Sources of Band Asymmetry and Tailing in Linear Chromatography

6 Sources of Band Asymmetry and Tailing in Linear Chromatography 

One empirical model that is widely used for the evaluation of band asymmetry is the exponentially modified Gaussian (EMG) fimction [111-113]. It is the convolution of the unit area Gaussian function and of an exponential decay of unit area. By definition, the convolution of two functions and (f) is the following function 

For the purpose of this convolution, the Gaussian profile is written 1 

In addition to the EMG fimction, many other mathematical models have been suggested to accoimt for the profiles of experimental peaks and to determine characteristic shape-parameters, such as a number of theoretical plates, a skew and an excess. These parameters are related to the second, third and fourth moments (Eq. 6.77) of the peak, respectively. For example, the Gram-Charlier series (GC) [96,114,115] and the Edgeworth-Cramer series (EC) [115,116] have been 

In the first case, since the active sites occupy a very small fraction of the surface and the interaction between active sites and solute molecules is very strong, the active sites will become saturated for a rather low concentration of the solute, a much smaller concentration than would be needed to saturate the ordinary types of sites. In other words, although the isotherm for the main adsorption sites could be considered linear in the range of concentrations achieved in the column, the same is no longer true for the high active site isotherm. It is not linear, and we should use a nonlinear isotherm instead, e.g., a Langmuir isotherm, to accoimt for the band profiles [119]. 

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