Response peaks

Method of Moments The first step in the analysis of chromatographic systems is often a characterization of the column response to sm l pulse injections of a solute under trace conditions in the Henry s law limit. For such conditions, the statistical moments of the response peak are used to characterize the chromatographic behavior. Such an approach is generally preferable to other descriptions of peak properties which are specific to Gaussian behavior, since the statisfical moments are directly correlated to eqmlibrium and dispersion parameters. Useful references are Schneider and Smith [AJChP J., 14, 762 (1968)], Suzuki and Smith [Chem. Eng. ScL, 26, 221 (1971)], and Carbonell et al. [Chem. Eng. Sci., 9, 115 (1975) 16, 221 (1978)]. 

Higher moments can also be computed and used to define the skewness of the response peak. However, difficulties often arise in such computations as a result of drifting of the detection system. 

The preceding eqiiations are accurate to within about 10 percent for feed injections that do not exceed 40 percent of the final peak width. For large, rec tangiilar feed injections, the baseline width of the response peak is approximated by ... 

In a DTA study [1193] of decomposition reactions in Ag2C03 + CaC03 mixtures, the presence of a response peak, absent on heating the silver salt alone, resulted in the identification of the double salt Ag2C03 2 CaC03, stable at <420 K. One important general consideration which arises from this observation is that the formation of a new phase, by direct interaction between the components of a powder mixture, could easily be overlooked and, in the absence of such information, serious errors could be introduced into attempts to formulate a reaction mechanism from observed kinetic characteristics. Due allowance for this possibility must be included in the interpretation of experimental data. 

A continuation of the preceeding diaphragm integration indicates a seemingly resonant condition after several cycles of the applied wall reactions. This result has little effect on the first response peaks and disappears with the application of a reasonable amount of damping. 

The linearity of a method is its ability to obtain test resnlts that are directly proportional to the sample concentration over a given range. For HPLC methods, the relationship between sample concentration and detector response (peak area or height) is nsed to make this determination. 

Figure 3. Analytical response. Peaks 1, 2, and the shaded portion of 3 correspond to organic C the unshaded portion of Peak 3 is elemental C. The shaded portion of Peak 3 constitutes the correction for pyrolytic conversion of organic to elemental C. Peak 4 is the calibration peak.

The uncertainties of K and the response ratio depend upon the ability to measure detector response. Peak height ratios can be used with excellent results (generally better than areas) when the peaks are symmetrical and sharp. Peak hight ratios are also more useful than peak areas in overlapping peaks. For automated systems, peak areas are preferred since the ratios are readily measured, and the data are calculated with electronic integrators and computers. 

Calibration curves are constructed using the standard solutions, as shown in Figure 7.20, by plotting the ratio of the detector response (peak area or peak height) for each solute relative to that of the internal standard against the solute concentration. The peak area ratio for each component in the sample is then calculated, and from these ratios the amount of each constituent in the sample may be determined. 

The principle of the method may be illustrated by considering the response to the injection of a perfect pulse of sorbate at the column inlet at time zero. The mean retention time t is given by the first moment of the response peak and is related to the dimensionless Henry constant by ... 

T Tw = internal, resp. external tube wall temperature [ °C] tR = retention time of a response peak [ sec]... 

ECD RESPONSES (PEAK AREA/MASS OF DERIVATIVE) OF HALOGENATED STEROID DERIVATIVES RELATIVE TO TESTOSTERONE 17/3-HFB [392]... 

The analysis of chromatographic data is commonly carried out by determining the first and second moments of the response peak. This method is simple and convenient although somewhat more accurate results may be obtained by more sophisticated methods such as Fourier transform or direct matching of the response curves in the time domain(10-14). 

Figure 6 illustrates the progressive overloading of an anti-HSA polyclonal antibody column after repeated injections of 2 /zg of HSA. At first injections, impurities elute from the column at the dead volume, while HSA is totally adsorbed. The gradual emergence of the nonretained HSA elution peak is due to two different effects, the saturation of the support and the slow adsorption kinetic process. The unretained fraction is calculated from peak area measurements, subtracting the area of the impurity response peak. 

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