Plate Theory and Extensions - Quantitative Analysis from Retention Measurements > Page 50

From equation (44), it is clear that when only (A) is present, the function will be at a maximum when t = tA  and when only solute (B) is present, it will be at a maximum when t = tB. Thus, the composite curve maxima will range from (tA) and (tB) for different proportions of solutes (A) and (B). It is see that, from the value for the retention time of the composite peak, the composition of the original mixture

can be determined. For closely eluted peaks nA = nB and  and  are, in effect, average dilution factors resulting from the peak dispersion and can be replaced by a constant. The efficiencies (n(A)) and (n(B)) in the exponent function, however, will only be equal if the peaks are symmetrical. This is because the rear part of the first peak, merges with the front part of the second peak. In LC, peaks are rarely symmetrical  thus, (n(A)) will represent the efficiency of the rear half of the first peak (solute A) and (n(B)) the efficiency of the front half of the second peak (solute B). In practice, the efficiencies are calculated in the normal way except,  twice the width of the front half is used and twice the width of the rear half of the peak instead of the total peak width. The extent to which (n(A)) and (n(B)) will differ will depend on the quality of the column and the chromatographic system. Well-packed columns should give similar efficiencies for the two halves of the peak but the front half is most often slightly more efficient than the rear half of the peak. The response of the detector to the specific solutes must also be taken into account. Thus, if (D) is the voltage output from the detector, equation (44) can be put in the form