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Thin-layer chromatography : reagents and detection methods in SearchWorks catalog
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What the riverscape has has together the freedom of the culture, but the gradient of vaudeville. Liquid scintillation counting is widely utilised to analyze samples containing radioactively labelled substances. Typically a sample in solution is mixed with a liquid scintillation cocktail and the light events produced from the sample and cocktail mixture are detected according to their energy and number of events.
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The light events occur when the energy of the particles, emitted form 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 instruments to perform liquid scintillation counting have been widely discussed in a multitude of publications and patents. The separation and determination of radioactive species which are present in a multi-component mixture can therefore be accomplished using a separation scheme such as HPLC followed by a radio-assay. Collection of column effluent fractions followed by liquid scintillation counting has been used extensively, but has the disadvantage of being time consuming, requires manual manipulation of samples and compromises resolution.
A continuous flow-through radioactivity detector minimises these disadvantages and several such continuous-flow radioactivity monitors have been described in the literature; see, e. Reeve and A. Crozier J. Bransome, Jr.
Grune and Stratton, New York, pp ; and L. Snyder and J. Parvez et al. Roberts Elsevier. Other flow systems include amino acid analyzers where the amino acids are separated by e. Prior liquid scintillation cocktails known to applicants for use in flow-through cells for measuring radioactivity in fractions from HPLC procedures have certain drawbacks. These scintillation flow cocktails are based on conventionally used solvents which include toluene, xylenes, cumenes, ethylbenzene and pseudocumene.
Other solvents which have been used are mono-, di- and tri-alkyl benzenes. These solvents suffer from the disadvantage that they have relatively high vapour pressures, relatively low flash points and relatively high toxicity. A further disadvantage is that they are not biodegradable. Although these solvents suffer from the above mentioned disadvantages their relatively low viscosity makes them suitable as media either alone or in conjunction with alcohol-type diluents for scintillation cocktails usable in flow systems. The advent of safer solvents suggested that all the disadvantages associated with the conventionally used solvents would be overcome.
The safer solvents now in use in liquid scintillation counting include di-isopropylnaphthalene, linear alkylbenzenes and phenylxylylethane and these solvents are characterised by their low toxicity, low flammability, high flash point, low vapour pressure and biodegradability. These safer solvents have been tried as bases for scintillation cocktails usable in flow systems but they all suffer from having too high a viscosity.
The high viscosity causes high back-pressure in the flow system, which can exceed the maximum design pressure of the pump, fittings and even the flow cell. Another disadvantage of a high viscosity is the inhibition of fast mixing of the scintillation flow cocktail with the HPLC eluent containing the radioactive species, which fast mixing is necessary for an accurate and reproducible determination of the radioactivity.
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The much higher viscosity of these solvents has therefore precluded their widespread use even after modification with alcohol-type diluents. The amount of diluent necessary to reduce the viscosity to instrument-usable levels results in a substantial reduction in detection sensitivity with respect to counting efficiency due to the quenching effect of the alcohol-type diluents.
It is an object of the invention to provide a scintillation medium for homogeneous flow systems that is based on a solvent system having the advantageous properties of the above-mentioned safer solvents but having a much lower viscosity. Another object of this invention is to provide a scintillation medium for homogeneous flow systems that is liquid at temperatures of use, i. The invention now provides such a scintillation medium for homogeneous flow systems which comprises benzyltoluene.
Figure 1 contains a graph of viscosity in centiStokes versus temperature for various solvents. Figure 2 contains a graph of viscosity in centiStokes versus water-cocktail mixing ratio for various solvents. Figure 3 contains a graph of back-pressure in psi versus water-cocktail mixing ratio for various solvents. Similar to conventional scintillation media, those of this invention will generally comprise a solvent or solvent system, a fluor or fluor system, and a surfactant or surfactant system. The scintillation media of this invention distinguish from those of the prior art in that they use a solvent or solvent system which comprises at least one benzyltoluene compound.
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Benzyltoluene can exist in three different forms that are positional isomers: ortho, meta and para benzyltoluene. These positional isomers may be used individually, or as a mixture. Most preferably, however, the solvent system used in the scintillation media of the invention comprises a mixture of positional isomers of benzyltoluene. We prefer the mixture to contain from 0. We prefer the mixtures of benzyltoluene which contain ortho, meta and para isomers.
The solvent system used in scintillation media according to the invention preferably has a viscosity of lower than 3.
Preferably it has a viscosity of less than 3. Preferably, the solvent system used in scintillation media of the invention consists of at least one benzyltoluene without additional scintillation solvents. However, said benzyltoluene may also be used in admixture with one or more other liquid scintillation solvents, e.
The solvent system of scintillation media according to this invention may also comprise another non-scintillating solvent wherein the said non-scintillating solvent may act as a co-solvent, diluent or viscosity modifier. Such non-scintillating solvents that may be used include alcohols, cellosolves and esters. Scintillation detection media for flow systems according to the invention preferably contain at least one fluor fluorescent compound.
Suitable fluors for scintillation media are well known in the art and include, for example, 2,5-diphenyloxazole, p-terphenyl, 2- 4 1 -t-butylphenyl 4 11 -biphenyl -1,3,4-oxadiazole and 2,5-bis 5-t-butyl-benzoxaloyl -thiophene. The scintillation detection medium may also contain a secondary scintillator fluor e. When the scintillation medium is employed as a liquid scintillation counting medium for flow systems, particularly when aqueous samples are employed, the medium may also contain a surfactant or a mixture of surfactants e.
The proportion of the various optional components in the scintillation detection medium can vary over a wide range and will be well known to those skilled in the art. Several methods are available in the prior art for the preparation of benzyltoluene, such as by a Friedel-Crafts reaction between benzylchloride and toluene using a Friedel-Crafts catalyst such as AlCl 3 , FeCl 3 , BF 3 , etc.
The preparation of benzyltoluene is described in e. EP-A For a detailed description of the preparation of benzyltoluene, we simply refer to descriptions given in the prior art, such as the above mentioned references. According to the invention there is provided a scintillating medium for flow systems exhibiting substantially low viscosity and is therefore particularly suited for use in flow systems. This substantially lower viscosity is unexpected given that the compound of the invention - benzyltoluene:. The scintillation media of the invention are advantageous in that they have relatively low viscosities, relatively high flash points, relatively low vapour pressures and are particularly suited for use in flow systems.
The invention is illustrated but in no way limited by the following examples.
Related Radiochromatography: the chromatography and electrophoresis of radiolabelled compounds
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