The previous installment of “GC Connections” (1) discussed the effects of peak-detection choices on gas chromatography (GC) peak quantification. For situations with less than ideal peak resolution—where resolution (Rs) is less than 2.0, and especially when the peaks are of unequal size—it becomes easy to choose peak detection settings that may compromise peak size-measurement accuracy and repeatability. While proper peak detection will help remedy such a situation, sometimes no amount of data system tweaking will provide the desired improvements. In most cases it is better to increase peak resolution, thereby allowing the data system to perform as expected.
Peak resolution is the difference between the retention times (tR) of a pair of adjacent peaks, divided by their average widths at base (wb). In a practical sense, the peak width at half-height—which is 1.699 times smaller than the width at base—usually is employed for the calculation. If the two peaks’ widths are not too different then the width of the second peak can be substituted for the average width:
This classical resolution equation is predicated upon the assumption that the peaks are Gaussian, symmetrical, and equal in size. In that case, “baseline” resolution of 1.5 will occur when the amount of overlap between the two approaches about 0.15%. Any data handling system can easily measure such a pair accurately and repeatably. However, real-world peaks are non-Gaussian, asymmetrical, and often of significantly different sizes. Resolution as measured by equation 1 will exaggerate the quality of the separation. This approach can cause much greater inaccuracies and worse precision than expected for peaks that apparently measure as baseline-resolved. These effects were detailed in a previous “GC Connections” series (2–4). Overall, it is better to strive for higher as-measured resolution beyond the baseline level for a couple of reasons. First, the excess resolution will yield more-accurate and more-repeatable peak measurements and second, it will provide a bit of a buffer for eventual degradation of the separation as the column ages.
I remember using early gas chromatographs that had actual knobs for adjustment of the carrier-gas pressure, oven temperature, and detector electronics. Figure 1 shows a Fisher-Gulf Partitioner GC system from circa1955. This is the first kind of GC system that I was exposed to as an undergraduate in the 1970s. These days I no longer have as many GC knobs to adjust in my lab, really only the tank pressure regulators, but I still have a mental map between GC parameters on a computer screen and these first-generation commercial instruments. Turning a pressure controller knob and feeling its smooth rotational resistance is not the same as dragging an on-screen slider, but eventually an enterprising software developer will close the circle by creating a virtual reality GC control system with haptic feedback that appears something like Figure 1.
Read more: Improving GC Performance Systematically