Channels ratio method

In column 110, it is also theoretically possible that glycine com-plexed with the added humic acid and that it was sequestered in the aqueous phase of the Teflon eluate and bound to the Teflon bed. To test this explanation, a 1/10-scale parfait column was constructed and 4 liCi of 14C-glycine, 40 fig total, was applied in 800 mL of synthetic hard water (column 123). In this experiment, the alcohol and solvent 1 conditioning washes were combined with the standard eluates of each bed before counting. These solutions were not concentrated before counting. Quench correction was by the channels ratio method. 

For general purpose tracer work, however, and particularly in polymer chemistry, the liquid scintillation counter surpasses all other instruments in its sensitivity and adaptability. There is no question on the author s mind that at the present time such an instrument would be the first choice, particularly where tritium, carbon-14 or sulphur-35 were involved. Samples for assay are dissolved in a phosphor whose major solvent usually consists of toluene, toluene-alcohol, or dioxan. Many polymers and low molecular weight compounds are readily soluble in these solvents. Prospective users should not be deterred by alleged complications due to "variable quench effects" as these effects are readily corrected for via internal or external standards or the channels ratio method (7, 46, 91). Dilution quench corrections, though valid, are tedious and unnecessary. Where samples are insoluble in phosphor they may be suspended (e.g. as gels or as paper cut from chromatograms, etc.) or they can be burnt and the combustion products absorbed in a suitable phosphor solution. A modification of the Schoniger flask combustion technique is particularly suitable for this purpose (43—45). 

OF THE RADIOACTIVITY CONCENTRATION Radioactivity measurements are carried out by the liquid scintillation counting procedure in -spectrometers using an external standard device which permitted the counting efficiency to be determined by the channel ratio method (explained for instance by Dyer (1980)). 

The channels ratio method makes use of existing counts within the sample vial. This method is suitable when large numbers of counts are present, but it becomes very time consuming with samples containing few counts, because a long time is required to accumulate sufficient counts for statistical accuracy. Most modern scintillation counters therefore employ an automatic external standardization system of quench analysis to avoid the time required for the internal channels ratio method. This method utilizes a specially selected external y radiation source carried in a lead-shielded chamber that is buried in the instrument. Before the regular counting of the sample, the external standard is... 

This experiment demonstrates two methods for analysis of quenching within samples, namely, the channels ratio method and the automatic external standardization method. Either or both of the methods may be demonstrated with the quench series of bottles described in the following protocol, depending on the capabilities of your particular scintillation counter. 

If the channels ratio method is used, proceed with Steps 3 to 8. If the automatic external standardization method is used, proceed with Steps 9 to 13. 

The channel 1/channel 2 ratio is a value that will be provided by the instructor. It is a correction factor that takes into account the fact that some 14C counts per minute were counted in channel 1. The instructor determined this ratio by scintillation counting of a sample containing a known number of 14C disintegrations per minute in channel 1 and channel 2 under conditions identical to your experiment. Refer to Section I, Experiment 3 for further discussion of the channel ratio method of quantifying radioactivity. 

In practice a sample is counted normally as described for the channels ratio method and then counted a second time with a source of y-rays (usually Cs or Ba) positioned in the center of the counting chamber just below the sample vial (see Figure 3-19). Any quenching that occurs has the same effect on the efficiency and spectrum of the Compton electrons as it does on those of the sample j8 particles. Since this technique has incorporated the procedures of both internal standardization and channels ratio correction methods, it is no surprise that the data obtained must be treated as described above for both of these techniques. The first or normal count rate obtained in each channel arises only from the sample and may be represented as follows ... 

E. T. Bush, Anal. Chem., 35 1024 (1963). General Applicability of the Channels Ratio Method of Measuring Liquid Counting Efficiencies. 

Tl. Takahashi, I. T., and Blanchard, F. A., Counting quenched liquid scintillation samples by using an outside-the-instrument gamma source and an external-standard channels-ratio method. Anal. Biochem. 35, 411-423 (1970). 

The radioactivity of samples was measured by liquid scintillation with toluene+POPOP or with Triton X-100 (Isocap-300).Counting efficiencies,determined via the channel ratio method and quench correction curve,were sufficiently constant (about 70-80%).Gas exchange measurements of respiration of leaf were conducted by manometric tech-nique(9).It was found that the chosen concentrations of exogenous substrates had no effect on dark respiration of leaves studied plants. 

Synthetic samples were prepared containing known amounts of beta, beta-gamma, and electron capture activity. The samples were then measured using the channels ratio method and the results are listed in Table 2. These results indicate that various combinations of low, medium, and high energy activity can be estimated within 1-8% by this method. 

The channels ratio method is satisfactory for all types of quenching. It is also less time consuming since only one coxmt is required in two-channel scintiUation counter. Moreover, it is the most accurate method for counting efficiency determination. 

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