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

Abstract: A liquid, homogeneous mixture for use in the LSC (Liquid Scintillation Counting) analysis technique, comprising a scintil­ lation liquid, a scintillator and a surfactant, which mixture in­ cludes 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 mate­ rial having a pK a of at least 5, and in which of the organic phos­ phoric acid esters having formula 1, R″ and R‴ may be the same or different.

Description: The invention relates to a liquid, homogeneous 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 β 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. Generally, LSC compositions comprise three types of constituents: (a) a solvent (or scintillation liquid) which usually consists of aromatic hydrocarbon compounds such as xylenes, toluene, alkylated benzenes, etc. (b) a surfactant or a mixture of surfactants, usually ethoxylated alkyl phenols such as ethoxylated nonylphenol having about 9 ethoxy moieties. (c) one or more scintillation solutes (or scintillators) such as 2,5-diphenyl oxazole (PPO) and 1,4-bis(o-methylstyryl)benzene (bis-MSB). Such prior art LSC compositions are well-known, as is confirmed by US patents 3,999,070 and 4,001,139, for example. Said US patents 3,999,070 and 4,001,139 acknowledge certain shortcomings of prior art LSC compositions, and contain proposals to remedy said shortcomings. US patent 3,999,070, for example, suggests to add a substituted ethoxylated carboxylic acid, either in the form of the free acid to reduce chemiluminescence, or in the form of a tertiary amine or quaternary ammonium salt to enhance water miscibility, or both. The other example, US patent 4,001,139, suggests to use certain surfactant combinations [for example a combination of Triton X-100 (ethoxylated isooctylphenol) and Aerosol MA-80 (sodium dihexyl sulfosuccinate)] and a filter dissolving or transparentizing agent, such as tetrahydrofuran. Other documents concerning the state of the art for LSC compositions are British patent 1,222,111, International Journal of Applied Radiation and Isotopes, 21, 1970, pp. 487-490, and Instruments and Experimental Techniques 14 (2), 1971, pp. 439-440. 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. Another disadvantage of the known LSC mixtures, when they are used in the presence of strongly alkaline samples, e.g., samples comprising 2 M 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. D.A. Kalbhen, in "Liquid Scintillation Counting", Vol. 1, 1970, pages 1-14, discusses the problem of chemiluminescence in liquid scintillation counting. It is reported therein that organic peroxides react in an alkaline medium to produce chemiluminescense. An alkaline medium is considered to be essential for the chemiluminescence reaction in standard scintillation solutions and addition of an acid to a neutral pH or lower than pH 7 generally stops luminescence. To avoid chemiluminescence, it is suggested that biological samples, which have been digested in basic solubilising agents, should be neutralised or acidified by the addition of an acid, and that, if the scintillation mixture is alkaline, it should be checked for chemiluminescence and stored until luminescence has decayed to a tolerable level. It was therefore generally thought that to prevent or reduce chemiluminescence in the case of an alkaline sample, said sample had to be neutralised by the addition of acid, previous to its addition to the LSC mixture, or the LSC mixture had to contain free acid. 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 also includes one or more mono- and/or di-esters of phosphoric acid, which phosphoric acid esters have been neutralized with an alkaline material having a pK a of at least 5 to a pH, at which the neutralization product comprises a mono- and/or diphosphate salt, with the proviso that the alkaline material is not ammonia. It is noted that an LSC mixture containing mono- and/or di-esters of phosphoric acid neutralized with ammonia was sold before the priority date. Said LSC mixture was advertised in the 1982 catalogue of Koch-Light Laboratories Ltd under the designation "0003-57 Liquid Scintillator Supersolve". Although the composition of said product was not described in written publications, it actually contained the scintillant compounds PPO and BisMSB, some organic solvents and several surfactants including Gafac PE510, which is a mixture of mono- and di-esters of phosphoric acid. The product also contains ammonia. The neutralization of the phosphoric acid ester requires the use of an alkaline material having a pK a value sufficiently high to enable proton absorption, and which is therefore at least 5 according to the invention. The upper limit of the pK 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 compatability with strongly concentrated salt solutions. It is preferred, however, that-within the framework of the invention-the neutralization of the phosphoric acid ester is effected by using an alkaline material having a pK a value, or in case of polyacidic alkaline materials a maximum pK a value, of 12-5. The incorporation of phosphoric acid ester neutralized with such an alkaline material 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 compatability 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 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 cm³ 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-(O-methylstyryl/benzene); mixture B: 55 cm³ of xylene and 30 g of ethoxylated 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 cm³ 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 cm³ a measurement was carried out for 2 minutes by using a Tricarb 3380 LCS counting device. There was added 1 cm³ of a quaternary ammonium hydroxide solution (approximately 0.60 N) 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 1.25×10⁷ 1982 33 8×10⁴ 211 66 22778 70 132 7700 44 There could also be observed an improved absorption capacity for a 2 M NaCI solution: see the graphic representation in the drawing (Fig. 1). In it, the number of cm³ of the 2 M NaCI solution added to 10 cm³ 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 cm³ 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-(O-methylstyryl)benzene); and mixture B, comprising 50 cm³ 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. Example III: Determination of the reduction of chemiluminescence when a 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 cm³ 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 Hüls) and 1.22 g of Dabco (1,4-diaza-bicyclo(2,2,2)octane), supplemented to 200 cm³ 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 cm³ of the samples listed in the following table to 10 cm³ 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. 3×10³ 86 1 cm³ of hyamine 36 min. 295 45 hydroxide solution 69 min. 152 31 approximately 1 N 3 min. 1.3×10⁴ 65 1 cm³ of quaternary 36 min. 1.2×10³ 26 ammonium hydroxide solution, approximately 0.55 N 3 min. 193.5 155 1 cm³ of 0.5 N sodium 36 min. 144.5 26 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 µl 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 cm³ 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 cm³ 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 Hüls) and 0.3 g of ethylene diamine. The changes that could be observed with respect to the absorption capacity for a 0.15 M NaCI solution and a 0.5 M 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 cm³ (X axis) of the 0.15 M NaCI solution to 10 cm³ of mixture A, B or C, and series II represents the observations in case of addition of 0; 0.5; 1 and 1.5 cm³ of the 0.5 M 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 cm³ of toluene; mixture B, comprising 19.2 g of alkyl phosphoric acid ester of an ethoxylated fatty alcohol (MARLOPHOR ND of Chemische Werke Hüls); 5.2 g of dimethyl cyclohexylamine and 40 cm³ of toluene. Measurements of the background noise in case of addition of 1 cm³ of 0.2 M sodium hydroxide solution to 10 cm³ 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

Claims: A liquid, homogeneous mixture for use in the Liquid Scintillation Counting analysis technique, comprising a scintillation liquid, a scintillator and a surfactant, characterized in that said mixture also includes one or more mono- and/or di-esters of phosphoric acid, which phosphoric acid esters have been neutralized with an alkaline material having a pK a of at least 5 to a pH, at which the neutralization product comprises a mono- and/or diphosphate salt, with the proviso that the alkaline material is not ammonia. A mixture according to claim 1, characterized in that of the alkaline material used for neutralization the pK a value or maximum pK a value is 12-5. A mixture according to claim 2, characterized in that the organic phosphoric acid ester is neutralized with an organic amine. A mixture according to claims 1-3, characterized in that the weight amount of neutralized organic phosphoric acid ester in the mixture is at least from 0.1% to 40% or more. A mixture according to claim 4, characterized in that the weight amount is 1-20%.

Inventors: HEGGE THEODORUS CORNELUS JOZEF, TER WIEL JAN, HEGGE, THEODORUS CORNELUS JOZEF M., TER WIEL, JAN
Applicants: PACKARD INSTR BV, PACKARD INSTRUMENT B.V.
Application year: 1984
Main classification: G01T1
IPC codes: C09K11, C07F9, G01T1