ChromFAQ:Tailing

From ChromFAQ

What's the difference between Tailing Factor and Asymmetry Factor?


In principle, chromatographic peaks should be very close to a "Gaussian" or "bell-shape" (normal distribution) curve. In practice, most peaks show a certain amount of asymmetry. If the leading edge of the peak is shallower than the trailing edge, the phenomenon is referred to as "Fronting". If the trailing edge is shallower than the leading edge, it is referred to as "Tailing". Tailing is far more common than fronting; in fact, it can be argued that the majority of chromatographic peaks tail to some extent.


There are two "standard" ways to measure tailing or fronting, each defined by a different standards organization:



The Asymmetry Factor (ASF or As) is defined by IUPAC and by ASTM as ratio of the distance between the center of the peak and the trailing edge to the distance between the center of the peak and the leading edge, with both measurements made at 10% of the maximum peak height.

The Tailing Factor (TF or T) is defined by the USP as the ratio of the distance between the leading and the tailing edge of the peaks to twice the distance between the center of the peak and the leading edge, with both measurements made at 5% of the maximum peak height.

For pharmaceutical analyses, regulatory agencies suggest that that Tailing Factors should be less than 2.0 (see, for example, the US FDA Reviewer Guidance: Validation of Chromatographic Methods (http://www.fda.gov/cder/guidance/cmc3.pdf).

As a matter of personal (Tom Jupille) opinion, both ASF and TF are single measurements which provide only approximate information about peak shape; it is quite possible for two peaks to have the same Asymmetry Factor (or Tailing Factor) and have quite different shapes, depending on the cause of the tailing. Both parameters date back to the "strip chart recorder" days of chromatography; they are distinguished by being relatively easy to measure with a pencil and ruler.

Both measurements represent peak shape qualitatively the same way: a perfectly symmetrical peak has ASF and TF both equal to 1.0; fronting peaks have ASF and TF less than 1.0; tailing peaks have ASF and TF greater than 1.0. Because the measurements are made at different places on the peak, however, (10% vs. 5% of peak height), it is impossible to exactly calculate one given a value of the other. Within the range of commonly encountered values for tailing peaks, ASF and TF will generally be similar, with the Asymmetry Factor typically being 10 - 20% or so higher than the Tailing Factor.