Plate Theory and Extensions - Retention Measurements on Close Eluting Peaks > Page 45


Figure 10. Errors Involved in Measuring Retention Data From Merging Peak Envelopes

The composite envelope is shown plotted over the envelope of each individual peak. The actual retention difference, if taken from the maxima of the envelope, will give a value of less than 80% of the true retention difference. In addition, as the peaks become closer this error  rapidly increases. Most data processing software do not take this type of error into account. Consequently, if such data was used for solute identification, or column design, the results may be in gross in error.

Another serious source of error can arise when two peaks are unresolved, and the retention time of the maximum of the envelope is taken as the mean retention time of the two individual solutes. This measurement can only be accurate if the peaks are absolutely symmetrical and the two peaks are of equal height. The result of different proportions of each isomer on the retention time of the composite envelope is shown in Figure 11. It is quite obvious that the position of the peak maximum of the composite envelope is very different from the mean retention time of the individual peaks.

Figure 11. A Composite Peak Formed by Two Closely Eluting Peaks of Different Size


Additionally, in the example given, the peaks were taken as truly Gaussian in shape. The peak maximum of the envelope are distorted to an even greater extent if the peaks are asymmetric. The retention time of a composite peak must never be assumed to have a specific relationship with those of the unresolved pair.