ by treatment with activated carbon and silica gel, and by the procedures of Green er c7l.l” by treatment with Dowex-I X-S (hydroxide form). The source of Triton X-100 or purification procedures did not alter the count rate obtained, hence. all studies utilized Triton X-100 as obtained from Sigma Chemical Co. The “P was obtained as HI-“PO, (carrier-free, less than 5:a “P) and S6Rb as “‘RbCl (0.5-10 Ci g) from Neu England Nuclear. Boston. Mass. All samples were counted in a Beckmann LS7500 liquid scintillation counter. The index of refraction of Triton X-100 was obtained with a Bausch and Lomb Model 369KV Refractometer. The fluorescence emission spectrum was obtained with an American Instrument Co. Spectrophotofluorometer.
Results
and Discussion
This study was initiated to identify a cocktail that was both economical and applicable to counting aqueous preparations of ionic mineral nutrients commonly utilized in plant nutrition studies. Cerenkov counting in water has the advantage of both economy and solubility of ionic compounds. but cannot detect efficiently lower energy /? emitting radionuclides, such as “Ca, >?P, ?‘?ia. ‘W, which are often utilized in plant nutrition studies. In the initial studies, T&on X-100 was tested because of its ability to solubilize large quantities of water containing salts. and be-
its higher index of refraction (no = 1.43) might lower the Cerenkov threshold to allow more efficient detection. Both “‘Rb (92’; 1.77 MeV. 9”; 0.68 MeV), which is easily detected in water by Cerenkov radiation, and ‘>P (LOO”, 0.245 MeV), Hhich is just below the Cerenkov threshold of 0.263 MeV in water. were tested. The efficiencies observed were much higher than expected (Table I). The index of refraction of Triton X- 100 was measured to be 1.rlS, which was not high enough to explain the efficiencies obtained by cerenkov radiation alone.“’ Triton X-100 war shown to yield a fluorescence spectrum with a peak at 360nm (Fig. I). These data confirm other reports which have shown that Triton X-100 can act as a scintillator in the presence of radioactive emissions.“.“’ To test if this is peculiar to Triton X-100 or if it is a property of other surfactants. a wide variety of classes of commercially available surfactants were tested. Of the 10 tested, 9 yielded count rate increases over water alone, with Triton X-100 yielding the highest efficiency (Table 1). This indicates that many cause
rlt
surfactants offer potential as scintillators. Higher counting efficiency can be obtained by adding wavelength shifters such as sodium salicylate to the Triton X- 100 cocktail.‘“’ Further investigations on the use of Triton X-100 were conducted due to its unique ability to act as both a scintillator and a solvent capable of solubilizing large quantities of aqueous salt solutions. There was little effect of volume on counting efficiency (Fig. 2). with the highest efficiency in the range of 2-10 mL for ZO-mL vials. The “P was more sensitive to volume than ‘6Rb. Adding increasing percentages of water to the Triton X-100 gradually decreased the counting efficiency of both “P and S6Rb (Fig. 3). In loo:,; water (no = 1.33). counting efficiency of “6Rb was SO”.,, which would reflect cerenkov counting, and the counting efficiency of “P was O.ly;;, because “P is just below the (5erenkov threshold in water. Adding increasing percentages of toluene to the Triton X-100 increased the counting efficiency of both “P and ‘“Rb over Triton X-100 alone up to 30:: Triton X-100/700; toluene, after which counting efficiency decreased (Fig. 4). At 100% toluene
II’, II I II 1 (1 ’ ‘I ’ II : ‘I \
// (I
II ‘I ’ I I 1 ’ I : ’ I I ’ I _- I 300
360
100
: \ 1 : ‘, I \ \
I. Fluorescence with maximum
\
\
\
f nm
I 600
500
400
emission excitation
X’P\
‘._
/
t
.-.-.._* \
1
spectrum of Triton X-100 wavelength at 345 nm.
.
%’
l’ 7s
Wavelength
Fig.
klj--*--*--.--.__.____
oi 0
_ *- .- . -.-..__
5
Volume
Triton
X - 100 (mL)
Fig. 2. Effect of volume of Triton X-i&j on percent etficiency (compared to a standard cocktaii) of”P and “Rb in Triton X-100.
Tnton
75
‘\
i_
.X-l00 cocktail
for aqueous solutions
, : j=Rb -.
-.
.
“P -‘-.._ .-.
I
0
-.-.-•.
I
iOcW./O% @3%/20% 60%/40% % Triton
X -1001%
4c%/60%
woter
L__
.i-
20%./80% O%/lOO%
( IO mi
)
volume
Fi_e. 3. Effect of increasing amounts of water on etkiency (compared to a standard cocktail) of ‘:P and ‘“Rb in Triton X-100.
(no = 1.5) “‘Rb was detected at 5700 efficiency by Cerenkov counting, whereas ‘IP was barely detected (1.7”,). The detection of Y6Rb and “P was slightly greater in loo?/, tolucne than water. due to the higher index of refraction of toluene. All combinations of Triton X-100 with both water and toluene were completely micible and one phase. These data indicate that toluene is a more ethcicnt energy transfer agent than Triton X-100. These data also indicate that water can be added to Triton X-100 to produce a cocktail suited for counting solutions of salts and hydrophilic compounds, vvhereas toluene can be added to Triton X-100 to produce a cocktail suited for counting hydrophobic compounds. One of the biggest advantages of a Triton X-100 cocktail is its stability over time with aqueous solution of ionic salts. One milliliter of 1.0 M potassium “P-phosphate was added to Triton X-100 and a standard cocktail and the count rate was followed over time (Fig. 5). After 3 weeks in the standard cocktail the count rate had decreased by 353; more than predicted from radioactive decay. indicating a change in counting geometry caused by salts either absorbing to the vial wall or precipitating. Triton
75
2
=‘ 50’ G G -
z i-f
\
=P\
t r* 5
/
*..
./
_..-•
_
-
\
-..-.. \
\ i
.
\.
\ i ;
1
,
100Y~0%
,
BoY./20%
%Triton
X-
/
60%/4OY../6w.
1001%
toluene
,
izQ%AcT/
‘i.,,, i O%/Icxw.
( IOmLvolume
1
Fig. 3. Effect of increasing amounts of toluene on efficiency (compared to a standard cocktail) of “P and “Rb in Triton x-100.
0.1
L
0
I 4
I
I
I
I
!
B
I2
I6
20
24
Doys
Fig. 5. Stability of count rate over time of potassium :‘P-phcsphate in a standard cocktail compared to Triton X- 100 alone.
X-100 resulted in no count rate decrease over time, other than that due to radioactive decay. indicating complete cocktail stability. The increased viscosity of Triton X-100 also would allow stable counting of suspensions of fine light-weight particulate samples such as plant ash or thin layer chromatographs, especially if the mixture is cooled. Triton X-100 allows sufficient spectral separation of low and high energy B-particle emitters to allow double-label counting (Fig. 6). The photomultiplier tube gain and windows would need to be adjusted on individual counters to allow maximum spectral separation with minimum cross-over. Routinely. samples double-labeled uith J6Rb and “P were counted with an error of 1.8”; or less. which compares favorably to standard cocktails containing PPO and POPOP. One of the disadvantages of Triton X-100 appears to be its greater sensitivity to chemical quenching agents, especially with the lower b-particle energy of “P (Table 2). This problem could be overcome partially by adding toluene to act as a more efficient energy-transfer agent. Triton X-100 also has the dbadvantage of being difficult to handle due to its viscosity, but this can be overcome easily by heating the solutions slightly or using syringe-type pipeting equipment. Also. Triton X-100 is a mixture of compounds produced in large quantities for industrial uses. Considerable variation has been reported between production lots,““.“’ however, no differences were found in these studies from Triton X-100 obtained from various sources (Si_ma Chemical Co.. Rohm and Haas). with or without purification.““.“’ However, caution should be exercised and each lot of Triton X-100 should be tested for stability.
Conclusions
r ,i
I/
/-
\,
/’ /
I,‘%
\
Triton X-100 as a one-component liquid scintillation cocktail \vvas found to be ideally suited for counting aqueous solutions of ionic nutrient salts.
\ /--=R:,
Triton X-100 is economical and safe. its count rate is stable over long periods of time, it does not undergo phase separation with increasing water added, it allows double-label counting. and it can be mixed with water or toluene to form either a hydrophilic or hydrophobic based cocktail. 97-7 .; 0
\ I ..
I
I 400
200
Oiscrimmator
600
\ \ 600
setting
Fig. 6. Spectra of “P and ‘“Rb in Triton X-100 and cross-over with gain (605) and windows (0-ISO, 150-1000) of Beckman LS7500 set for double-label counting.
Table 2. Chemical quenching of Triton X-100 compared to il standard cocktail of 2 :olusne:l Triton X-100 plus PPO and POPOP
~. _ .~~ “P friron .x-100 Srandard cocktail
Efficiency !“,,) -. .~__ Plus 0.1 mL chloroform - .~
Nvrhmg added __~~
Ye quenching
43.2
11.6
66
YO.0
71.8
II
100.0
98. I
‘“Rb Triron S-100 Standard cockiail
2
References I. Benson R. H. Inr. J. dppl. Rndiat. Isot. 27, 667 (1976). 2. Peirson D. R. Can. J. EM. 62, I17 (1974. 3. Parker R. P. and Elrick R. H. In The Current Srurrrsof’ Liquid Scintifiutiorz Counting. (Ed. Bransome E. D.) p. 110 (Grune & Stratton. New York. 1970). 4. Green R. C. In The Cwrent Status ofliquid Scit~tillaiion Counting. (Ed. Bransome E. D.) p. IS9 (Grime & Stratton, New York. 1970). 5. Collins K. E.. Farris M. G.. Estrazulas 0. A. S. and Collins C. G. ht. J. Appl. Radiat. Isot. 28, 732 (1977). 6. Turner J. C. lat. J. Appl. Radial. Isot. 20, 499 (1969). 7. Chow P. N. P. In Liquid Scintillation Counting: Recent Application and Derelopment Vol. I. Practical Aspects. (Eds Peng C.. Horrocks D. L. and Alpen E. L.) p. 387 (.Academic Press, New York. 19SO). 8. Kellogg T. F. ht. J. Appl. Radiat. Isot. 33, I65 f 1982). 9. Sharpe G. E. III and Bransome E. D. Jr In Liquid Scintkation Counting: Recent Derelopmen~s. (Eds Stanleq P. E. and Scoggins B. A.) p. 133. (Academic Press, New York. 1971). IO. Patterson M. S. and Green R. C. Anal. Chetn. 37. 85-l (1965). II. Greene R. C.. Patterson %I. S. and Estes A. H. .dnal. C/WF~.40, 2035 f.1968).
X-100 as a Complete
Liquid
Scintillation Cocktail for Counting Aqueous Solutions and Ionic Nutrient Salts* D.A\-ID Department of Hsrticultumi
WM.
REED
Sciences. Texts A&M Uni\erGty. Colizge Station. TX 77843, U.5.4.
Triton X-100+, used alone. was found to act as a complete liquid scintillation cocktail. Triton X-100 acted as a scintillator and the eiTect was not due to Cerenkov radiation. A variety of other commercially available surfactants also acted as scintillators. but with different levels of efficiency. Triton X-100 water combinations were suitable for counting aqueous solutions of “P and ‘“Rb and the count rate was stable over extended periods of time. Triton X-100 toluene combinations also yielded high counting efftcien&. Triton N-100 was more sensitive to quenching than standard cocktails containing Huors.
Introduction Current liquid scintillation cocktails are organic based. which results in instable mixtures when combined with aqueous solutions of ionic salts. The mixtures may be stabilized by emulsion formation”’ or the use of solid supports or suspensions.“’ For strong p-emitters, aqueous solutions of ionic salts can be counted effectively by Cerenkov radiation in u.ater.‘:“ Of these techniques the use of surfactants, such as Triton X-100, as an emulsifier is the most widely utilized. Triton X-100 was first added to toluene or xylene based cocktails to increase micibility with aqueous solutions.“.‘“’ Others have eliminated toluene and utilized Triton X-100 as the only solvent, demonstrating that it can act as an energy transferring agent in conjunction with PPO and POPOP.‘s,6’ Recent studies have shovvn that Triton X-100 can act as a primary scintillator in a Triton X-100,water system.“.” Other surfactants also have been shown to act as scintillators.‘” These studies report on the use of Triton X-100 alone as a compete liquid scintillation cocktail for counting aqueous solutions of ionic nutrient salts.
Materials
and Methods
A standard cocktail of 7: 1 (v,v) toluene: Triton X- 100 containing 5 g,‘L PPO and 0.1 g L POPOP was used for comparison of efficiencies with al1 other * Texas Agricultural Experiment Station Article So. TA I7?55. + Triton X-100 is a regstered trademark of Rohm and Haas Corporation. 367
cocktails. All samples were counted to at least 0.5”, preset error in 20-mL volume low-background glass vials containing IO-mL cocktail. Samples of Triton S-100 (Sigma Chemical Co.. RPI. Rohm and Haas) were purified according to the procedures of Patterson and Greene”“’ by treatment with activated carbon and silica gel, and by the procedures of Green er c7l.l” by treatment with Dowex-I X-S (hydroxide form). The source of Triton X-100 or purification procedures did not alter the count rate obtained, hence. all studies utilized Triton X-100 as obtained from Sigma Chemical Co. The “P was obtained as HI-“PO, (carrier-free, less than 5:a “P) and S6Rb as “‘RbCl (0.5-10 Ci g) from Neu England Nuclear. Boston. Mass. All samples were counted in a Beckmann LS7500 liquid scintillation counter. The index of refraction of Triton X-100 was obtained with a Bausch and Lomb Model 369KV Refractometer. The fluorescence emission spectrum was obtained with an American Instrument Co. Spectrophotofluorometer.
Results
and Discussion
This study was initiated to identify a cocktail that was both economical and applicable to counting aqueous preparations of ionic mineral nutrients commonly utilized in plant nutrition studies. Cerenkov counting in water has the advantage of both economy and solubility of ionic compounds. but cannot detect efficiently lower energy /? emitting radionuclides, such as “Ca, >?P, ?‘?ia. ‘W, which are often utilized in plant nutrition studies. In the initial studies, T&on X-100 was tested because of its ability to solubilize large quantities of water containing salts. and be-
its higher index of refraction (no = 1.43) might lower the Cerenkov threshold to allow more efficient detection. Both “‘Rb (92’; 1.77 MeV. 9”; 0.68 MeV), which is easily detected in water by Cerenkov radiation, and ‘>P (LOO”, 0.245 MeV), Hhich is just below the Cerenkov threshold of 0.263 MeV in water. were tested. The efficiencies observed were much higher than expected (Table I). The index of refraction of Triton X- 100 was measured to be 1.rlS, which was not high enough to explain the efficiencies obtained by cerenkov radiation alone.“’ Triton X-100 war shown to yield a fluorescence spectrum with a peak at 360nm (Fig. I). These data confirm other reports which have shown that Triton X-100 can act as a scintillator in the presence of radioactive emissions.“.“’ To test if this is peculiar to Triton X-100 or if it is a property of other surfactants. a wide variety of classes of commercially available surfactants were tested. Of the 10 tested, 9 yielded count rate increases over water alone, with Triton X-100 yielding the highest efficiency (Table 1). This indicates that many cause
rlt
surfactants offer potential as scintillators. Higher counting efficiency can be obtained by adding wavelength shifters such as sodium salicylate to the Triton X- 100 cocktail.‘“’ Further investigations on the use of Triton X-100 were conducted due to its unique ability to act as both a scintillator and a solvent capable of solubilizing large quantities of aqueous salt solutions. There was little effect of volume on counting efficiency (Fig. 2). with the highest efficiency in the range of 2-10 mL for ZO-mL vials. The “P was more sensitive to volume than ‘6Rb. Adding increasing percentages of water to the Triton X-100 gradually decreased the counting efficiency of both “P and S6Rb (Fig. 3). In loo:,; water (no = 1.33). counting efficiency of “6Rb was SO”.,, which would reflect cerenkov counting, and the counting efficiency of “P was O.ly;;, because “P is just below the (5erenkov threshold in water. Adding increasing percentages of toluene to the Triton X-100 increased the counting efficiency of both “P and ‘“Rb over Triton X-100 alone up to 30:: Triton X-100/700; toluene, after which counting efficiency decreased (Fig. 4). At 100% toluene
II’, II I II 1 (1 ’ ‘I ’ II : ‘I \
// (I
II ‘I ’ I I 1 ’ I : ’ I I ’ I _- I 300
360
100
: \ 1 : ‘, I \ \
I. Fluorescence with maximum
\
\
\
f nm
I 600
500
400
emission excitation
X’P\
‘._
/
t
.-.-.._* \
1
spectrum of Triton X-100 wavelength at 345 nm.
.
%’
l’ 7s
Wavelength
Fig.
klj--*--*--.--.__.____
oi 0
_ *- .- . -.-..__
5
Volume
Triton
X - 100 (mL)
Fig. 2. Effect of volume of Triton X-i&j on percent etficiency (compared to a standard cocktaii) of”P and “Rb in Triton X-100.
Tnton
75
‘\
i_
.X-l00 cocktail
for aqueous solutions
, : j=Rb -.
-.
.
“P -‘-.._ .-.
I
0
-.-.-•.
I
iOcW./O% @3%/20% 60%/40% % Triton
X -1001%
4c%/60%
woter
L__
.i-
20%./80% O%/lOO%
( IO mi
)
volume
Fi_e. 3. Effect of increasing amounts of water on etkiency (compared to a standard cocktail) of ‘:P and ‘“Rb in Triton X-100.
(no = 1.5) “‘Rb was detected at 5700 efficiency by Cerenkov counting, whereas ‘IP was barely detected (1.7”,). The detection of Y6Rb and “P was slightly greater in loo?/, tolucne than water. due to the higher index of refraction of toluene. All combinations of Triton X-100 with both water and toluene were completely micible and one phase. These data indicate that toluene is a more ethcicnt energy transfer agent than Triton X-100. These data also indicate that water can be added to Triton X-100 to produce a cocktail suited for counting solutions of salts and hydrophilic compounds, vvhereas toluene can be added to Triton X-100 to produce a cocktail suited for counting hydrophobic compounds. One of the biggest advantages of a Triton X-100 cocktail is its stability over time with aqueous solution of ionic salts. One milliliter of 1.0 M potassium “P-phosphate was added to Triton X-100 and a standard cocktail and the count rate was followed over time (Fig. 5). After 3 weeks in the standard cocktail the count rate had decreased by 353; more than predicted from radioactive decay. indicating a change in counting geometry caused by salts either absorbing to the vial wall or precipitating. Triton
75
2
=‘ 50’ G G -
z i-f
\
=P\
t r* 5
/
*..
./
_..-•
_
-
\
-..-.. \
\ i
.
\.
\ i ;
1
,
100Y~0%
,
BoY./20%
%Triton
X-
/
60%/4OY../6w.
1001%
toluene
,
izQ%AcT/
‘i.,,, i O%/Icxw.
( IOmLvolume
1
Fig. 3. Effect of increasing amounts of toluene on efficiency (compared to a standard cocktail) of “P and “Rb in Triton x-100.
0.1
L
0
I 4
I
I
I
I
!
B
I2
I6
20
24
Doys
Fig. 5. Stability of count rate over time of potassium :‘P-phcsphate in a standard cocktail compared to Triton X- 100 alone.
X-100 resulted in no count rate decrease over time, other than that due to radioactive decay. indicating complete cocktail stability. The increased viscosity of Triton X-100 also would allow stable counting of suspensions of fine light-weight particulate samples such as plant ash or thin layer chromatographs, especially if the mixture is cooled. Triton X-100 allows sufficient spectral separation of low and high energy B-particle emitters to allow double-label counting (Fig. 6). The photomultiplier tube gain and windows would need to be adjusted on individual counters to allow maximum spectral separation with minimum cross-over. Routinely. samples double-labeled uith J6Rb and “P were counted with an error of 1.8”; or less. which compares favorably to standard cocktails containing PPO and POPOP. One of the disadvantages of Triton X-100 appears to be its greater sensitivity to chemical quenching agents, especially with the lower b-particle energy of “P (Table 2). This problem could be overcome partially by adding toluene to act as a more efficient energy-transfer agent. Triton X-100 also has the dbadvantage of being difficult to handle due to its viscosity, but this can be overcome easily by heating the solutions slightly or using syringe-type pipeting equipment. Also. Triton X-100 is a mixture of compounds produced in large quantities for industrial uses. Considerable variation has been reported between production lots,““.“’ however, no differences were found in these studies from Triton X-100 obtained from various sources (Si_ma Chemical Co.. Rohm and Haas). with or without purification.““.“’ However, caution should be exercised and each lot of Triton X-100 should be tested for stability.
Conclusions
r ,i
I/
/-
\,
/’ /
I,‘%
\
Triton X-100 as a one-component liquid scintillation cocktail \vvas found to be ideally suited for counting aqueous solutions of ionic nutrient salts.
\ /--=R:,
Triton X-100 is economical and safe. its count rate is stable over long periods of time, it does not undergo phase separation with increasing water added, it allows double-label counting. and it can be mixed with water or toluene to form either a hydrophilic or hydrophobic based cocktail. 97-7 .; 0
\ I ..
I
I 400
200
Oiscrimmator
600
\ \ 600
setting
Fig. 6. Spectra of “P and ‘“Rb in Triton X-100 and cross-over with gain (605) and windows (0-ISO, 150-1000) of Beckman LS7500 set for double-label counting.
Table 2. Chemical quenching of Triton X-100 compared to il standard cocktail of 2 :olusne:l Triton X-100 plus PPO and POPOP
~. _ .~~ “P friron .x-100 Srandard cocktail
Efficiency !“,,) -. .~__ Plus 0.1 mL chloroform - .~
Nvrhmg added __~~
Ye quenching
43.2
11.6
66
YO.0
71.8
II
100.0
98. I
‘“Rb Triron S-100 Standard cockiail
2
References I. Benson R. H. Inr. J. dppl. Rndiat. Isot. 27, 667 (1976). 2. Peirson D. R. Can. J. EM. 62, I17 (1974. 3. Parker R. P. and Elrick R. H. In The Current Srurrrsof’ Liquid Scintifiutiorz Counting. (Ed. Bransome E. D.) p. 110 (Grune & Stratton. New York. 1970). 4. Green R. C. In The Cwrent Status ofliquid Scit~tillaiion Counting. (Ed. Bransome E. D.) p. IS9 (Grime & Stratton, New York. 1970). 5. Collins K. E.. Farris M. G.. Estrazulas 0. A. S. and Collins C. G. ht. J. Appl. Radiat. Isot. 28, 732 (1977). 6. Turner J. C. lat. J. Appl. Radial. Isot. 20, 499 (1969). 7. Chow P. N. P. In Liquid Scintillation Counting: Recent Application and Derelopment Vol. I. Practical Aspects. (Eds Peng C.. Horrocks D. L. and Alpen E. L.) p. 387 (.Academic Press, New York. 19SO). 8. Kellogg T. F. ht. J. Appl. Radiat. Isot. 33, I65 f 1982). 9. Sharpe G. E. III and Bransome E. D. Jr In Liquid Scintkation Counting: Recent Derelopmen~s. (Eds Stanleq P. E. and Scoggins B. A.) p. 133. (Academic Press, New York. 1971). IO. Patterson M. S. and Green R. C. Anal. Chetn. 37. 85-l (1965). II. Greene R. C.. Patterson %I. S. and Estes A. H. .dnal. C/WF~.40, 2035 f.1968).