Adhesive plastic scintillator

An adhesive plastic scintillator which can be attached onto a solid support medium such as a microtiter plate, either by melting it in and/or on a solid support medium because the plastic scintillator is capable of being changed between solid and liquid phases, whichever is desired, by temperature control, or by deposition from a solution of said adhesive plastic scintillator. The plastic scintillator is for analyzing radioactive samples and comprises fluorescent substances and optionally energy transfer compounds for converting radiation energy into light energy. The plastic scintillator remains transparent upon transition from the liquid to the solid state, and upon the same transition adheres to the solid support medium.

FIELD OF THE INVENTION 
The present-invention relates to scintillation counting and more 
particularly to the integration of a plastic scintillator onto a support 
medium, especially a microtiter plate. 
BACKGROUND OF THE INVENTION 
Liquid scintillation counting and automated instruments known as liquid 
scintillation counters are widely utilized to analyze samples containing 
radioactively labelled substances. 
Typically, a sample in solution is mixed with a liquid scintillator, 
commonly referred to as a cocktail, and the light events produced from the 
sample and cocktail mixture are detected according to their energy and 
number of events. The light events occur when the energy of the particles, 
emitted from the radioactive isotope component of the sample in solution, 
is transferred to the molecules of liquid scintillator. This produces a 
light emission of a specific energy range which is characteristic of the 
radioactive isotope. 
Detecting both the energy and number of light events in a particular energy 
range provides the information necessary to construct a spectrum. Using 
this information the radioactive species can be quantitatively analyzed. 
Liquid scintillation counting and automated instruments to perform liquid 
scintillation counting have been widely discussed in a multitude of 
publications and patents. 
Scintillation counting of liquid samples has certain disadvantages 
attributable to the nature of the liquid solution used. One is a 
phenomenon known as quench. Quench commonly refers to an effect in the 
scintillation process of a chemical or optical nature which results in 
loss of light events or reduction in light emission energy. The chemical 
nature of the solution in which the sample and scintillator are mixed and 
the color of the liquid sample solution are the causative agents. The 
result is inefficiency in the ability of the liquid scintillation counter 
to accurately count the particle disintegrations of the isotopes, and 
therefore interference with sample analysis. 
Another disadvantage is that after analysis the liquid produced by mixing 
the radioactive sample with the cocktail must be disposed of. The 
regulations and controls governing the disposal of liquid radioactive 
materials are particularly rigorous. Due to the volume of liquid 
radioactive materials that require disposal of, the costs can be 
considerable. 
In many cases a solid material having a radioactive nature is easier to 
dispose of and incurs less expense. Plastics are often used as such solid 
scintillation materials and previously mentioned in the literature are the 
thermosetting plastics which include polystyrene, polyvinyltoluene, 
various acrylic polymers and copolymers. The following patents further 
illustrate the use of plastics as scintillation materials. 
U.S. Pat. No. 3,010,908 issued Nov. 28, 1961, discloses the use of 
dialkylstyrene polymers as the primary absorber in a solid solution 
scintillation counting composition. 
U.S. Pat. Nos. 2,985,593 and 3,356,616 disclose styrene-derived monomers 
polymerized or copolymerized with vinyl or methacrylate monomers to form 
the solvent for a solid solution scintillation counting composition. 
U.S. Pat. No. 3,457,180 issued July 1969 discloses as the solvent for a 
solid solution scintillator copolymerized paravinyltoluene and 
methylmethacrylate. 
U.S. Pat. No. 3,513,102 discloses a fluorescent coating in which a fluor 
and a copolymer of an acrylate and styrene are dissolved in an organic 
solvent, and the solution is emulsified in an aqueous dispersion of a 
hydrophilic colloid. The copolymer is not derived from a latex, but is a 
solution polymer isolated, redissolved and blended by high-speed milling 
for dispersion in a gel binder. 
U.S. Pat. No. 3,886,082 issued May 27, 1975 discloses an example of one 
such plastic scintillator material. The scintillator employs acrylic 
polymers and copolymers as the host plastic and 
bis(O-methylstyryl)-benzene, perylene, tetraphenyl-butadiene, diphenyl 
anthracene, bis(-phenyloxazolyl benzene) and dimethyl bis(phenyl oxazolyl 
benzene) as the fluorescent additive. 
U.S. Pat. No. 4,180,479 issued Dec. 25, 1979 discloses the use of various 
stilbene derivatives as fluorescent agents in scintillators. 
U.S. Pat. No. 4,495,084 discloses plastic scintillators in which a 
scintillating substance is incorporated into a matrix resin which 
comprises a copolymer of a styrene type compound and various unsaturated 
copolymers including unsaturated esters. 
U.S. Pat. No. 3,068,178 discloses plastic scintillators based on 
polystyrene and polyvinyltoluene. 
More recently there have been further advances in the field of solid 
scintillators. 
U.S. Pat. No. 4,713,198 describes the preparation of a polymethylpentene 
thermoplastic scintillator capable of functioning at high temperatures. 
International Patent application WO 90/16002 describes a detection material 
that is solid at room temperature but optionally meltable to fluid. This 
material is composed of a low molecular weight plastic, a hot melt 
copolymer and a paraffin wax. 
U.S. patent application Ser. No. 07/499,434 refers to a solid-liquid 
reversible scintillator used for solid support sample counting. This 
scintillator is composed of fluors, paraffin and p-xylene and is fluid 
above 40.degree. C. but reverts to a translucent waxy solid upon cooling. 
French Patent No. 1,590,762 describes the use of polyolefin resins and 
solvents to form gels which can be used as scintillation materials. These 
materials are solid-liquid reversible. 
International Patent application WO 89/02088 describes the use of an 
inorganic solid scintillator which is attached to a solid support medium 
by a binder material. 
U.S. Pat. No. 4,692,266 issued Sep. 8, 1987 describes a dry solid 
scintillator counting composition for the detection of radiative 
substances in a liquid. 
U.S. Pat. No. 3,491,235 describes a method for producing fluorescent layers 
by dispersing organic solution of fluorescent compounds in aqueous colloid 
solution, coating and drying. 
Japanese Patent Publication Sho 63-101787 describes multi-layer 
scintillators made by piling up either mixed monomolecular films 
consisting of radiation absorbing compounds and compounds emitting 
ultraviolet, visible or infrared radiation, or monomolecular films 
consisting of radiation absorbing compounds and separate monomolecular 
films consisting of compounds emitting ultraviolet, visible or infrared 
radiation. The layers are deposited from a solution of the compounds in 
chloroform. 
U.S. Pat. No. 4,258,001 describes an element for analysis or transport of 
liquid, which contains a structure comprising a plurality of heat-stable, 
organo-polymeric particles non-swellable in and impermeable to the liquid, 
and an adhesive concentrated at particle surface areas contiguous to 
adjacent particles bonding the particles into a coherent, 
three-dimensional lattice that is non-swellable in the liquid. 
Interconnected void spaces among the particles provide for transport of 
the liquid. 
The prior art scintillators have the disadvantage that it is not possible 
to cohesively bond them onto plastic support media. The plastic support 
media can be polystyrene, polyvinylchloride, polyethylene, polypropylene, 
other polyolefins, acrylonitrile copolymers and combinations of these. The 
plastic support media can also be clear, translucent, white or black or a 
combination of these. The plastic support media can be fabricated into 
microplates, petri dishes, culture flasks, test tubes and stand-alone 
single cups. As well as plastic support media, other media, e.g. glass and 
metals are suitable host support media. Further disadvantages of the prior 
art scintillators are that they do not possess the properties necessary 
for producing a scintillating plastic coating on a solid support medium. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a plastic 
scintillator which can be attached from a liquid state onto a solid 
support medium. 
It is another object of the present invention to provide a plastic 
scintillator already attached to a solid support medium for analyzing a 
wet or dry sample. 
It is a further object of this invention to provide a plastic scintillator 
which can be melted in and/or on (abbreviated herein as melted-in-on) a 
solid support medium. 
It is a further object of this invention to provide a plastic scintillator 
which is capable of becoming fluid at a temperature low enough to prevent 
distortion of the solid support medium. 
It is a still further object of this invention to provide a plastic 
scintillator which acts as a hot melt adhesive. 
A still further object of this invention is to provide a plastic 
scintillator which remains transparent upon transition from liquid to 
solid state. 
A still further object of this invention is to provide a plastic 
scintillator which has sufficient solubility for the fluorescent agents 
and thus demonstrates good scintillation properties. 
This invention relates to the use of alpha-methylstyrene polymer or 
alpha-methylstyrene/vinyltoluene copolymer or a (low molecular weight) 
styrene polymer to provide the scintillator plastic material which will 
melt-in-on a solid support medium. In one embodiment of the present 
invention, the solid support medium is a microplate, and the inside 
surfaces of the sample wells of the microplate are already coated with the 
plastic scintillator before introducing samples to be analyzed in the 
sample wells. 
Primary, and optionally, secondary scintillating agents are added to the 
host plastic such as wavelength shifters and energy transfer compounds. 
This invention also relates to a method for producing the plastic 
scintillator composition, which comprises melting the host plastic, adding 
and mixing the fluorescent agents and any other additives while the 
composition is maintained at a temperature above the melting point, and 
cooling the composition. 
Furthermore, the invention relates to a method for producing the 
melted-in-on plastic scintillator bonded to the solid support medium, and, 
in one embodiment, the solid support medium is a microtiter plate. This is 
effected by judicious selection of application temperature which ensures 
that the solid support medium retains its structural integrity. 
Additionally, the present invention involves a method of analyzing a 
sample using the plastic scintillator bound to a solid support medium. In 
one embodiment, inside surfaces of sample wells in a microtiter plate are 
already coated with the plastic scintillator before introducing a sample 
to be analyzed, and the plastic scintillator coating remains solid at 
least until after the sample is analyzed. 
It is a further object of this invention to provide a plastic scintillator 
which can be attached or bound to a solid support medium from a solution 
in a suitable solvent. 
It is still a further object of the present invention to provide a method 
of analyzing a wet or dry sample using the plastic scintillator already 
attached to a solid support medium. 
A still further object of the present invention is to provide a microplate 
with plastic scintillator attached to the inside surfaces of the sample 
wells for analyzing samples. 
It is a further object of this invention to provide a plastic scintillator 
which is capable of dissolving in suitable solvent media. 
A further object of this invention is to provide a method of 
bonding/attaching the plastic scintillator to a solid support medium by 
contacting the solid support medium with a solution of the plastic 
scintillator in a suitable solvent followed by evaporation of the solvent. 
Furthermore, the solution of the plastic scintillator may also be applied 
by spraying, e.g. using conventional spray-can technology. 
The solid support medium can be made of plastic or any other suitable 
material, such as glass or metal. It is also possible to use 
plastic-coated or metal-coated support medium. In one embodiment, the 
solid support medium is a microtiter plate, and the plastic scintillator 
adheres to the inside surfaces of the sample wells and forms a hard 
surface coating. 
This invention further relates to a method for producing the plastic 
scintillator composition, which comprises dissolving the host plastic, the 
fluorescing agents and any other additives in a suitable solvent medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As indicated above, this invention relates to plastic scintillators and a 
method of producing a melted-in-on coating, a coating from solution or a 
coating from spraying, onto a solid support medium. A further aspect of 
the present invention involves the plastic scintillator and a method of 
using the plastic scintillator where the plastic scintillator forms a 
solid coating attached to the solid support medium before exposing a 
sample to the plastic scintillator. In a preferred embodiment of the 
present invention, the solid support medium is a microtiter plate. 
According to the present invention it has been found that 
alpha-methylstyrene/vinyltoluene copolymer finds particular advantage as a 
plastic scintillator because the material has a desirable melting point 
(75.degree. C.), retains its transparency upon transition from liquid to 
solid state, has good scintillation properties, is a hot melt adhesive and 
is soluble in organic solvents. Alpha-methylstyrene/vinyltoluene copolymer 
is a known thermoplastic and is available commercially. For example 
Piccotex 75 is available from Hercules B. V. in The Netherlands. 
Examples of suitable alpha-methylstyrene polymers are Kristalex F100 
(having a weight-average molecular weight Mw of 1400, a number-average 
molecular weight M.sub.N of 800 and a Z-average molecular weight M.sub.Z 
of 2350) and Kristalex F85 (M.sub.W =1200, M.sub.N = 750 and M.sub.Z 
=1950) from Hercules B. V. in The Netherlands. An example of a suitable 
low molecular weight styrene polymer is Piccolastic A75 from Hercules B. 
V. in The Netherlands. 
For use in the present invention the plastic scintillator is processed to 
provide the scintillation properties described. In the method for forming 
the product of the invention, the plastic of the preferred embodiment is 
either subjected to an elevated temperature to melt it and form a plastic 
melt, or dissolved in a suitable solvent medium. Solvents suitable for 
such a medium include aromatic hydrocarbons, aliphatic hydrocarbons, 
chlorinated hydrocarbons, ethers, esters and nitroparaffins. 
Fluors useful in the successful practice of the present invention include 
any of the organic fluors well-known in the scintillation counting art 
which are compatible with the host plastic. Generally, suitable organic 
fluorescent compounds may be selected, for example, from those described 
as "organic fluors" and "organic scintillators" in Organic Scintillation 
Detection, E. Schram and R. Lombaert, Elsevier Publishing Co., 1963. 
Useful wavelength shifters (i.e. secondary fluors) are also well-known in 
the scintillation counting art. Preferred among these classes of materials 
are 2,5-diphenyloxazole (PPO) as the primary fluor and either 
bi(O-methylstyryl)benzene (bis-MSB) or 9,10-diphenyl-anthracene or 
9,10-dimethylanthracene as the secondary fluor. The primary fluor is 
preferably present in the range 0.01 to 5.0 wt %. The secondary fluor is 
preferably present in the range 0.001 to 0.5 wt %. 
Energy transfer compounds which enhance the scintillation properties are 
optional and include compounds like mono- and di-alkylnaphthalenes, 
naphthalene, anthracene, and durene. They are added in the range 0.01 to 
15 wt %. 
After completion of the addition of fluorescent agents and the optional 
additives the melt is cooled to room temperature. The plastic scintillator 
can now be readied for integration onto the solid support medium in 
various ways. The solid plastic scintillator, which preferably forms a 
solid solution, can be ground to a fine white powder or granulated into 
regular size granules. Alternatively, the plastic scintillator can be hot 
melt extruded and chopped into regular sized pellets. A further 
alternative, for larger sized solid support media, is to re-melt the 
plastic scintillator and pour the plastic melt onto or into the support 
medium. 
For small support media, the powdered or granular plastic scintillator is 
weighed into the desired receptacle and carefully heated to the required 
temperature which will be specific for each solid support medium. The 
plastic scintillator is maintained at this temperature for such a period 
as to ensure that all the plastic scintillator is transformed into a 
homogeneous melt. Upon cooling to room temperature (about 25.degree. C.), 
the melted-in-on plastic scintillator is firmly attached to the solid 
support medium before exposure to the sample to be analyzed. 
A yet further alternative is the integration of the plastic scintillator 
onto the solid support medium using a solution of the plastic scintillator 
(in the range of from 5 to 80 wt %) in a suitable solvent medium. After 
dissolving the host plastic, the fluorescent agent and the optional 
additives in the desired solvent medium, the desired receptacle is filled 
with the plastic scintillator solution and then emptied again. A layer of 
plastic scintillator solution remains on the internal surface and as the 
solvent evaporates, a solid layer of plastic scintillator is attached to 
the internal surface. 
FIG. 1 shows a scintillating solid support medium 10 having a preferably 
homogeneous plastic scintillator 12 attached or bound to a solid support 
medium 14 as described above. In accordance with another aspect of the 
present invention, the plastic scintillator 12 is already attached to the 
solid support medium 14 when the plastic scintillator 12 is exposed to 
sample 16. In the illustrated embodiment, the sample 16 includes 
radiolabeled constituents 18 producing radiation energy which interacts 
with the plastic scintillator 12. The plastic scintillator 12 converts the 
radiation energy 17 into light energy 19. Because the scintillating solid 
support medium 10 can be used with a variety of solid support medium, it 
is not intended for the light energy to be internally reflected within the 
solid support medium 14 to propagate along said solid support medium for 
detection at one end (not shown) of the solid support medium 14. As can be 
seen from FIG. 1, the plastic scintillator 12 forms a distinct layer 
between the solid support medium 14 and the sample 16. The layer of 
plastic scintillator 12 preferably remains solid after application to the 
solid support medium 14. 
In accordance with the principles of the present invention, FIG. 2a shows a 
scintillating solid support medium in the form of a scintillating 
microplate 20 having a plurality of sample wells 22 with a solid coating 
24 attached to at least portions of the inside surfaces 26 of the sample 
wells 22. The solid plastic scintillator coating 24 is attached to the 
microplate 20 before wet or dry samples 28 are introduced into the sample 
wells 22. Generally, samples 28 are introduced into the sample wells 22 
for analysis in a liquid medium. For each sample well 22, the solid 
coating 26 forms a layer of plastic scintillator between the inside 
surface of an individual well 22 and the sample 28 to be analyzed. A 
typical sample 28 comprises radioactive constituents 29 of interest 
labelled with a radioactive substance (denoted as an "*"), and the plastic 
scintillator converts the radiation energy 31 into light energy or 
scintillations 33 when the plastic scintillator is exposed to the sample 
28. As part of the analysis, a scintillation counter 30 detects the 
scintillations from the plastic scintillator 24. 
The microplate 24 can perform scintillation counting on wet or dry samples. 
FIG. 2a shows a wet sample 28 in which the constituents 29 of interest are 
in the liquid medium. In such a sample, it is preferable that a relatively 
higher energy radioactive substance, such as I.sub.125, is used as the 
radioactive label for the constituents 29 of interest so that the 
radiation energy 31 is sure to reach the plastic scintillator 24 through 
the liquid medium. 
Furthermore, the surface 32 of the plastic scintillator is a good site for 
the attachment of receptors or antibodies to perform proximity assays with 
radio-labelled ligands. As shown in FIG. 2b, receptors 34 can be attached 
to the surface 32, and the receptors 34 would bond with the radio-labelled 
ligands 29 in the sample, thereby bringing the radio-labelled ligands 29 
in close proximity to the plastic scintillator 24. Once the radio-labelled 
ligands 29 are bond to the plastic scintillator 24, the liquid medium can 
be removed from the wells 22 as shown in FIG. 2b, thereby enabling dry 
counting of the scintillators from the plastic scintillator 24. 
The scintillations of both FIGS. 2a and 2b are not totally internally 
reflected within the microplate 20 to propagate along the microplate 20 
for detection at one end of the microplate 20. Such a detection scheme for 
the scintillating microplate 20 could cause "cross-talk" between the 
different samples 28 in the sample wells 22. Accordingly, in this 
particular embodiment, the light energy diffuses from the surface of the 
plastic scintillator, and the emitted light should remain within the 
confines of each individual well 22 for optimum performance. 
The microplate 20 of FIGS. 2a and 2b can be made from a variety of 
materials having a variety of optical characteristics, including different 
colors and opaqueness. In the microplate 20, the support surfaces for the 
plastic scintillator 24 are preferably opaque to prevent "cross-talk" 
between the sample wells 22. The plastic scintillator 24 can adhere to 
various surfaces without permeating the generally non-permeable solid 
support medium, thereby forming a distinct layer between the solid support 
medium 20 and the sample 28 to be analyzed. 
The solid coating 24 remains fixed to the microplate surface 26 as a solid, 
thereby enabling the microplate to be re-used. The plastic scintillator 24 
of the present invention forms a relatively distinct, hard surface 32 
which is exposed to the sample 28. The sample 24 contacts the hard surface 
32 without permeating the generally non-permeable surface 32 of the 
plastic scintillator 24. As previously stated, where the plastic 
scintillator is attached to the surface 21, the plastic scintillator 24 
forms a distinct layer between the sample medium and the sample 28. In a 
particular embodiment, the plastic scintillator layer is within the range 
of 16-50 microns on the vertical wall of an individual well and 15-80 
microns on the base of an individual well. 
The plastic scintillator 24 forms a relatively hard, distinct surface 32 
which is distinguishable from the soft, non-distinct surface of 
scintillators that are used to impregnate sorption sheets. These 
scintillators generally form a crystalline structure that would not stick 
to the walls of the microplates. Such scintillators produce a very 
fragile, powdery, flaky end product. These scintillators are used with 
different methods of sample analysis that include melting the plastic 
scintillator to surround the sample with a mobile scintillator. As such, 
the low melting point properties of these scintillators are beneficial, 
and having the surface characteristics and adhesive nature of the plastic 
scintillator of the present invention would be detrimental. For example, 
the adhesive plastic scintillator would not be used to impregnate a 
typical sorption sheet for use as a filter because the adhesive plastic 
scintillator, applied to a sorption sheet, would form a sorption sheet 
with an impermeable layer of plastic scintillator. 
The solid plastic scintillator of the present invention is attached to the 
scintillating solid support before any samples are introduced and remains 
fixed on the scintillating solid support, thereby enabling the potential 
re-use of the microplate. Thus, the surface characteristics and the 
adhesive nature of the solid plastic scintillator coating are essential to 
the present invention in forming a fixed, durable layer of plastic 
scintillator. 
FIGS. 3a and 3b show a microplate 35 having a plurality of sample wells 37 
with a solid plastic scintillator coating 39 attached to at least portions 
of the transparent bottom 41 of the microplate 35. The walls 40 of the 
microplate 35 are opaque to prevent cross-talk between the sample wells 
37. As described for the microplate 20 (FIGS. 2a and 2b), the solid 
scintillator coating 39 is attached to the microplate 20 before the sample 
43 is introduced into the sample well 37. A typical sample 43 comprises 
radioactive constituents 45, and the plastic scintillator 39 converts the 
radiation energy 47 into light energy 49. FIG. 3a shows a wet counting 
arrangement as described in FIG. 2a, and FIG. 3b shows a dry counting 
arrangement as described for FIG. 2b. A scintillation counter 51a or 51b 
can be positioned at the bottom or top of the sample well 37 to detect 
scintillators 49 from the transparent plastic scintillator 39. 
Scintillation counters 51a and 51b can also be positioned at the top of 
the sample well 37 or at both locations for coincidence counting 
arrangements. Again, the scintillations should not be internally reflected 
within the bottom 41 to propagate along the bottom for detection at one 
end. 
The plastic scintillator, the scintillating solid support medium, and the 
methods of producing them are illustrated in the following examples, where 
parts are by weight unless otherwise indicated. However, the invention is 
not to be considered as limited thereto. 
EXAMPLE 1 
The preferred embodiment is made by a straightforward melting, mixing and 
cooling process. To 100 gms of plastic (Piccotex 75) is added 0.9 gms 
2,5-diphenyl oxazole (PPO) and 0.1 g bis(methylstyryl)-benzene (bis-MSB). 
This mixture is heated at 110.degree. C.-120.degree. C. for 1 hour. When 
the plastic melt has formed it is mixed thoroughly to ensure complete 
homogeneity. After mixing the plastic melt is cooled to ambient 
temperature and then broken down to a fine white powder. This procedure 
produces the plastic scintillator. 
EXAMPLE 2 
The plastic scintillator powder produced by the process disclosed in 
Example 1 is weighed into the wells of a polystyrene microplate. The 
polystyrene microplate is of the 96 well configuration and 50 mgm are 
weighed into each well. A Dynatech Microfluor 96 well plate obtained from 
Dynatech (USA) was used. This plate is made of polystyrene and is stable 
up to 85.degree. C. After dispensing the plastic scintillator into each 
well the plate is heated at 80.degree. C. for 1 hour and then cooled to 
ambient temperature. 
EXAMPLE 3 
To 90 gms plastic (Piccotex 75) is added 10 gms 
2,6-di-isopropylnaphthalene, 0.9 gms 2,5-diphenyloxazole and 0.1 gms 
bis-(methylstyryl)benzene (bis-MSB). This mixture is heated at 110.degree. 
C.-120.degree. C. for 1 hour. When the plastic melt has formed it is mixed 
thoroughly to ensure complete homogeneity. After mixing, the plastic melt 
is cooled to ambient temperature and then broken down to a fine white 
powder. This procedure produces the plastic scintillator containing the 
optional energy transfer compound. This plastic scintillator can be 
processed by the process disclosed in Example 2. 
EXAMPLE 4 
The plastic scintillator produced by the process disclosed in Example 1 can 
be heated to a plastic melt and used to replace the liquid or solid 
scintillator used in some large detectors. Upon cooling to ambient 
temperature the large detector is now allowed a greater degree of 
positional orientation than is possible with a liquid scintillator. A 
pourable plastic melt offers advantages over a cast or machined solid 
plastic scintillator. 
EXAMPLE 5 
To 61.4 gms plastic (Piccotex 75) is added 0.31 gms 2,5-diphenyloxazole, 
0.06 gms bis-(methylstyryl)benzene and 38.25 gms n-heptane. This mixture 
is stirred at room temperature until all the components have dissolved. 
This procedure produces a plastic scintillator solution containing 
fluorescent agents. 
EXAMPLE 6 
The plastic scintillator solution produced by the process disclosed in 
Example 5 is dispensed into the wells of a polystyrene microplate which is 
of the 96 well configuration. Each well is filled with approximately 250 
.mu.l of plastic scintillator solution and then emptied again. The 
residual layer which adheres to the inner surface of the wells is allowed 
to stand for 2 hours at ambient temperature (20.degree. C.) during which 
time the solvent evaporates. The final traces of solvent are removed by 
further heating at 40.degree. C. for a further 2-4 hours. 
The invention has been defined in detail with particular reference to 
certain embodiments thereof, but it will be understood that variations and 
modifications can be effected within the spirit and scope of the 
invention.