WO1991011735A1

WO1991011735A1 - Scintillation medium and method

Info

Publication number: WO1991011735A1
Application number: PCT/US1991/000008
Authority: WO
Inventors: Susan Eileen Feierberg; Duk Hi Lee
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1990-01-25
Filing date: 1991-01-04
Publication date: 1991-08-08
Also published as: IE910262A1

Abstract

A scintillation medium is described which comprises a fluor and a solvent of formula (I), wherein R is H, sulfonic acid, or a salt of sulfonic acid. A method of using the scintillation medium to detect β-particle emission is also described.

Description

Title. Scintillation Medium and Method Background of the Invention Field of Invention:

This invention relates to new scintillation media and to methods of using them to detect β-particle emissions. State of the Art: The term liquid scintillation counting is used because the radioactive material to be assayed is dissolved in a suitable solvent containing scintillators. This mixture is commonly referred to as a liquid scintillation cocktail (LSC) . Non-volatile radioactive samples may be applied to solid scintillator mixtures and detected with the same instruments as liquid scintillator mixtures. The following sequence of events occurs during liquid scintillation counting. The kinetic energy of a beta particle emitted from a radioactive source is absorbed by the solvent molecules, causing them to become excited. The energy is propagated within the solvent and transferred to the scintillator, causing the scintillator molecules to become excited. When the scintillators return to their ground state they emit photons. These photons can be detected by a photomultiplier tube. The efficiency of the cocktail is determined by adding a calibrated radioactive standard to the scintillators. The efficiency is calculated by dividing the number of counts detected per minute by the known number of disintegrations per minute. When using solid scintillator mixtures, it is sometimes advantageous to detect the emitted photons with photographic film. This method is commonly referred to as fluorography.

^SUB^STITUTE ^SHEET In the preparation of liquid scintillation cocktails, choice of the solvent is most important. The most inefficient step of the process leading to photon emission is the initial transfer of energy from the beta particle to the solvent molecules. The majority of the energy is lost as heat during this first step. Only a small percentage (5-10%) of the total absorbed energy results in photon production. It is therefore critical to choose the most efficient solvent possible. The best solvents have aromatic structures.

Aliphatic solvents (e.g., dioxane, cyclohexane) will have, even at a high concentration of dissolved scintillator, about half the photon production of aromatic solvents. Dioxane has been used because of its complete iscibility with water, which allows the introduction of aqueous samples. It is no longer used because it has been classified as a carcinogen by the Environmental Protection Agency (EPA) . Commonly used solvents, listed in the order of increasing photon production, are: benzene < toluene < ji- xylene=pseudocumene. Benzene has also been classified - as a carcinogen by the EPA. The chronic toxicity of toluene and ji-xylene is not well defined but is believed to be hazardous. Pseudocumene is the most common liquid scintillation solvent today. All of these solvents require ventilation and special safety precautions during use because of their low flashpoints. The disposal of these solvents is also becoming difficult and expensive. It is therefore desirable to create a high efficiency liquid scintillation counting cocktail with a high flashpoint. High flashpoint LSC systems have been formulated using mineral oil. These systems detect β particles with a very low efficiency. High flashpoint alkylbenzenes have also been used for LSC but do not

su_BSTiTuτ E SHEET have the high efficiency of p_-χylene and pseudocumene formulations. It is therefore advantageous to find a solvent that has both a high efficiency and a high flash point. This will provide for safe and convenient handling of the scintillation cocktail without compromising performance. Researchers can perform very sensitive experiments and minimize waste disposal and safety problems. Information Disclosure: In U.S. Patent 4,651,696, issued April 14, 1987, to Thomson, scintillation media are described which use diisopropylnaphthalenes as solvents.

Summary of the Invention

According to the present invention there is provided a scintillation medium comprising: a fluor and a solvent having the formula:

wherein R is H, sulfonic acid, or a salt of sulfonic acid.

There is also provided a method of detecting β-particle emission using the aforesaid scintillation medium. Further provided is a sulfonic acid or a salt of a sulfonic acid derivative of the aforesaid solvent as a novel compound.

Detailed Description of the Invention The solvents used in the new scintillation media have the formula:

SUBSTITUTE SHEET

wherein R is H, sulfonic acid, or a salt of sulfonic acid.

The .preferred solvent is the one where R is H.

The preferred solvent, 1,1-di(o_-xylyl)ethane (DXE), is available commercially.

Synthesis of sulfonated DXE, ammonium salt: Materials: l,l-di(o_-xylyl)ethane 76.1 gms (0.32 mol) chlorosulfonic acid 20.6 ml (0.31 mol) methylene chloride 175 ml

Chlorosulfonic acid (dissolved in 25 ml methylene chloride) was added dropwise with stirring at room temperature to a flask containing a solution of DXE in 150 ml methylene chloride. After the addition was complete, it was refluxed for 30 minutes. The solvent was then stripped off and the product was dissolved in 150 ml toluene. Ammonia gas was bubbled into the solution. As the neutralization point was reached, white precipitate formed in the flask. After the ammonia gas addition was complete, the toluene suspension was heated to boiling and the suspension was vacuum filtered. It is not known which positions on the ring are sulfonated.

Other preferred salts of sulfonic acid are the alkali metals (e.g., Na and K) . Other salts will be apparent to those skilled in the art. Pseudocumene is considered to be the best known solvent for scintillation counting. Fluorescence data

TITUTE SHEET indicates that DXE has a photon yield which is equal to or greater than the photon yield of pseudocumen . This is shown as follows:

Solvent (NEAT) Emission Max (nm) Intensity (cps)

Pseudocumene 314 2.5314 X 10⁴

DXE 362 4.1798 X 10⁴

The scintillation media of the invention are advantageous in that they have low vapor pressure (and therefore high flashpoints) , efficient photon production, do not readily permeate plastics, have relatively low toxicity, and do not react to form colored compounds with basic materials. The solvents meet all of the requirements for a good solvent: 1) high solubility of the scintillators, 2) efficient energy transfer from the radioactive source, 3) high flashpoint for safe handling and storage, 4) ability to dissolve the radioactive sample with or without the aid of solubilizing agents, 5) remain liquid at the working temperature of the- instrument (generally between 4°C and ambient temperature) , 6) transparency to the photons emitted by the scintillator, 7) relatively low toxicity for safe handling and ease of disposal and 8) no photoluminescence.

These solvents can be used alone or in combination with other solvents. The other solvents may be fluorescent, diluents or water miscible solvents to aid sample preparation. The cocktails may have a total solvent content of 30 to 99.9% by weight (preferably 50-

99.9%) by weight for various applications. Examples of other solvents are mineral oil, dioxane, cyclohexane, benzene, toluene, r>-χylene, pseudocumene, diisopropylnaphthalenes, terpenes which do not contain quenchers, white spirits and the like. ^~

SUBSTITUTE SHEET The solvents, whether used alone or in combination with other solvents, should be free of impurities which will act as quenchers.

The scintillation media contain, in addition to a solvent, a fluor or scintillator. Thus, the solvents can be used with one fluor or in combination with various mixtures of primary (1°) or secondary (2^β) fluors. Examples of such fluors are 2,5-diphenyloxazole ("PPO"); p_-bis(c-methyIstyryl)benzene ("bis-MSB"); naphthalene; 2-methylnaphthalene; 1,4-bis-2-(4-methyl-5- phenyloxazolyl)benzene; 1,4-bis-2-(5-phenyloxazolyl)- benzene; 1,4-bis-2-(5-phenyloxazolyl)benzene; 2-(l- naphthyl)-5-phenyloxazole; 2-phenyl-5-(4-biphenyl)- 1,3, -oxadiazole; r>-terphenyl; and many other common scintillators. Examples of these fluors and others can be found in Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press (1971) .

When the scintillation medium is used to analyze aqueous samples such as buffers (either acidic or basic), urea, plasma, urine, sucrose and the like, the medium may also contain one or more surfactants to aid in solubilizing the aqueous samples. Examples of useful surfactants are such as polyethoxylated alkyl phenols, ethoxylated alcohols, dialkyl sulphosuccinates, quaternary ammonium compounds, alkylbenzene sulfonates, and other common surfactants. Many scintillation medium formulations can be found in Bransome "The Current Status of Liquid Scintillation Counting", Grune and Stratton, New York (1970); Kobayashi, et al., "Biological Applications of Liquid Scintillation

Counting", Academic Press, New York (1974) .

The proportions of the components in the scintillation medium can vary over a wide range and are well known to those skilled in the art. In general, the scintillation medium will contain about 0.001 to 2% by

^SUB^ST^ITUTESHEET weight of fluor, preferably about 0.2 to 1% by weight, with the balance being solvents with or without surfactants and other additives.

The invention can be further understood by the following examples in which parts and percentages are by weight unless otherwise indicated.

Example 1 The following formulation was used in a liquid scintillation counter opposite a standard to measure efficiency. This formulation can be used for assaying organic soluble compounds:

(w/w)%

99. 39 Solvent 0. 60 PPO

0 . 01 bis-MSB

Efficiency is defined as:

% Efficiency = (counts per minute) X 100

(disintegrations per minute)

Standard: calibrated SOLVENT tritiated dihydrotestoserone pseudocumene 60.60%

DXE 62.28%

Example 2 The following formulation was used in a liquid scintillation counter using an internal standard to measure efficiency. This formulation can be used for assaying aqueous samples: (w/w)%

60.00 Solvent 39.17 Nonylphenolethoxylate mixture

00.60 PPO

SUBSTITUTESHEET 00 .20 bis-MSB

00.03 hypophosphorous acid (50% solution)

Standard: calibrated ³H₂θ, 50 μl Various amounts of water were added to 10 mis of scintillation cocktail.

Mis of H₂O/IO mis Cocktail pseudocumene DXE

0.0 0.5 1.0 1.5

The solvents were tested for sensitivity to chemical quenching agents. The addition of certain chemicals such as CCI₄ drastically reduces photon productions. CCI₄ was added to the cocktail in 10 μl aliquots. The cocktail formulations are the same as in Example 2. The vials were spiked with 50 μl ³H₂θ and 450 μl H2O.

EFFECTOFCHEMICALQUENCH

DOCUMENE

μl CCI4 / 10 mfs COCKTAIL

SUBSTITUTE SHEET DXE is slightly less sensitive to chemical quenching agents than pseudocumene.

Example 4 When plastic vials are used for LSC, a counting problem known as the Wall Effect can occur. Solvent and fluors are slowly absorbed by the plastic. If vials are allowed to sit long enough the external standard efficiency will change. The Compton spectrum will reflect the combined efficiencies of the cocktail and the "plastic scintillator". In this example, high density polyethylene vials were counted over four days.

Very little change in efficiency occurred as shown in the following:

SOLVENTS Hours pseudocumene DXE

0 58.33% 57.83% 94 58.33% 57.73%

SUBSTITUTE SHEET

Claims

WHAT IS CLAIMED IS:

1. A scintillation medium comprising: a fluor and a solvent having the formula:

wherein R is H, sulfonic acid, or a salt of sulfonic acid.

2. The scintillation medium of Claim 1 wherein R is H.

3. The scintillation medium of Claim 1 wherein the fluor is at least one of 2,5-diphenyloxazole; p_-bis(o_-methylstyryl)benzene; naphthalene; 2-methylnaphthalene; 1,4-bis-2-(4-methyl-5- phenyloxazolyl)benzene; 1,4-bis-2-(5- phenyloxazolyl)benzene; 2-(l-naphthyl)-5-phenyloxazole; 2-phenyl-5-(4-biphenyl)-l,3,4-oxadiazole; or p_-terphenyl.

4. The scintillation medium of Claim 3 wherein the solvent is 1,l-di(o_-xylyl)ethane.

5. A method of detecting beta-particle emissions using a scintillation medium of Claim 1.

6. A method of detecting beta-particle emissions using a scintillation medium of Claim 4.

SUBSTITUTE SHEET

7. A compound having the formula:

wherein R is sulfonic acid or a salt of sulfonic acid.

SUBSTITU ^T*= SHEET