What is Tailing factor ?
Before going to the topic – Allow me to briefly address the topic of an Ideal peak, often known as an Ideal Chromatography Peak.
A symmetrical peak is defined as having two equal halves. This makes it an Ideal Peak.
A peak can deviate from this ideal in several different ways:
It can become Asymmetrical, flatten, and become broader (or) baseline can rise.
Peak tailing:
If the peak were split into two vertically, the latter half (B) would be wider than the first half (A) of the peak. This effect is most clearly seen close to the baseline and is known as Peak tailing.
The quality of separation and analytical data can be affected by peak tailing.
There are two main methods for defining peak tailing.
1. Tailing factor– widely used in the pharmaceutical industry.
-Let a and b be the peak half widths at 5% of the peak height, a is the front half-width, b is the back half
Tf = (a+b)/2a
2. Asymmetry factor(As) – used in other industries
Let a and b the peak half widths, but at 10% of peak height.
As= b/a
Acceptable Tailing:
Asymmetry factor (or) Tailing factor is an acceptable value between 0.9 -2.0 depending on the separation and resolutions of peaks.
Large Asymmetry factor value might have a reduced peak height. This affects the analysis when wide ranges of different concentrations are used.
How to avoid Peak tailing?
· Selection of the correct column and instrument parameters.
· Avoiding overloading of the column with a poor injection volume.
· Operate at a lower pH (mobile phase).
· Consider the possibility of mass overload.
· Working at high pH when analysing basic compounds.
· Use a sample clean-up procedure.
Peak Fronting:
Asymmetric peak having a wider front half of peak compared to back half is known as peak fronting.
If the tailing factor is less than 0.9- the peak is unacceptable due to peak fronting.
It occurs when the sample capacity of the analytical column is exceeded and channeling in a packed column.
How to avoid Peak fronting:
· Reduce the solute concentration in the sample
· Reduce the injection volume
· By using end caps during column storage and degassing of the mobile phase can help to prevent channel formation.
Resolution:
The resolution of a elute is a quantitative measure of how well two elution peaks can be differentiated in chromatographic separation.
It is defined as the difference in retention times between the two peaks (or) distance between the two peaks
Acceptable resolution:
Resolution is more than 1.5
Points to be noted:
· The most ideal peak resolution is aimed at As (Asymmetry factor) of 1.0
· Asymmetry factor is calculated at 10% of peak height.
· Tailing factor is calculated at 5% of peak height as per USP.