Typical Analytical Working Curve

This section will describe, in step-by-step detail, the procedures involved in the establishment of a spectrographic working curve and its use in the determination of the concentration of the element in an unknown. Use will be made of the principles described in earlier sections of this chapter. 

To obtain the working curve, the following conditions and procedures were used. Standard solutions containing 2, 4, 8, 10, and 20 ppm germanium were prepared, all containing chromium at the same concentration, 50 ppm. The chromium serves as the internal standard. 

High-purity, 3/16-in. diameter graphite electrodes were drilled to a depth of 1/4 in. to serve as anodes. Counter electrodes were blunt-nosed 

Emulsion Calibration Data Using a Seven-Step Filter and the Germanium Emission Line at 3039 A 

FIGURE 8-10. A photographic emulsion calibration curve using a seven-step neutral filter and the germanium spectral emission line at 3039 A. 

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The second aspect of method validation is the experimental work. This typically involves initial experiments with analytical standards to confirm the reliability and repeatability of calibration of the system using only standards. The next step usually involves a series of analytical runs, conducted over several days or weeks, in which one or more analysts prepare calibration curves and analyze replicates of the typical analyte/matrix combinations and concentrations that are to be routinely analyzed using the method. The final phase of validation typically includes several runs in which fortified or incurred materials, again representing typical analyte/matrix combinations and concentrations, are provided blind to the analyst(s). The results are then summarized in a validation report, which again should receive appropriate peer review within the laboratory prior... 

Typical parameters that are generally considered most important for validation of analytical methods are specificity, selectivity, precision, accuracy, extraction recovery, calibration curve, linearity, working range, detection limit, quantification limit, sensitivity, and robustness. 

A major detraction for LS AAS has always been the relatively short linear region of the calibration curves, typically not more than two orders of magnitude in concentration. The limits of the linear working range arise from stray radiation and the finite width of the emission lines of the radiation source, which is not monochromatic and just three to five times narrower than the absorption profile. With HR-CS AAS, there is no theoretical limit to the calibration range, only the practical limits imposed by the size of the array detector, the increasing possibility of spectral interferences, and the ability to clean the atomizer after extremely high analyte concentrations have been introduced. 

Amperometric titration curves typically take one of the forms shown in Figure 23-14. The curve in part a represents a titration in which the analyte reacts at the electrode while the titrant does not. Figure 23-14b is typical of a titration in which the reagent reacts at the electrode and the analyte does not. Figure 23-14c corresponds to a titration in which both the analyte and the titrant react at the working electrode. 

In a previous work (Leon and Atanasiu 2(X)7), we noticed that many fragility curves have a typical shape, similar to a sigmoid function. Through regression, we estimated them with this simplified analytical model which takes into account only one coefficient ... 

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