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SOLUTE OPTIMIZATION
SOLUTE OPTIMIZATION by R. L. Litle and M. P. Neary Beckman Instruments, Inc. Fullerton, California ABSTRACT A standardized procedure for studying the effects of concentration of primary and secondary solutes on liquid scintillation counting efficiencies for tritium has been devised. Examples are presented showing the application of this procedure to the study of quenching of PPO and diMePOPOP in toluene and dioxane/naphthalene systems. INTRODUCTION Although numerous reports devoted to the problem of optimal fluor concentrations have appeared, these have, in general, not yielded results directly applicable to practical counting situations. Screening and evaluation studies of potential fluors are usually carried out in argon-purged toluene solutions and thus avoid the main problem, that of chemical quenching. To our knowledge the most comprehensive and systema tic evaluation of fluor concentration effects is that of Bush and Hansen (1). These workers showed that optimal fluor concentrations are strongly dependent on the con centration of quenching agents, and hence, can only be defined with respect to a given type of quenched sample. The situation has been further complicated by a per sistent tendency to regard so-called secondary fluors as "wave length shifters" only, despite both published evi dence to the contrary and the easily demonstrable fact that solutions containing only secondary fluors can give high counting efficiencies.
431
R. L. LITL E AND M. P. NEAR Y
An additional consideration in evaluating fluor com positions for large numbers of samples may be cost. White (2) has evaluated the cost/efficiency ratio for a number of systems and has found, e.g., that the use of expensive scintillation grade chemicals is seldom justi fied. The problem of determining optimal concentrations can be a complicated one. For a single solvent and two fluors the efficiency is a surface in 3-space, and as secondary solvents, solubilizers and quenchers are added the dimen sionality of the problem correspondingly increases. During some recent work on evaluation of experimental fluors and fluor/solvent combinations we adopted a stan dard procedure which has been found convenient for deter mining and displaying the effects of solute composition and additives on counting efficiency. In this paper we describe the procedure and present some illustrative re sults . PROCEDURE In our standard procedure samples are prepared and counted in groups of 100. Such a group constitutes a 10 χ 10 array in which primary fluor concentration in creases column-wise, and secondary concentration increases row-wise. To insure uniform activity in the samples a master batch of solvent, containing activity, is prepared, and this is used for preparing stock fluor solutions and for dilutions. Each sample is made up to the same total volume and thus contains identical activity. Using a multiple dispensing pipette assembly individual "row" and "column" additions can be made with a single setting. A sample group can normally be prepared in two hours. The samples are counted to the desired precision (17o in our case) and the relative efficiencies calculated. These are then entered on a grid at the intersection of lines representing the corresponding primary and secondary fluor concentrations. Appropriate contours of equal effi ciency can then easily be interpolated. A fixed amount of additive is then added to each sample, the counting repeated, etc.
432
O R G A N I C SCINTILLATORS
If appropriate computer facilities are available the contours can easily be computed, and drawn by automatic plotting devices. EXPERIMENTAL Scintillation grade solvents and solutes were used throughout. Samples were counted at ambient temperatures in Beckman LS-250 and LS-230 counters using an Isoset tritium window. Data were processed on the General Electric Mark II Time Sharing Service using data files created from paper tapes produced by the LS counters. RESULTS The first set of results show the effect of the potent quencher, nitromethane, or toluene solutions of PPO and diMePOPOP* containing *H-labelled toluene. Figure 1 shows results for "unquenched" samples (i.e. air-saturated, but without nitromethane). One can see that over a quite broad range of primary and secondary fluor concentrations efficiencies of at least 95% of the maximum available can be achieved, and this range includes compositions containing only primary or only secondary fluor. A smaller range will give efficiencies within 1% of the maximum **. Typically used fluor compositions give ca. 98% of the available efficiency.
PPO
=
diMePOPOP
**
2,5 - diphenyloxazole =
p-bis (2-(4-methyl-5-phenyloxazolyl)) benzene
The counting error limits resolution of contours to 1%, hence the "flat-topped" peak.
433
R. L. LITL E AND M. P. NEAR Y
Figures 2 and 3 show the results of successive addi tions of 0.2% nitromethane to these samples. The composi tions giving maximum efficiencies are displaced toward higher concentrations of both primary and secondary fluor. The highest relative efficiencies observed in these two cases (74 and 59%, resp.) were at 30 g/1 PPO and 3 g/1 diMePOPOP. Since this latter value represents a nearly saturated solution at 25°C, maximum efficiencies would be somewhat reduced for subambient counting due to lower secondary fluor solubility. In any event it is clear that fluor compositions selected on the basis of unquenched samples would be far from optimum for these highly quenched samples. In a secomd series of samples the effect of water on the system: dioxane, naphthalene, PPO, diMePOPOP was studied as an example of a commonly used cocktail for aqueous samples. A fixed concentration of 80 g/1 naphtha lene was used. Twelve ml. of the dioxane mixture and one ml of ^Η-labelled water were used initially for each sam ple. Samples were recounted after each of two successive additions of one ml water. The highest concentration of diMePOPOP used was one g/1, this being the solubility limit in the presence of the highest water content used. The effect of water is quite different from that of nitromethane on the toluene system as can be seen in fig ures 4 through 6. The shape of the efficiency surface is hardly altered by addition of water; rather, the effi ciency is decreased uniformly at each fluor composition. It should be noted that, following common practice, the values given for fluor concentrations are those of the dioxane solution before addition of water. Thus, some of the decrease in efficiency is caused by dilution. The data presented represent the effect of water per se on the system. Of course, the total activity will rise proportionally with the amount of aqueous sample used, hence the figure of merit (efficiency χ water content) is actually rising at each fluor composition with the addi tion of water.
434
ORGANIC SCINTILLATORS The effect of water alone is such that a composition selected for maximum efficiency at the lowest water con centration will remain near optimal at the highest. In practice, however, aqueous samples generally contain quenching agents whose concentrations will vary with the amount of sample used. In this case optimal compositions will likely require higher fluor concentrations. Such a trend can be seen in Figure 7 which shows results obtained by adding 10 μ 1 of nitromethane to each of the samples con taining 3 ml of water. The optimal composition is now displaced toward higher primary concentrations in accord with the results obtained in the toluene system. SUMMARY It is worth repeating here that the determination of efficiencies as a function of scintillator composition is an individual problem for each type of sample. We hope that the methods presented here will serve as a guide for workers evaluating scintillator mixtures for their parti cular samples. REFERENCES 1.
Bush, Ε. T. and Hansen, D. L., "Improvement of Liquid Scintillation Counting Efficiencies by Optimization of Scintillator Composition. Amersham/Searle Corp., Des Plaines, Illinois. (Reprints).
2.
White, D. R., Int. J. Appl. Rad. Isot., 19, 49 (1968).
435
R. L. LITLE A N D M. P. N E A R Y
ˇ, Fig. 1
’
Contours of equal relative counting efficiency for the system: Toluene, PPO, DiMePOPOP. Airsaturated samples containing ^H-labelled toluene. 3
Fig. 2
Quenching effect of nitromethane. Samples as in figure 1 with the addition of 0.2% nitromethane.
436
O R G A N I C SCINTILLATORS
PPO Fig. 3
Quenching effect of nitromethane. Samples as in figure 1 with the addition of 0.4% nitromethane.
/
Vjp°
/
ˇ
ιy
95
Cl.
0·5 Cl. CD
•Xi
90 0
\ /
90
/ 12
24
36
ˇ, ˆ’ Fig. 4
Relative efficiencies for the system: dioxane, naphthalene (80g/1) PPO, diMePOPOP. Each sample contains 12 ml dioxane scintillator plus 1 ml ^H-labelled water.
437
R. L. LITLE AND M. P. NEARY
Fig. 5
Quenching effect of water. 4 plus 1 ml water.
Samples as in figure
Fig. 6
Quenching effect of water. 4 plus 2 ml water.
Samples as in figure
438
O R G A N I C SCINTILLATORS
Fig. 7
Quenching effect of nitromethane. figure 6 plus 10 JLÇL nitromethane.
439
Samples as in
431
R. L. LITL E AND M. P. NEAR Y
An additional consideration in evaluating fluor com positions for large numbers of samples may be cost. White (2) has evaluated the cost/efficiency ratio for a number of systems and has found, e.g., that the use of expensive scintillation grade chemicals is seldom justi fied. The problem of determining optimal concentrations can be a complicated one. For a single solvent and two fluors the efficiency is a surface in 3-space, and as secondary solvents, solubilizers and quenchers are added the dimen sionality of the problem correspondingly increases. During some recent work on evaluation of experimental fluors and fluor/solvent combinations we adopted a stan dard procedure which has been found convenient for deter mining and displaying the effects of solute composition and additives on counting efficiency. In this paper we describe the procedure and present some illustrative re sults . PROCEDURE In our standard procedure samples are prepared and counted in groups of 100. Such a group constitutes a 10 χ 10 array in which primary fluor concentration in creases column-wise, and secondary concentration increases row-wise. To insure uniform activity in the samples a master batch of solvent, containing activity, is prepared, and this is used for preparing stock fluor solutions and for dilutions. Each sample is made up to the same total volume and thus contains identical activity. Using a multiple dispensing pipette assembly individual "row" and "column" additions can be made with a single setting. A sample group can normally be prepared in two hours. The samples are counted to the desired precision (17o in our case) and the relative efficiencies calculated. These are then entered on a grid at the intersection of lines representing the corresponding primary and secondary fluor concentrations. Appropriate contours of equal effi ciency can then easily be interpolated. A fixed amount of additive is then added to each sample, the counting repeated, etc.
432
O R G A N I C SCINTILLATORS
If appropriate computer facilities are available the contours can easily be computed, and drawn by automatic plotting devices. EXPERIMENTAL Scintillation grade solvents and solutes were used throughout. Samples were counted at ambient temperatures in Beckman LS-250 and LS-230 counters using an Isoset tritium window. Data were processed on the General Electric Mark II Time Sharing Service using data files created from paper tapes produced by the LS counters. RESULTS The first set of results show the effect of the potent quencher, nitromethane, or toluene solutions of PPO and diMePOPOP* containing *H-labelled toluene. Figure 1 shows results for "unquenched" samples (i.e. air-saturated, but without nitromethane). One can see that over a quite broad range of primary and secondary fluor concentrations efficiencies of at least 95% of the maximum available can be achieved, and this range includes compositions containing only primary or only secondary fluor. A smaller range will give efficiencies within 1% of the maximum **. Typically used fluor compositions give ca. 98% of the available efficiency.
PPO
=
diMePOPOP
**
2,5 - diphenyloxazole =
p-bis (2-(4-methyl-5-phenyloxazolyl)) benzene
The counting error limits resolution of contours to 1%, hence the "flat-topped" peak.
433
R. L. LITL E AND M. P. NEAR Y
Figures 2 and 3 show the results of successive addi tions of 0.2% nitromethane to these samples. The composi tions giving maximum efficiencies are displaced toward higher concentrations of both primary and secondary fluor. The highest relative efficiencies observed in these two cases (74 and 59%, resp.) were at 30 g/1 PPO and 3 g/1 diMePOPOP. Since this latter value represents a nearly saturated solution at 25°C, maximum efficiencies would be somewhat reduced for subambient counting due to lower secondary fluor solubility. In any event it is clear that fluor compositions selected on the basis of unquenched samples would be far from optimum for these highly quenched samples. In a secomd series of samples the effect of water on the system: dioxane, naphthalene, PPO, diMePOPOP was studied as an example of a commonly used cocktail for aqueous samples. A fixed concentration of 80 g/1 naphtha lene was used. Twelve ml. of the dioxane mixture and one ml of ^Η-labelled water were used initially for each sam ple. Samples were recounted after each of two successive additions of one ml water. The highest concentration of diMePOPOP used was one g/1, this being the solubility limit in the presence of the highest water content used. The effect of water is quite different from that of nitromethane on the toluene system as can be seen in fig ures 4 through 6. The shape of the efficiency surface is hardly altered by addition of water; rather, the effi ciency is decreased uniformly at each fluor composition. It should be noted that, following common practice, the values given for fluor concentrations are those of the dioxane solution before addition of water. Thus, some of the decrease in efficiency is caused by dilution. The data presented represent the effect of water per se on the system. Of course, the total activity will rise proportionally with the amount of aqueous sample used, hence the figure of merit (efficiency χ water content) is actually rising at each fluor composition with the addi tion of water.
434
ORGANIC SCINTILLATORS The effect of water alone is such that a composition selected for maximum efficiency at the lowest water con centration will remain near optimal at the highest. In practice, however, aqueous samples generally contain quenching agents whose concentrations will vary with the amount of sample used. In this case optimal compositions will likely require higher fluor concentrations. Such a trend can be seen in Figure 7 which shows results obtained by adding 10 μ 1 of nitromethane to each of the samples con taining 3 ml of water. The optimal composition is now displaced toward higher primary concentrations in accord with the results obtained in the toluene system. SUMMARY It is worth repeating here that the determination of efficiencies as a function of scintillator composition is an individual problem for each type of sample. We hope that the methods presented here will serve as a guide for workers evaluating scintillator mixtures for their parti cular samples. REFERENCES 1.
Bush, Ε. T. and Hansen, D. L., "Improvement of Liquid Scintillation Counting Efficiencies by Optimization of Scintillator Composition. Amersham/Searle Corp., Des Plaines, Illinois. (Reprints).
2.
White, D. R., Int. J. Appl. Rad. Isot., 19, 49 (1968).
435
R. L. LITLE A N D M. P. N E A R Y
ˇ, Fig. 1
’
Contours of equal relative counting efficiency for the system: Toluene, PPO, DiMePOPOP. Airsaturated samples containing ^H-labelled toluene. 3
Fig. 2
Quenching effect of nitromethane. Samples as in figure 1 with the addition of 0.2% nitromethane.
436
O R G A N I C SCINTILLATORS
PPO Fig. 3
Quenching effect of nitromethane. Samples as in figure 1 with the addition of 0.4% nitromethane.
/
Vjp°
/
ˇ
ιy
95
Cl.
0·5 Cl. CD
•Xi
90 0
\ /
90
/ 12
24
36
ˇ, ˆ’ Fig. 4
Relative efficiencies for the system: dioxane, naphthalene (80g/1) PPO, diMePOPOP. Each sample contains 12 ml dioxane scintillator plus 1 ml ^H-labelled water.
437
R. L. LITLE AND M. P. NEARY
Fig. 5
Quenching effect of water. 4 plus 1 ml water.
Samples as in figure
Fig. 6
Quenching effect of water. 4 plus 2 ml water.
Samples as in figure
438
O R G A N I C SCINTILLATORS
Fig. 7
Quenching effect of nitromethane. figure 6 plus 10 JLÇL nitromethane.
439
Samples as in