Dynamic pressure gradient modulation for comprehensive two-dimensional gas chromatography.

We report the discovery, preliminary investigation, and demonstration of a novel form of differential flow modulation for comprehensive two-dimensional (2D) gas chromatography (GC×GC). Commercially available components are used to apply a flow of carrier gas with a suitable applied auxiliary gas pressure (P) to a T-junction joining the first (D) and second (D) dimension columns. The D eluate is confined at the T-junction, and introduced for D separation with a cyclic rhythm, dependent upon the relationship of the modulation period (P) to the pulse width (p), where p is defined as the time interval when the auxiliary gas flow at the T-junction is off. We refer to this flow modulation technique as "dynamic pressure gradient modulation" (DPGM) since a pressure gradient oscillates with the P along the D and D column ensemble providing temporary stop-flow conditions and fast D flow rates, resulting in 100% duty cycle and full modulation. A 90-component test mixture was used to evaluate the technique with a p of 60 ms and a P of 750 ms. The resulting peaks were narrow, with W ranging from about 20-180 ms. With an average W of 3 s and a n of 10, a 2D peak capacity, n, for the 25 min separation was 5000. The detector response enhancement factor (DREF) is reported, defined as the peak height of the highest modulated D peak divided by the unmodulated D peak height (DREF = h/h). The DREF ranged from about 7-87, depending on the W and W for a given analyte. A diesel sample was analyzed to demonstrate performance with a complex sample. Based upon the average W of 5 s and an average W of 168 ms, a n of 8640 was obtained for the 60 min diesel separation. Finally, the modulation principle was investigated as a function of P, p, and the volumetric flow rates, F and F. The measured W correlate well with the theoretical D injected width, given by W = (F/F) ·P. However, the relevant F appears to be dictated by the D flow rate when no pressure is applied (during the p interval), instead of F being the average flow rate on D (defined by the D dead time). The findings provide strong evidence for a differential flow modulation mechanism.