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

Abstract The relationship between the phase ratio, β , of the primary ( 1 D) and secondary ( 2 D) 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, 1 β for 1 D and 2 β for 2 D. Six column sets were studied using common, commercially available β values. The β ratio, β R  =  1 β / 2 β , 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 2 D column, 2 k , and thereby changing the modulation period, P M , necessary for proper 2 D column separations. The necessary changes to P M modify the modulation ratio, M R , which affects the 1 D column peak widths and 1 n c due to the impact of undersampling. Through changes to 1 β , the range of 2 k can be controlled, with subsequent effects to both 2 n c and 1 n c . These effects were opposite in direction, such that improvements to 2 n c may result in declines in 1 n c . It is observed that due to the pseudo-isothermal nature of the 2 D separation, there are diminishing returns to extending the 2 n c at the cost of 1 n c . In this particular study, column set 3 ( 1 D: 20 m length, 250 μm i.d., 0.25 μm film; 2 D: 2 m, 180 μm i.d., 0.2 μm film; β R  = 1.11) with a P M of 3 s provided the highest theoretical n c,2D of ∼8200, though this was at a relatively low M R of ∼1.8. Column set 2 ( 1 D: 20 m length, 250 μm i.d., 0.5 μm film; 2 D: 2 m, 180 μm i.d., 0.2 μm film; β R  = 0.56) with a P M of 1.5 s 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 1 D column efficiency (i.e., narrowing the 1 D column peak widths) to improve 1 n c , can result in an increased theoretical n c,2D .