Implications of phase ratio for maximizing peak capacity in comprehensive two-dimensional gas chromatography...

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, n_c,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, β_R=¹β/²β, is defined as a quantitative metric to facilitate this study. It is demonstrated that β_R plays a key role in maximizing n_c,2D. Overall, β_R substantially affected n_c,2D by influencing retention factors on the ²D column, ²k, and thereby changing the modulation period, P_M, necessary for proper ²D column separations. The necessary changes to P_M modify the modulation ratio, M_R, which affects the ¹D column peak widths and ¹n_c due to the impact of undersampling. Through changes to ¹β, the range of ²k can be controlled, with subsequent effects to both ²n_c and ¹n_c. These effects were opposite in direction, such that improvements to ²n_c may result in declines in ¹n_c. It is observed that due to the pseudo-isothermal nature of the ²D separation, there are diminishing returns to extending the ²n_c at the cost of ¹n_c. In this particular study, column set 3 (¹D: 20m length, 250μm i.d., 0.25μm film; ²D: 2m, 180μm i.d., 0.2μm film; β_R=1.11) with a P_M of 3s provided the highest theoretical n_c,2D of ∼8200, though this was at a relatively low M_R of ∼1.8. Column set 2 (¹D: 20m length, 250μm i.d., 0.5μm film; ²D: 2m, 180μm i.d., 0.2μm film; β_R=0.56) with a P_M of 1.5s provided a high theoretical n_c,2D of ∼5800, at a much higher M_R of ∼3.7. Though column set 2 had a lesser total peak capacity than column set 3, its higher M_R suggests that by improving the ¹D column efficiency (i.e., narrowing the ¹D column peak widths) to improve ¹n_c, can result in an increased theoretical n_c,2D.