Quenched standards

Count a set of and C quenched standards (see steps 3-35 to 3-38 for preparation of the two sets of standards) in each channel in the presence and absence of the external standard. 

Plot the efficiency (in channel A) of each quenched standard as a function of the external standard channels ratio value it yielded. This type of plot is shown in Figure 3-20. 

The index of quenching with respect to sample is found from Compton spectrum created by y-ray irradiation to the sample. A set of quenched standards are employed for the construction of a quenching correction curve which represents the relationship between the index of quenching and the counting efficiency. The radioactivity of the sample to be measured can be determined by using the quenching correction curve. 

The environmental and dead water samples exhibit quenching characteristics falling into the range of 0.5 - 0.6 quenched standards (Packard Instrument Co.) for tritium and carbon-14. During each cycle an efficiency calibration using the quenched standards is made. A cycle consists of counting the 15 sample vials, 2 or more vials of dead water, and the 4 quenched standards (two carbon-14 and two tritium standards). The selected count period is 20 minutes. 

The numerous short counting periods, the large number of vials coupled with the efficiency calibration data from the external standard count, the quenched standards, and the vials with mixtures of either scintillator-sample or scintillator-dead water make this a program of data organization and reduction. To eliminate the need to change the source progreun due to different numbers of observations from one sample run to another, the PL/1 feature of dynamic allocation of storage has been used. This allows the user to define the dimensions of the arrays for the input data. It is not uncommon for the arrays for the quenched standards, the dead water, and the sample data to have different dimensions. 

The data from the quenched standards and the AES for the sample and dead water are used to calculate the efficiency of... 

Through the external standard counts of the quenched standards and those of the dead water and of the sanple, the counting efficiencies are calculated by subroutine calls. 

The activity of the external standard is irrelevant to the preparation of a calibration curve as it is used merely as a source of radioactivity. Its coimt rate is first determined in each of a series of quenched standards of known counting efficiency. The count rate of the external standard is then related to this efficiency. The external standard count rate obtained in a sample of unknown activity can then be used to determine the efficiency of counting for the particular sample. 

Quench curves are usually recorded in the fiill window mode. This means that the window up to the maximum beta energy of the nuclide at a given quench level is used to record the count rate. In practice however, a smaller and optimized window is applied in LS measurements of real samples. Therefore SpecWork software was used to re-count the count rates of all quench standards in this optimized window. This software gives the possibility to change window settings without recounting the sample. 

Nonradiative reiaxation and quenching processes wiii aiso affect the quantum yieid of fluorescence, ( )p = /cj /(/cj + Rsiative measurements of fluorescence quantum yieid at different quencher concentrations are easiiy made in steady state measurements absoiute measurements (to detemrine /cpjj ) are most easiiy obtained by comparisons of steady state fluorescence intensity with a fluorescence standard. The usefuiness of this situation for transient studies... 

The Scleroscope scale ranges from 0 to 140 the cahbration point of 100 is the hardness of fully quenched but untempered steel. Standard test blocks embodying this condition are used for cahbration. 

Steels having adequate hardenabiHty develop martensitic stmctures in practical section sizes. Molybdenum is a potent contributor to hardenabiHty, and has been shown to be even more effective in the presence of carehiUy selected amounts of other alloying elements (26). The end-quench test has become the accepted method for measuring hardenabiHty, and the data can be correlated with section size. Technical societies worldwide have standardized hardenabiHty limits (bands) for a large number of carbon and alloy steels standards of the Society of Automotive Engineers are examples (27). 

Chemical methods involve removing a portion of the reacting system, quenching of the reaction, inhibition of the reaction that occurs within the sample, and direct determination of concentration using standard analytical techniques—a spectroscopic metliod. These methods provide absolute values of the concentration of the various species that are present in the reaction mixture. However, it is difficult to automate chemical mediods, as the sampling procedure does not provide a continuous record of tlie reaction progress. They are also not applicable to very fast reaction techniques. 

Snccessful full-scale tests on quenching of deflagrations and detonations using expanded metal cartridge flame arresters were performed to USCG standards on Group C and D gases by Westech Industrial Ltd. (Lapp 1992, Lapp and Vickers 1992). 

An important aspect of quenching in analysis is that the fluorescence exhibited by the analyte may be quenched by the molecules of some compound present in the sample, i.e. this is an example of a matrix effect. If the concentration of the quenching species is constant this may be allowed for by using suitable standards (i.e. containing the same concentration of quenching species), but difficulties occur when there is an unpredictable variation in the concentration of quenching species. 

Requirements of Standards. The general requirements for luminescence standards have been discussed extensively (3,7-9) and include stability, purity, no overlap between excitation and emission spectra, no oxygen quenching, and a high, constant qtiantum yield independent of excitation wavelength. Specific system parameters--such as the broad or narrow excitation and emission spectra, isotropic or anisotropic emission, solubility in a specific solvent, stability (standard relative to sample), and concentration--almost require the standard to be in the same chemical and physical environment as the sample. 

The quenching effects of these esters and the phenolic products were also measured using standard Stern-Volmer quenching procedures. 

Chomiak, J. and Jarosinski, J., Flame quenching by turbulence. Combust. Flame, 48, 241, 1982 Jarosinski, J., Strehlow, R.A., and Azarbarzin, A., The mechanisms of lean limit extinguishment of an upward and downward propagating flame in a standard flammability tube, Proc. Combust. Inst., 19,1549,1982. 

---