GC019 Peak Shape and Width – Part 1

The ideal peak shape, or the distribution of the molecules of a compound, can be approximated as being normal or Gaussian. The presence of asymmetry in peaks are an indication of the presence of undesirable interactions during the chromatographic process. In packed columns, broad peaks are common and usually indicate that mass transfer kinetics are slow.

Asymmetric peaks can be classified as tailing or fronting depending on whether the peak seems to be compacted on the right side (fronting) or on the left side (tailing).

The tailing factor (TF) is used to evaluate the extent of the asymmetry. It is defined as

TF = b / a

Where a and b are the peak width at the left and at the right of the maximum intensity of the peak. a and b are usually measured at 10% of the peak height. US Pharmacopeia recommends 5% of the peak height and the following formula:

TF = (a+b) / 2a

Care must be taken when using tailing factor for column comparisons, because there are several common definitions.

If TF is greater than one, the peak is tailing, and a TF smaller than one means the peak is fronting.

Doublet peaks usually occur when a pair of solutes was not separated completely. However, the repeatability of the doublet peak should be verified, because there are other sources for this type of peaks, like injecting too much sample, a faulty injection or degraded columns.

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