Asymmetry Factor As

In the elucidation of retention mechanisms, an advantage of using enantiomers as templates is that nonspecific binding, which affects both enantiomers equally, cancels out. Therefore the separation factor (a) uniquely reflects the contribution to binding from the enantioselectively imprinted sites. As an additional comparison the retention on the imprinted phase is compared with the retention on a nonimprinted reference phase. The efficiency of the separations is routinely characterized by estimating a number of theoretical plates (AO, a resolution factor (Rs) and a peak asymmetry factor (As) [19]. These quantities are affected by the quality of the packing and mass transfer limitations, as well as of the amount and distribution of the binding sites. 

Plate number N (as for Gaussian peaks) Resolution factor Rs Asymmetry factor As... 

Fig. 6-7. Asymmetry factor (As) of the L-enantiomer versus sample load (A) and versus flow rate (B) on L-PA-imprinted polymers. Flow rate 1.0 ml min-1. Mobile phase MeCN/[potassium phosphate 0.05 M, pH 7] (7/3, v/v).

The point at which a column used for HPLC will fail depends largely upon how the operator uses it. Eventually, however, all HPLC columns will fail. The onset of column failure can be monitored by two common failure parameters the peak asymmetry factor, As, and the peak tailing factor. These parameters are defined according to the figure below ... 

Peak Asymmetry Factor (As, 10%) Peak Tailing Factor (5%)... 

Asymmetry factor, Ai The plate number, N, assumes that the peak shape is Gaussian, but in practice this is rare. It is more hkely that the peak is asymmetrical, i.e. it tails . This is quantified using the asymmetry factor. As, calculated as shown in Fig. 31.7. 

Keeping records of column backpressure and important chromatographic parameters [number of theoretical plates (AO, peak asymmetry factor (As), retention factor k ), resolution factor (Rs)] helps to monitor the required column performance, while storage in the appropriate organic solvent extends column lifetime. Table 8 presents the preventive actions for column protection. 

Peak Symmetry Asymmetry Factor (As) and Tailing Factor (Tf) 24... 

Under ideal conditions, chromatographic peaks should have Gaussian peak shapes with perfect symmetry. In reality, most peaks are not perfectly symmetrical and can be either fronting or tailing (Figure 2.8). The asymmetry factor (As) is used to measure the degree of peak symmetry and is defined at peak width of 10% of peak height (W0.i). Note that Tf is used here instead of T, as in the USP, because T often stands for temperature. 

Most chromatographic peaks cannot be characterized with a perfect Gaussian distribution and some peaks are tailing, while others are fronting. Peak asymmetry can be measured as the asymmetry factor, at 10% of the peak height (Figure 1.5). Well-made columns are expected to have an asymmetry factor as close to 1 as possible. 

Under non optimal conditions, separations often yield non-symmetrical peaks, i.e., other than Gaussian in shape. Asymmetry can be measured in terms of the band asymmetry factor As, that is calculated from measures taken at 10% of the peak height (see Fig. 3) by dividing the width segment after the peak maximum projection and the width segment before the peak maximum projection. 

The clustering of 24 variables of 30 columns is depicted in Fig. 32. Two clusters can be discerned. The first cluster ( a ) includes k values, values for the specific surface area, and the carbon content. The other variables, e.g. the asymmetry factor (AS), the plate number (N), and the diameter of the pores are in the second cluster ( b ). 

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