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Pinning Down Tailing Peaks, Part II: Quantitative Measurement
Jul 1, 2009
By: John V. Hinshaw
LCGC North America

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In the world of practical chromatography, peak shapes only approach an ideal Gaussian statistical distribution. Solute band shapes are perturbed by extracolumn volumes, nonideal retention processes, and strong adsorptive interactions as they move from inlet to column to detector. The first installment in this series discussed some reasons for the appearance of distorted peak shapes in gas chromatography (GC) and presented various ways of expressing the degree of peak tailing (1). Beyond just cosmetic appearance, however, chromatographers have good reasons for concern over the effects of peak tailing: distorted peak shapes affect the measurement of peak height and area, and they degrade the resolution of adjacent solutes. The extent of these effects depends not only upon the degree of peak tailing but also on the size of peaks relative to the noise level and baseline drift, as well as in relation to each other. Significant peak tailing increases uncertainties due to noise and drift; even large isolated peaks can be misquantified when they tail. This "GC Connections" installment discusses the effects of tailing on the quantitative measurement of single peaks.


John V. Hinshaw
Of course, few peaks stand alone in complete resolution from their neighbors. The determination of the size of partially merged symmetrical peaks is a difficult task that is complicated by disparities in peak sizes as well as by peak tailing. Selective response such as that provided by diode-array detection (DAD) or mass-selective detection (MSD) can deconvolve merged peaks, but when provided with nonselective detection methods analysts must resort to baseline approximations. This will be the subject of a subsequent installment.

Which Peak Tailing Measurement to Use?




The previous installment (1) presented several peak tailing measurements: the ratio of peak back to front widths at a percentage of the peak height, a related pharmacopoeial measurement, the ratio of the back and front peak areas, and some statistical determinations. Back to front ratios are the simplest and most widely recognized expressions of peak asymmetry — this metric, taken at 10% of the peak height, is utilized in this installment.

For the interested reader, Ettre wrote about the origins and significance of various peak tailing measurements in LCGC in 2003 (2).

Measuring the Size of Tailing Peaks


Figure 1: Effect of peak tailing on baseline allocations: (a) asymmetry = 1.1; (b) asymmetry = 1.4; (c) asymmetry = 2.0; (d) asymmetry = 2.5; and (e) asymmetry = 3.0. The green "B" symbol indicates baseline start and the orange "B" baseline stop. The baseline is drawn over the peak in blue. Inset: Peak from (e) expanded 20 times in the y-direction to show missing area in gray. The vertical dashed green line shows how the peak apex shifts as tailing changes compared to the symmetrical peak in (a).
A 2004 "GC Connections" installment (3) discussed the basics of peak detection with chromatography data systems. In that article, synthetic generated peak areas were measured as a function of the size of the peaks in relation to their minimum detectable quantities (MDQ) and the noise level for peaks with constant asymmetries of 1.5, which were intended to model an average amount of peak tailing. This month's "GC Connections" extends the measurement of peak areas across a range of peak asymmetries in another paper experiment.


Figure 2: Effect of peak asymmetry on measured area counts for the six peaks in Figure 1.
The determination of peak starting and ending baseline points for integration and height measurements is affected by peak tailing. Figure 1 illustrates what happens when peak tailing increases with a constant baseline — no noise, no drift. In Figure 1e, an inset shows the peak magnified by 20 times in the y-direction. Peak detection found the end of the peak while a significant portion had yet to be eluted, so that a small area, filled in gray, is missing from the peak area measurement that spans the base points. Figure 2 is a graph of the measured peak area as a function of the peak asymmetry: at an asymmetry of 3.0 the measured peak area has decreased by 800 ľV-s, or a little more than 5%, compared with a symmetrical peak.

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Source: LCGC North America, 7/1/2009
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