Mixture for use in the LSC (liquid scintillation counting) analysis technique

A liquid, homogeneous mixture for use in the LSC (Liquid Scintillation Counting) analysis technique, comprising a scintillation liquid, a scintillator and a surfactant, wherein the surfactant includes one or more phosphoric acid esters having formula 1 and/or formula 2 of the sheet of formulae, in which R', R" and R'" represent an organic group, which phosphor acid esters are neutralized to a pH, at which the neutralization product comprises a mono- and/or diphosphate with an alkaline material having a pK.sub.a of at least 5, and in which of the organic phosphoric acid esters having formula 1, R" and R'" may be the same or different.

The invention relates to a mixture for use in the LSC (Liquid Scintillation 
Counting) analysis technique comprising a scintillation liquid, a 
scintillator and a surfactant. 
The LSC analysis technique is generally known and one of the most applied 
techniques for the quantitative determination of usually low-energetic 
radioactivity in inorganic, organic and biological materials. 
The possibility of effectively and reproducibly determining in particular 
soft .beta. radiation within the framework of the LSC analysis technique 
without many preparatory measures is the result of a development which 
started some decades ago. 
In the LSC analysis technique the radioisotope to be determined is to be 
brought into close contact with the scintillator molecules. Consequently, 
many researches have been conducted into mixtures suitable for this 
purpose, in which the determination can be carried out, both at 
homogeneous and heterogeneous counting samples. 
The majority of the known LSC mixtures is satisfactory when used with water 
or with materials containing diluted salts. 
It is a drawback of the known materials, however, that their general 
usability is limited. For instance, there are limits to the possibility of 
involving concentrated aqueous salt solutions in the determination, unless 
there are developed special products therefor or there is diluted with 
water, which has the drawback that many preparatory measures must be 
taken. 
It is another disadvantage of the known LSC mixtures, when they are used in 
the presence of strongly alkaline samples, e.g., samples comprising 2M 
sodium hydroxide or quaternary ammonium hydroxide solutions is formed by 
the occurrence of prolonged background noise owing to chemiluminescence. 
In practice, the solution of this problem has been sought in the 
composition of LSC mixtures, the action of which is based on the principle 
of acidifying. This, however, has the important disadvantage of a strongly 
reduced counting efficiency. 
The object of the invention is therefore to provide an improved mixture for 
carrying out the LSC analysis technique in it, which does not show the 
disadvantages of the known mixtures and can be used directly and 
universally without or with taking only a few preparatory measures.

According to the invention there is provided for this purpose a mixture of 
the type set forth in the first paragraph, which is characterized in that 
said mixture includes one or more inorganic phosphoric acid esters having 
formula 1 and/or 2 of the sheet of formulae, in which R', R" and R'" 
represent an organic group, which phosphoric acid esters are neutralized 
to a pH, at which the neutralization product comprises a mono- and/or 
diphosphate with alkaline material having a pK.sub.a of at least 5, and in 
which of the organic phosphoric acid esters having formula 1, R" and R'" 
may be the same or different. 
The neutralization of the phosphoric acid ester requires the use of an 
alkaline material having a pK.sub.a value sufficiently great to enable 
proton absorption, and which is therefore at least 5 according to the 
invention. The upper limit of the pK.sub.a value has proved to be less 
critical. Also when a strong base is used, e.g. potassium hydroxide, as a 
neutralizing agent, it has surprisingly been found that an improved 
usability of the LSC mixture is obtained. This means a reduced 
chemiluminescence when LSC determination is carried out with the 
occurrence of conditions favouring chemiluminescence, as well as an 
improved compatibility with strongly concentrated salt solutions. 
It is preferred, however, when, within the framework of the invention, the 
neutralization of the phosphoric acid ester is effected by using an 
alkaline material having a pK.sub.a value, or in case of polyacidic 
alkaline materials the maximum pK.sub.a value, of 12 to 5. The 
incorporation of phosphoric acid ester neutralized with such an alkaline 
material, e.g., ammonia, into an LSC mixture according to the invention 
will result in a reduction of the chemiluminescence, within the framework 
of an LSC determination, in case of addition of a strongly alkaline 
solution or of a quaternary hydroxide solution to the LSC mixture, while 
an improvement in the compatibility with strongly concentrated salt 
solutions can also be observed. In combination, these advantageous 
properties occur to a higher degree if the phosphoric acid ester is 
neutralized by an organic amine as the alkaline material, so that the use 
of such a compound is especially preferred. 
In formula 1 and formula 2 of the sheet of formulae R', R" and R'" may 
represent an alkyl, aryl, alkoxylated alkyl or alkoxylated alkyl phenyl 
group, in which of the organic phosphoric acid ester having formula 1 of 
the sheet of formulae R" and R'" may be the same or different. The alkyl 
group may be, e.g., methyl, butyl, octyl or cycloalkyl, the aryl group, 
e.g., phenyl; the alkoxylated alkyl group may be a group having the 
formula C.sub.n H.sub.2n+1 O--(CH.sub.2 --CH.sub.2 --O).sub.m --H, in 
which n and m are integers and in which an appropriate group of this type 
is, e.g., the one in which n=12 and m=6. For instance, the alkoxylated 
alkyl phenyl group may have the formula C.sub.n H.sub.2n+1 --C.sub.6 
H.sub.4 --O--(CH.sub.2 --CH.sub.2 --O).sub.m --H, in which n and m 
likewise represent integers. 
When an amine is used for neutralization of the organic phosphoric acid 
esters, compounds are suitable having the formula R.sub.1 --NH.sub.2, in 
which R.sub.1 =alkyl, e.g., methyl, butyl, benzyl; compounds having the 
formula R.sub.1 R.sub.2 --NH, in which R.sub.1 and R.sub.2 may be the same 
or different and as defined above, e.g., (CH.sub.3).sub.2 N--C.sub.12 
H.sub.25 or N(C.sub.4 H.sub.9).sub.3 ; a diamine having the formula 
R.sub.1 R.sub.2 N--(CH.sub.2).sub.n --NR.sub.3 R.sub.4 in which R.sub.1, 
R.sub.2, R.sub.3 and R.sub.4 may be the same or different and as defined 
above, and n represents an integer, e.g., aminooctane NH.sub.2 
--(CH.sub.2).sub.8 --NH.sub.2 ; a cyclic amine, e.g., Dabco, morpholine or 
substituted morpholine, and an ethoxylated amine, e.g., the compound 
having formule 3 of the sheet of formulae. 
For the neutralization of the organic phosphoric acid ester there may also 
be used an aromatic amine in which the amino group is substituted direct 
to the aromatic ring, of course with the proviso that the pK.sub.a value 
satisfies the pertinent requirement of the invention. For instance, 
o-toluidine having pK.sub.a =4.4 is not suitable for the contemplated 
object, but 2-aminopyridine having pK.sub.a =6.81 is suitable indeed. 
In the neutralization of a phosphoric acid ester with an alkaline material, 
e.g., an organic amine, to obtain a phosphoric acid ester to be used 
according to the invention, the neutralization curve of phosphoric acid 
will in general be followed. The neutralization is continued until a pH 
value, at which the neutralization product comprises a mono- and/or 
diphosphate. Measured as a 1% solution or dispersion in water the pH is 
then approximately 4-8. 
According to the invention the resulting products may be incorporated into 
the known LSC mixtures. 
The invention is illustrated by the following examples. 
EXAMPLE I 
Determination of the reduction of chemiluminescence when a neutralized 
phosphoric acid ester is incorporated into a mixture comprising an 
alkylphenol ethoxylate, to which an alkaline solution was added. 
Two mixtures were compared: mixture A: 55 cm.sup.3 of xylene and 40 g of 
ethoxylated nonylphenol--average 9 moles of ethylene oxide--; 0.64 g of 
PPO (diphenyloxazole) and 0.06 g of bis--MSB 
(1.4-bis-(0-methylstyryl/benzene); mixture B: 55 cm.sup.3 of xylene and 30 
g of ethoxylated and nonylphenol--average 9 moles of ethylene oxide--; 10 
g of mono-diphosphoric acid ester of an ethoxylated nonylphenol--average 7 
moles of ethylene oxide--, neutralized with 4.2 cm.sup.3 of KOH solution 
(approximately 4M) up to pH 7.0 and 0.64 g of PPO and 0.06 g of bis--MSB. 
At each of the samples of 10 cm.sup.3 a measurement was carried out for 2 
minutes by using a Tricarb 3380 LCS counting device. There was added 1 
cm.sup.3 of a quaternary ammonium hydroxide solution (approximately 0.60N) 
to each of the samples. The standard background noise of the counting 
device was 25-35 counts/min. 
______________________________________ 
Background after min. 
mixture A mixture B 
______________________________________ 
3 1.25 .times. 10.sup.7 
1982 
33 8 .times. 10.sup.4 
211 
66 22778 70 
132 7700 44 
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There could also be observed an improved absorption capacity for a 2M NaCl 
solution; see the graphic representation in the drawing (FIG. 1). In it, 
the number of cm.sup.3 of the 2M NaCl solution added to 10 cm.sup.3 of 
mixture is plotted on the X axis. The hatched area indicates an 
unserviceable area in which a precipitate is formed. In FIG. 1, 
designations A and B relate to mixture A and mixture B, respectively. 
EXAMPLE II 
Determination of the reduction of chemiluminescence in an LSC mixture based 
on toluene. 
The background noise was determined by means of a TriCarb 3380. 
Two mixtures were prepared, namely, mixture A from 50 cm.sup.3 of 
toluene+scintillator Permablend III in an amount of 5 g/l of mixture, 
which means, per liter of mixture, 4.55 g of PPO (diphenyl oxazole) and 
0.45 g bis--MSB (1,4-bis-(0-methylstyryl)benzene); and mixture B, 
comprising 50 cm.sup.3 of toluene, 5.32 g of 
mono-di-2-ethyl-hexyl-phosphoric acid; 3.7 g of tributylamine+scintillator 
Permablend III in an amount of 5 g/l of mixture, which means, per liter of 
mixture, 4.55 g of PPO and 0.45 g of bis--MSB. 
Measurement gave the following results. 
______________________________________ 
Background 
noise after: 
mixture A, counts/min. 
mixture B, counts/min. 
______________________________________ 
3 min. 6183 112 
6 min. 1210 70 
9 min. 559 40 
12 min. 337 .ltoreq.ST 
27 min. 125 .ltoreq.ST 
30 min. 115 .ltoreq.ST 
______________________________________ 
ST=standard background=30-40 counts/min. 
EXAMPLE III 
Determination of the reduction of chemiluminescence when an neutralized 
phosphoric acid ester is incorporated into a mixture containing 
alkylphenol ethoxylate, to which an alkaline solution was added. 
Two mixtures were compared: 
mixture A, comprising 55 g of ethoxylated nonylphenol--average 9 moles of 
ethylene oxide--, supplemented to 200 cm.sup.3 with xylene+1 g of 
Permablend III: (0.91 g of PPO+0.09 g of bis--MSB); 
mixture B, comprising 50 g of ethoxylated nonylphenol--average 9 moles of 
ethylene oxide--, 9.02 g of ethoxylated fatty alcohol phosphate (MARLOPHOR 
FC of Chemische Werke Huls) and 1.22 g of Dabco 
(1,4-diaza-bicyclo(2,2,2)octane), supplemented to 200 cm.sup.3 of xylene+1 
g Permablend III: 0.91 g of PPO+0.09 of bis--MSB). 
At each of the samples a measurement was carried out for 2 minutes by using 
a Tricarb 3380 LSC counting device. There was added 1 cm.sup.3 of the 
samples listed in the following table to 10 cm.sup.3 of the LSC mixture. 
The background noise of the counting device was 25-35 counts/min. 
______________________________________ 
mixture A mixture B 
Background after 
counts/min. 
counts/min. 
sample 
______________________________________ 
3 min. 3 .times. 10.sup.3 
86 1 cm.sup.3 of hyamine 
36 min. 295 45 hydroxide solution 
69 min. 152 31 approximately 1 N 
3 min. 1.3 .times. 10.sup.4 
65 1 cm.sup.3 of quaternary 
36 min. 1.2 .times. 10.sup.3 
26 ammonium 
hydroxide solution, 
approximately 
0.55 N 
3 min. 193.5 155 1 cm.sup.3 of 0.5 N 
36 min. 144.5 26 sodium hydroxide 
solution 
______________________________________ 
Counting measurements made by using a TriCarb 3380 Liquid Scintillation 
Spectrometer, which was adjusted to conditions for optimum tritium 
measurement, gave the following results (the counting device with which 
the measurements were carried out was capable of counting a sealed 
unquenched standard for tritium at about 50.6% efficiency. There was 
injected 8 .mu.l of tritiated toluene corresponding to a value of 14,518 
ppm in the samples. The determination was carried out in triplicate): 
There was found for 
mixture A: 5871 counts/min., corresponding to an absolute tritium counting 
efficiency of 40.4%; 
mixture B: 5450 counts/min., corresponding to an absolute tritium counting 
efficiency of 37.5%. 
EXAMPLE IV: 
Determination of the improvement in the absorption capacity of highly 
concentrated salt solutions. 
Measurements were carried out at three mixtures: 
mixture A: 55 g of nonylphenol ethoxylate--average 9 moles of ethylene 
oxide--, supplemented to 200 cm.sup.3 with xylene; 
mixture B: 2.66 g of mono-di-2-ethylhexyl phosphoric acid ester, 0.3 g of 
ethylene diamine, 50 g of nonylphenol ethoxylate--average 9 moles of 
ethylene oxide--supplemented to 200 cm.sup.3 with xylene, 
mixture C: as mixture B, but with 8.8 g of ethoxylated fatty alcohol ester 
of phosphoric acid (MARLOPHOR FC of Chemische Werke Huls) and 0.3 g of 
ethylene diamine. 
The changes that could be observed with respect to the absorption capacity 
for a 0.15M NaCl-- solution and a 0.5M NaCl solution have been graphically 
represented in the drawing (FIG. 2). In it, series I represents the 
observations in case of addition of 0; 0.5; 1 and 1.5 cm.sup.3 (X axis) of 
the 0.15M NaCl solution to 10 cm.sup.3 of mixture A, B or C, and series II 
represents the observations in case of addition of 0; 0.5; 1 and 1.5 
cm.sup.3 of the 0.5M NaCl solution. Especially series II shows a 
substantial improvement in the absorption capacity for salt solutions. 
EXAMPLE V 
Improvement in the action of an LSC mixture according to O'Connor and 
Bransome (L.S.C., Recent Application and Developments, ed. C. T. Peng et 
al, 2, 245-257, 1980). 
Measurements were carried out at the following mixtures: mixture A, 
comprising 90 g of Triton X-100 (Rohm & Haas); 30 g Aerosol MA 80 
(Cyanamid) and 280 cm.sup.3 of toluene; mixture B, comprising 19.2 g of 
alkyl phosphoric acid ester of an ethoxylated fatty alcohol (MARLOPHOR ND 
of Chemische Werke Huls); 5.2 g of dimethyl cyclohexylamine and 40 
cm.sup.3 of toluene. Measurements of the background noise in case of 
addition of 1 cm.sup.3 of 0.2M sodium hydroxide solution to 10 cm.sup.3 of 
scintillator mixture are given in the following table. 
______________________________________ 
8 parts of mixture A 
mixture A 2 parts of mixture B 
Background after 
counts/min. 
counts/min. 
______________________________________ 
3 min. 196.5 43.5 
6 min. 73 28 
9 min. 74 24 
30 min. 39 24 
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