Fronting of chromatographic peaks

Kuge and Yoshikawa (3) related a change in the gas chromatographic peak shape to the beginning of multilayer adsorption on the surface of the solid. For small adsorbate volumes, the peak shape is symmetrical. As the adsorbate volume is increased, a sharp front, diffuse tail, and a defect at the front of the peak top is observed (Figure 11.2). It then acquires a diffuse front and sharp tail. This point corresponds to the B point of the BET Type II adsorption isotherm at which the relative surface area may be calculated. 

Fronting. A condition in which the front of a peak is less steep than the rear relative to the baseline. This condition results from nonideal equilibria in the chromatographic process. 

Fig. 3 Investigation of chromatographic peak purity by diode array detection (A) spectra of drug substance and impurity (B) coelution of a mixture containing about 10% impurity (C) coelution of a mixture containing about 0.5% impurity. The spectra were obtained in the peak maximum, at about 5% and 50% of each side of the peak. The normalization was performed with respect to the first spectrum at the peak front (a matchfactor of 1000 means identical spectra) using commercial software.

In defining the resolution using equation 23, one must note that in an annular chromatograph, the effluent streams are constrained to elute in 2tc radians. In our model we consider the range of 6 to be (0, < ) and therefore it was necessary to modify this definition of Rg for instances where the front of one peak containing smaller solutes overlaps the opposite front of another peak containing larger solutes. In each case, the resolution between peaks was calculated for the separation between the closest fronts. 

Having established that a finite volume of sample causes peak dispersion and that it is highly desirable to limit that dispersion to a level that does not impair the performance of the column, the maximum sample volume that can be tolerated can be evaluated by employing the principle of the summation of variances. Let a volume (Vi) be injected onto a column. This sample volume (Vi) will be dispersed on the front of the column in the form of a rectangular distribution. The eluted peak will have an overall variance that consists of that produced by the column and other parts of the mobile phase conduit system plus that due to the dispersion from the finite sample volume. For convenience, the dispersion contributed by parts of the mobile phase system, other than the column (except for that from the finite sample volume), will be considered negligible. In most well-designed chromatographic systems, this will be true, particularly for well-packed GC and LC columns. However, for open tubular columns in GC, and possibly microbore columns in LC, where peak volumes can be extremely small, this may not necessarily be true, and other extra-column dispersion sources may need to be taken into account. It is now possible to apply the principle of the summation of variances to the effect of sample volume. 

Figure 5.6. Mass spectra of para-fluorobenzophenone on the front side (a), on the back side (b), and at the top (c) of the chromatographic peak.

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