Peak Fronting Possible Causes

Peak fronting normally occurs because of an incompatibiUty between the sample solvent and the mobile phase or as a result of the sample being too concentrated. If a sample is prepared in 100% methanol and the mobile phase contains 5% methanol, then this might result in a fronting problem. Under normal conditions, when the sample is introduced into the flow of mobile phase, the sample solvent and the mobile phase quickly mix. The sample then 

Too much sample means that the capacity of the column will be exceeded and some of the compound cannot interact with the stationary phase. The material that does not interact with the stationary phase will travel at a faster rate through the column, resulting in peak fronting. 

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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. 

The use of a sample solvent other than the mobile phase is another cause of fronting peaks in IPC. In this case, the sample should only be injected as a solution in the mobile phase. No more than 25-50 /rl of sample should be injected, if possible. 

A perfect Gaussian peak is symmetrical and is hence equal to 1.0. Adequate symmetry is considered achieved as long as bands have asymmetry factors between 0.9 and 1.2. Tailing As > 1.5) and fronting (As < 0.7) can cause poor separations, and indicate possible problems of the column or the chromatographic system. 

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