Chromatography peak skew

Kirkland, J. J., Yau, W. W., Stoklosa, H. J., and Dilks Jr., C. H., Sampling and extra-column effects in high-performance liquid chromatography influence of peak skew on plate count calculations, /. Chromatogr. Sci., 15, 303, 1977. 

For EMG peaks, peak skew increases with the ratio xG/oG. Figure 16-32 illustrates the characteristics of such a peak calculated for xG/aG = 1.5. In general, with xG/aG > 1 (peak skew > 0.7), a direct calculation of the moments is required to obtain a good approximation of the true value of N , since other methods give a large error (Yau et al., Modems Size-Exclusion Liquid Chromatography, Wiley, New York, 1979). Alternatively, Eq. (16-165) can be fitted to experimental peaks to determine the optimum values of fG, aG, and xG. 

These considerations are particularly important for non-linear or concentration dependent relations and non-equilibrium conditions, such as those found in chromatographic systems showing markedly skewed peaks (6.). As these authors have shown, there is no identifiable solution to the problem of the thermodynamic properties of the highly skewed chromatogram peak. Thus, the elution method is only valid for equilibrium chromatography. 

Figure 1.4 Distribution isotherms, (a) The ideal situation corresponding to the invariance of the concentration isotherm, (b) Situation in which the stationary phase is saturated - as a result of which the ascent of the peak is faster than the descent (skewing factor greater than 1) (c) The inverse situation the constituent is retained too long by the stationary phase, the retention time is therefore extended and the ascent of the peak is slower than the descent apparently normal. For each type of column, the manufacturers indicate the capacity limit expressed in ng/compound, prior to a potential deformation of the corresponding peak. The situations (a), (b) and (c) are illustrated by authentic chromatograms taken out from liquid chromatography technique.

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