Sample channels ratio

The applicability of the sample channels ratio (SCR) and external standard channels ratio (ESCR) for determining the efficiency of counting of quenched samples was studied using phenolphthalein, chloroform, and alkaline extracts of sheep s liver as quenching agents when counting CO,. 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

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). 

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. 

Using the data obtained from samples 1 to 4, the known number of 14C disintegrations per minute added to each of these samples, and considering the counts per minute data obtained from the widest window as a reflection of overall counting efficiency (the total possible number of counts per minute that can be detected in each sample), construct a channels ratio quench correction curve similar to that shown in Figure 3-9. 

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. 

Detection is one of the most difficult problems in micro chemical processes. Because sample volume becomes extremely small in such systems, an ultrasensitive detection method is indispensable. For example, limited sample channel thickness causes very small signal-to-background ratio in absorption spectroscopy, thus only very concentrated samples can be analyzed. 

Three methods have evolved to ascertain the degree of efficiency loss both within the instrument and as a result of quenching. These techniques are termed (1) internal standardization, (2) channels ratio quench correction, and (3) external standard channels ratio quench correction. Determination of counting efficiency by internal standardization may be performed in two steps. The sample is first accurately counted followed by the addition of a precisely known quantity of radioactivity to the vial (50,000-80,000 dpm C or 100,000-150,000 cpm H). It is important for the amount of added radioactivity to be considerably larger than that originally present in the vial. The sample is then counted a second time. The first count is the sample cpm and the second count is the sample cpm + (efficiency)(standard dpm). That is. 

Figure 3-16. Channels ratio counting of quenched samples. Quenching increases sequentially from sample 1 to sample 4. Arrows above the figure indicate the range of the A and B channel discriminators. (Courtesy of Beckman Instruments, Inc.)...

Example. A sample of C was counted using the channels ratio technique and the following data were obtained ... 

What was the efficiency of counting and the absolute radioactivity (dpm) in the sample These data give a channels ratio of... 

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 ... 

Once these standard curves have been prepared the multiply labeled sample may be counted in the presence and absence of the external standard. The counting rate in channel B (cpm C). and the channels ratio value may be used in conjunction with the quench correction cutve (Figure 3-20) to calculate the absolute dpm of C using equation 3-24. 

Since we have already calculated dpm C above, equation 3-26 has only one unknown (the efficiencies are obtained from Figure 3-20 at the appropriate channels ratio values), the dpm of in the sample. If the amount of radioactivity in the sample is sufficiently low that the counter... 

Counting Quenched Samples Using the Channels Ratio Technique... 

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 ratio of the sample to waste electric field strength needed to prevent the sample from diffusing into the analysis channel depends on both the transit time of the sample through the crossintersection and the diffusion coefficient of the sample. Sample to waste electric field strength ratios from 0.4 to 0.75 are obtained empirically for a neutral marker, e.g., rhodamine B. This range allows the sample to pass through the valve from the sample channel to the waste channel without diffusing into the analysis channel. 

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. 

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