Peak Tailing Asymmetry

A symmetrical peak would have an asymmetry factor equal to 1.0. Most column manufacturers would consider a peak tailing factor of 0.9-1.2 to be acceptable. An unsymmetrical peak (Aj 1.2 or A 0.9) can have a detrimental effect on resolution and on the calculation of peak area for quantitative analysis. Steps can be taken to reduce peak tailing and these will be discussed further in Chapter 10. 

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The area of a peak is the integration of the peak height (concentration) with respect to time (volume flow of mobile phase) and thus is proportional to the total mass of solute eluted. Measurement of peak area accommodates peak asymmetry and even peak tailing without compromising the simple relationship between peak area and mass. Consequently, peak area measurements give more accurate results under conditions where the chromatography is not perfect and the peak profiles not truly Gaussian or Poisson. 

Determining peak-asymmetry and peak-tailing factors. Peak asymmetry = B/A, and peak tailing factor = (A + B)/2A. 

Fig. 5 Electropherogram of six sulfur anions illustrating peak fronting [thiosulfate (S2Ol-)] and peak tailing [tetrathionate (S4Ol-)] for better visibility of peak asymmetry, the perpendicular peak axis is drawn as a solid line. Separation in 20 mM chromate run buffer at pH 8.3.

Tailing. Asymmetry of a peak such that, relative to the baseline, the front is steeper than the rear. 

The point at which a column used for HPLC will fail depends largely upon how the operator uses it. Eventually, however, all HPLC columns will fail. The onset of column failure can be monitored by two common failure parameters the peak asymmetry factor, As, and the peak tailing factor. These parameters are defined according to the figure below ... 

Peak Asymmetry Factor (As, 10%) Peak Tailing Factor (5%)... 

To understand the role of the surface silanols and their contribution to retention, these compounds were separated on a column that was purposely coated to half of the Cjg level of the original Clg (Fig. 5-21b) (coated column had approximately 50% silanol content). If peak tailing is due solely to the silanol interaction, the peak symmetry should be worse than on the fully coated column. But there was no decrease in peak symmetry. In fact, retention of the tetracaine (peak 4) increased slightly while the peak symmetry improved. Thus, it appears that silanols do contribute to retention but the amount of silanols is not the main cause of peak asymmetry. The logical extension of this approach is to separate the compounds on unbonded silica gel, as shown in Figure 5-27c. In this situation, peak symmetry is quite good and retention is decreased. 

Peak Tailing (Peak Asymmetry Factor > 1.2). This is attributed to the wrong pH value, wrong column, wrong sample solvent (mobile phase is better to be used), void volumes at column inlet (the column may need repacking), as well as to active sites within the column which can be solved with the use of a competing basic or acidic modifier. 

Fig. 2 (A) Measurement of peak tailing using USP tailing factor and peak asymmetry factor. (B) Peak resolution. (From Merck...

A second calculation method for peak asymmetry can be found in the different pharmacopoeias, e.g. United States Pharmacopoeia (USP) or Pharmacopoeia Europea (PhEUR). These pharmacopoeias calculate the peak tailing at 5% of the peak height from the baseline and are therefore more stringent (Eq. 2.17b). 

Peak Symmetry Asymmetry Factor (As) and Tailing Factor (Tf) 24... 

Peak asymmetry is a common practical measure of the quality of a column. As columns age, the peak asymmetry usually deteriorates thus, one observes tailing peaks. Peak tailing is undesirable, since it can affect the quality of peak integration, especially when the ignal-to-noise ratio is low or when peaks are only partially resolved. This tailing can have multiple causes, as will be discu ed in later sections. 

There are several measures of peak tailing. A common one is the ratio of the width of the tail of the peak to the width of the front of the peak at either 5 or 10% of the height of the peak (see Fig. 2.3 for an illustration of the measurement of the various asymmetry factors) ... 

Acceptable levels of peak tailing should be indicated in the system suitability section of the method as peak asymmetry or tailing factor. Once the limits have been exceeded, a number of basic checks can be carried out that can assist in the diagnosis. A lot of problems of this nature are found to be due to analyst error, and the mobile phase composition and column choice should always be checked as a matter of course. 

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