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

- Modulation induced band broadening for GC × GC is studied.
- True modulation ratio is determined from the measurable effective modulation ratio.
- Effective primary separation peak widths are measured with Gaussian curve fitting.
- Modulation induced loss of primary separation peak capacity is calculated.
- Modulation periods of ∼1 to 2 s minimize broadening of primary separation peaks.

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 (2-4.5 s) and PM (0.25-8 s). Use of MR < 2 significantly broadens the 1D peak (MR* ≥ 1.2 MR) 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 (1-2 s) to minimize modulator induced band broadening and a subsequent reduction in the effective 1D peak capacity.