Implications of phase ratio for maximizing peak capacity in comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry

The relationship between the phase ratio, β, of the primary (1D) and secondary (2D) separation dimensions of comprehensive two-dimensional (2D) gas chromatography (GC × GC) separations, and the implications of β on realization of maximal 2D peak capacity, nc,2D, are examined. A GC × GC chromatographic system with time-of-flight mass spectrometry, TOFMS, was otherwise held constant for the separation of a multi-component test mixture spanning a range of chemical functionalities, while only the β of the two analytical columns were changed, 1β for 1D and 2β for 2D. Six column sets were studied using common, commercially available β values. The β ratio, βR = 1β/2β, is defined as a quantitative metric to facilitate this study. It is demonstrated that βR plays a key role in maximizing nc,2D. Overall, βR substantially affected nc,2D by influencing retention factors on the 2D column, 2k, and thereby changing the modulation period, PM, necessary for proper 2D column separations. The necessary changes to PM modify the modulation ratio, MR, which affects the 1D column peak widths and 1nc due to the impact of undersampling. Through changes to 1β, the range of 2k can be controlled, with subsequent effects to both 2nc and 1nc. These effects were opposite in direction, such that improvements to 2nc may result in declines in 1nc. It is observed that due to the pseudo-isothermal nature of the 2D separation, there are diminishing returns to extending the 2nc at the cost of 1nc. In this particular study, column set 3 (1D: 20 m length, 250 μm i.d., 0.25 μm film; 2D: 2 m, 180 μm i.d., 0.2 μm film; βR = 1.11) with a PM of 3 s provided the highest theoretical nc,2D of ∼8200, though this was at a relatively low MR of ∼1.8. Column set 2 (1D: 20 m length, 250 μm i.d., 0.5 μm film; 2D: 2 m, 180 μm i.d., 0.2 μm film; βR = 0.56) with a PM of 1.5 s provided a high theoretical nc,2D of ∼5800, at a much higher MR of ∼3.7. Though column set 2 had a lesser total peak capacity than column set 3, its higher MR suggests that by improving the 1D column efficiency (i.e., narrowing the 1D column peak widths) to improve 1nc, can result in an increased theoretical nc,2D.