Scintillation counting efficiency

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. 

Thus an attempt was made to correlate (liquid scintillation) counting efficiency determined as a function of the composition of the solution with structural changes occurring in the solution as reflected in positron annihilation parameters. 

Liquid scintillation counting is by far the most common method of detection and quantitation of -emission (12). This technique involves the conversion of the emitted P-radiation into light by a solution of a mixture of fluorescent materials or fluors, called the Hquid scintillation cocktail. The sensitive detection of this light is affected by a pair of matched photomultiplier tubes (see Photodetectors) in the dark chamber. This signal is amplified, measured, and recorded by the Hquid scintillation counter. Efficiencies of detection are typically 25—60% for tritium >90% for and P and... 

The addition of a secondary solute or wavelength shifter can serve to offset much if not all of the action of tagged nitrocompds in reducing counting efficiency. For expl nitrocompds, a shift of the emission spectrum considerably into the visible region where absorption effects are not so pronounced is clearly indicated. The secondary solute POPOP has been found to be most efficient for this purpose (Ref 2). This enhanced effect on the scintillation process is illustrated in Fig 2 for p-Nitrotoluene... 

To determine the efficiency of aminoacylation of [14C]Phe-tRNA, 5 fil aliquots of the aminoacylation mixture are withdrawn before and after the reaction the samples taken from the reaction mixture at the end of the incubation are spotted onto 3-MM paper discs (Schleicher Schuell) and processed by the cold TCA precipitation method, while the sample taken before the reaction is spotted on a paper disc pretreated empty by the same cold TCA procedure. Determination of the radioactivity present on these filters by liquid scintillation counting allows one to calculate the aminoacylation efficiency of the reaction (which, for phenylalanine, should be >2% of total tRNA). The specific activity of the [14C] Phe-tRNA can be determined after one-step purification of Phe-tRNA by BD cellulose chromatography (Gillam et al., 1968), followed by determination of the radioactivity and of the A260. 

Picer et al. [49] described a method for measuring the radioactivity of labelled DDT contaminated sediments in relatively large volumes of water, using a liquid scintillation spectrometer. Various marine sediments, limestone and quartz in sea water were investigated. External standard ratios and counting efficiencies of the systems investigated were obtained, as was the relation of efficiency factor to external standard ratios for each system studied. 

The efficiency of the cDNA probe synthesis is determined by scintillation counting of the flowthroughs from steps 9 and 11 5pL of each flowthrough are added to 5mL scintillation fluid in separate scintillation counter vials. The samples are counted on the P-channel, and the obtained counts are multiplied by a dilution factor of 20. Probes should yield a total of 5-25 x 10 cpm see Note 32). 

A lot of ready-to-use scintillation cocktails are available. Besides counting efficiency, chemical, biologic, and environmental hazards should be reflected when choosing a cocktail. Since vials and their contents are radioactive waste, number and volume of vials should be as small as possible. 

In recent years, several developments have generated new possibilities in liquid scintillation counting. The most important event was the development of the bialkali photomultipliers with considerably improved quantum efficiency. In addition, several new scintillators and solvents of higher purity have become available. With the resulting counting systems it has been possible to achieve sensitivities comparable with gas counters, with much less time and effort involved. 

Many successful suspension counting systems have been reported where water is suspended in a scintillation mixture by a detergent. Van der Laarse (8) reported a system consisting of 4 ml. of water and 16 ml. of a mixture of toulene containing 4 grams PPO, 0.3 gram POPOP/liter mixed with Triton X-100 in a ratio of 55 25. He obtained a counting efficiency of 21% and a Y-value of 1.3 nCi/liter. 

Reagents. Aqueous ethanol solutions of the triethylammonium salts of adenosine-5 -[a-S2P]triphosphate (a- PATP), adenosine-5 -[7-S2P]triphosphate (7-S2P-ATP), thymidine-5 -[a-S2P]triphosphate (a- p-TTP), cytidine-5 -[q-S2P]triphosphate (a-S2P-CTP), and guanosine-5 -[a-S2P]triphosphate (a-S2P-GTP) were purchased from Amersham (Arlington Heights, IL). Radioactive sample concentrations reported for detector efficiency determination were adjusted from the manufacturer s specifications after subjecting several diluted aliquots of the stock solution to liquid scintillation counting. 

---