External standard efficiency

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

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. 

To be able to produce quantitative data, an internal standard and external standards are required. Internal standardization corrects for possible instrument drift or changes in the efficiency of the ablation and thus improves the... 

The average error for the 105 data points is -7.1% with a standard deviation of the error of 26.1%. The data (65 points ) for external contacting efficiency in the trickle-flow regime (11. 18, 20, 21) can be correlated by ... 

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)). 

What do you notice about the relationship between the chloroform concentration in the sample and the counting efficiency for 14C What effect does increased quenching have on the external standard number reported by your instrument ... 

The colour quenching effect of carotenoids in liquid scintillation spectrometry has been studied in detail. The most efficient quenchers were lycopene [i/>, -carotene (225)] and echinenone [j8,/3-caroten-4-one (226)]. Use of the external standards ratio method to correct for colour quenching in the radioassay of carotenoids is justified.84... 

Difficulties are caused by the need to calibrate the electron beam energy scale using an external standard, often a rare gas. With the commonly used older methods available for processing ionization-efficiency data, it is necessary to assume some relationship between the ionization-efficiency curve for the compound under investigation and the... 

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. 

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. 

External Standard Method for the Determination of the Efficiency in Liquid Scintillation Counting. 

Measuring organic compounds, quantification can make use of internal and external standards. Different examples are described in this book. There is no general advice on the various organic compounds in soil that can be quantified. Due to the complex composition of soil, which influences the extractability, it is not customary to correct measured values for the incomplete extraction. There can be a high extraction efficiency for one soil and a lower extraction... 

The tritium measurement protocol Is relatively simplistic and consists of neutralization of the sample, single plate distillation, removal of an appropriate size aliquot (usually eight mL.) via reproducible automatic pipets and the addition of 15 mL. of dark adapted scintillation cocktail under Incandescent lighting conditions. After shaking to ensure a uniform gel, the sample is allowed to settle and dark adapt for up to twenty but no more than sixty minutes. The liquid scintillation unit efficiency Is determined dally on a previously prepared standard and background measurements are determined at least dally. Quench corrections are not applied to the system due to the lack of an external standards ratio capability and an effort to minimize the amount of hazardous waste which would be generated if an Internal standards approach were adopted. 

While convenient, especially if sample preparation is simple and an autosampler is used so that injections are reproducible, the external standard method does not account for sample variability. Take, for instance, two samples, A and B, in both of which an analyte is present at the same concentration. If the efhdency of extraction of the analyte is 100% from sample A but only 70% from sample B, then the signal intensities will vary by the same ratio (Figure 122a). When these values are inserted into the equation of the calibration curve, the calculated concentrations will vary by the same 30% that the extraction efficiencies differ, even though the concentrations of the analyte are the same in both samples. 

The internal standard (IS) method compensates for the recovery problem encountered with external standardization by adding a compound (the IS) of known concentration to each sample prior to extraction. The compound chosen as the IS should have the same extraction efficiency, chromatographic retention time, and mass spec-trometric properties as the analyte. These properties can be provided by an IS that has a chemical structure similar to that of the analyte. 

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