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

A new method is presented to determine the true modulation ratio, M_R, from the measurable effective modulation ratio, M_R*, 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 ¹W_b and the selected modulation period, P_M, 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, ¹W_b*. The relationship between ¹W_b and ¹W_b* is studied as a function of the effective modulation ratio, M_R*, which is ¹W_b* divided by P_M, in order to determine the true modulation ratio, M_R, which is ¹W_b divided by P_M. We explore how peak sampling phase (in-phase and out-of-phase) plays a role in the relationship between M_R and M_R*. Experimental validation of the simulated results is also provided, to span a range of commonly implemented conditions with typical ¹W_b (2-4.5s) and P_M (0.25-8s). Use of M_R<2 significantly broadens the ¹D peak (M_R*≥1.2M_R) corresponding to a loss in ¹D peak capacity, ¹n_c≥20%. The new method relies upon mapping from M_R* to M_R, which is discussed in relation to peak capacity theories for GC×GC. It is found that optimizing ¹n_c in GC×GC requires that ¹W_b is minimized and must be sampled with a sufficiently short P_M (1-2s) to minimize modulator induced band broadening and a subsequent reduction in the effective ¹D peak capacity.