Scintillation counting quenching

Measurement of radioactive decay can also be affected by various components present in, or added to, the scintillation cocktail. These components can cause quenching that is, they can decrease the efficiency of the scintillation process. Scintillation counting provides data in counts per minute (cpm). Quenching dictates that the counts per minute detected is less than the actual decay rate, or disintegrations per minute (dpm). Almost every sample encountered experimentally is quenched to some degree for example, 02 picked up by the scintillation fluid from contact with air serves as a quencher. Therefore, researchers frequently count an additional sample containing a standard of known de-... 

Preparation of Samples for Liquid Scintillation Counting. Quenching in its various forms constitutes the major concern during sample preparation for liquid scintillation counting. The following sections discuss some of the problems commonly encountered and also present some remedies to overcome these problems. 

Incomplete solubility and associated point quenching constitute a major problem in scintillation counting. Efficient scintillation counting requires that the sample be fully soluble in the excitable organic solvents of the scintillation fluid. However, biological systems, which are usually aqueous systems or assays, frequently contain water or hydrophilic molecules that will not dissolve in standard toluene-based scintillation cocktails. 

This method is laborious and expensive, as well as being irreversible after the standard has been added. With access to good scintillation counters, it is more usual to carry out measurements with an external standard in which a y emitting source is positioned dose to the stintillation vial. The y rays excite the solvent molecules and the resulting additional scintillation counts (which are also subject to quenching) are used for standardisation. 

G. A. Bruno and J. E. Christian, Anal. Chem., 33 650 (1961). Correction for Quenching Associated with Liquid Scintillation Counting. 

B. Scales, Anal. Biochem., 5 489 (1963). Liquid Scintillation Counting The Determination of Background Counts of Samples Containing Quenching Substances. 

In a similar study, Fe-labeled Fe(H20)6 was allowed to exchange with Fe(H20)e + in the presence of chloride ions [72]. The reaction was quenched by the addition of 2,2 -bipyridine which complexes Fe, and the activity of Fe in the complex was determined by scintillation counting. It was found that most of the exchange takes place by the reaction of Eq. 66... 

In these experiments a tritiated substrate was mixed with 1 -labeled protonated (for H/T KIE experiment) or deuterated (for D/T KIE experiment) substrate. The mixture was allowed to react in the presence of enzyme, under defined conditions of pH and temperature, and quenched at different fractional conversions. The quenched mixtures were analyzed by HPLC and liquid scintillation counting to determine the fractional conversion (f, determined from the counting) and tritium to ratio in the products ([ H/I cjy and pH/l4C]oo for the ratio at the time point and the infinity point, respectively). The L/T KIE (also denoted by (V/K)l or kt/kj) were calculated by equation 1 (Melander Saunders, 1987) ... 

The quenching is a phenomenon caused by the energy loss in the process of the energy transfer inside the liquid scintillator. It should be taken into account, whenever the liquid scintillation measurement is performed. Owing to the existence of a quencher, which means the material giving rise to the quenching, the counting efficiency finally decreases as follows. 

The presence of inert material or colored material in a sample may reduce the radioactivity observed in scintillation counting. This quenching effect can be corrected for quite easily by means of an internal or added standard. The first thing to be sure of is that the samples are counted under the same conditions as the standards. For example, if the samples are 0.5 ml of an enzyme assay mixture (in aqueous buffer) that are added to 5 ml of scintillation fluid, then the specific activity of the standard should be determined in 0.5 ml of the same buffer, counted in 5 ml of scintillation fluid. 

For liquid scintillation counting (LSC), a Packard Tri-Carb 460 CD microprocessor controlled liquid scintillation spectrometer, with automatic external standarization, quench correction, and conversion of counts per minute (cpm) into disintegrations per minute (dpm), was utilized. An appropriate scintillation cocktail was selected according to sample characteristics Insta-Gel II (Packard) was used for aqueous solutions and organic extracts, Dimilume-30 (Packard) for NaOH solutions and... 

Liquid Scintillation Detector, This detector measures radioactivity by recording scintillations occurring within a transparent vial that contains the unknown sample and liquid scintillator. Because the radionuclide is intimately mixed with, or actually dissolved in, the fiquid scintillator, the technique is ideal for the pure -emitters, such as H, and Typical efficiencies for liquid scintillation counting in the absence of significant quenching are 60% for tritium and 90% for... 

Other Components and Techniques. Other components of a liquid scintillator detector include (1) electronics, (2) a photomultiplier tube, (3) a preamplifier, and (4) a pulse-height analyzer. Description of these components and discussion of relevant topics such as (1) efficiency of scintillation counting, (2) quenching, (3) counting statistics, (4) assay optimization, and (5) radiation safety can be found in an earlier edition of this textbook. ... 

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