Peak tailing/fronting

The reader should be aware that these definitions are different from those normally used in chromatography, where peak tailing leads to values larger than unity and peak fronting to values smaller than unity. The ISO/EN/DIN definitions (22,23) are just reciprocal. 

Neutral species remain unaffected by electrodispersion, giving rise to symmetrical peaks. An example electropherogram illustrating peak fronting and peak tailing is shown in Figure 5. 

Fig. 5 Electropherogram of six sulfur anions illustrating peak fronting [thiosulfate (S2Ol-)] and peak tailing [tetrathionate (S4Ol-)] for better visibility of peak asymmetry, the perpendicular peak axis is drawn as a solid line. Separation in 20 mM chromate run buffer at pH 8.3.

Electromigration dispersion determines the shapes of the peaks. Unless the mobilites of the analytes match that of the electrolyte, or unless the concentration of the analytes is at least two orders or magnitude lower than that of the buffer, the peaks will front or tail slower analytes tail while faster analytes front. 

As columns are overloaded for preparative work, peak shape often deviates from the Gaussian shape typical of analytical work. In preparative work, the peaks can assume a triangular shape because the adsorption isotherm is nonlinear. A typical isotherm is shown in Figure 6-37, where CM is the concentration of sample in the mobile phase and Cs is the concentration of sample in the stationary phase. At low concentration of sample (CM) there is a linear adsorption isotherm which results in Gaussian peak shapes. At a point when either the sample adsorption in the stationary phase or the sample solubility in the mobile phase becomes limited, the isotherm becomes nonlinear, assuming either a convex or a concave shape. Convex isotherms are the most common and result in peak tailing. Conversely, concave isotherms cause fronting of the peaks. 

In general. As values between 0.9 and 1.2 are acceptable. If Ts > 1 peak tailing is in evidence if As < 1 peak fronting is evident. The practical impact of peak tailing or fronting is that adjacent peaks are not as well separated as they would be if they were symmetrical, leading to difficulties in peak quantitation. 

Broad peaks, ghost peaks, pseudo peaks, negative peaks, peak doubhng, peak fronting, peak tailing, spikes, no peaks. The major causes and their solutions are tabulated in Table 1. 

Figure 10.9. Comparative chromatograms of a good and a failed silica-base C18 column due to exposure to high pH mobile phase. Severe peak fronting indicated channeling of the packed bed while peak tailing of the basic analyte (amitriptyline) indicated increase of silanophilic activity due to loss of endcapping or bonded groups. Figure courtesy of Waters Corporation.

There are several measures of peak tailing. A common one is the ratio of the width of the tail of the peak to the width of the front of the peak at either 5 or 10% of the height of the peak (see Fig. 2.3 for an illustration of the measurement of the various asymmetry factors) ... 

Peak shape is poor (see Figure 10.8). Poor peak shape can take different forms and includes peak tailing, peak fronting, broad peaks, and peak splitting. Each of these different forms will be dealt with individually. 

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