Method to determine the true modulation ratio for comprehensive two-dimensional gas chromatography

Abstract

A new method is presented to determine the true modulation ratio, MR, from the measurable effective modulation ratio, MR*, in comprehensive two-dimensional gas chromatography, GC×GC, without the requirement for a detector at the end of the primary column. The method was developed through the investigation of modulator induced band broadening, as a function of 1Wb and the selected modulation period, PM, for simulated GC×GC data, by first defining primary column 1D peak(s) and simulating the modulation process. Gaussian curve fitting is used to model each modulated secondary column separation peaklet, 2D, in the unfolded GC×GC data to accurately determine the maxima of the peaklet distribution, followed by Gaussian curve fitting to the maxima to determine the effective 1D peak profile and width, 1Wb*. The relationship between 1Wb and 1Wb* is studied as a function of the effective modulation ratio, MR*, which is 1Wb* divided by PM, in order to determine the true modulation ratio, MR, which is 1Wb divided by PM. We explore how peak sampling phase (in-phase and out-of-phase) plays a role in the relationship between MR and MR*. Experimental validation of the simulated results is also provided, to span a range of commonly implemented conditions with typical 1Wb (2–4.5s) and PM (0.25–8s). Use of MR<2 significantly broadens the 1D peak (MR*≥1.2MR) corresponding to a loss in 1D peak capacity, 1nc≥20%. The new method relies upon mapping from MR* to MR, which is discussed in relation to peak capacity theories for GC×GC. It is found that optimizing 1nc in GC×GC requires that 1Wb is minimized and must be sampled with a sufficiently short PM (1–2s) to minimize modulator induced band broadening and a subsequent reduction in the effective 1D peak capacity.