Dynamic pressure gradient modulation for comprehensive two-dimensional gas chromatography

Abstract

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 (Paux) to a T-junction joining the first (1D) and second (2D) dimension columns. The 1D eluate is confined at the T-junction, and introduced for 2D separation with a cyclic rhythm, dependent upon the relationship of the modulation period (PM) to the pulse width (pw), where pw 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 PM along the 1D and 2D column ensemble providing temporary stop-flow conditions and fast 2D flow rates, resulting in 100% duty cycle and full modulation. A 90-component test mixture was used to evaluate the technique with a pw of 60 ms and a PM of 750 ms. The resulting peaks were narrow, with 2Wb ranging from about 20–180 ms. With an average 1Wb of 3 s and a 2nc of 10, a 2D peak capacity, nc,2D, for the 25 min separation was 5000. The detector response enhancement factor (DREF) is reported, defined as the peak height of the highest modulated 2D peak divided by the unmodulated 1D peak height (DREF = 2h/1h). The DREF ranged from about 7–87, depending on the 1Wb and 2Wb for a given analyte. A diesel sample was analyzed to demonstrate performance with a complex sample. Based upon the average 1Wb of 5 s and an average 2Wb of 168 ms, a nc,2D of 8640 was obtained for the 60 min diesel separation. Finally, the modulation principle was investigated as a function of PM, pw, and the volumetric flow rates, 1F and 2F. The measured 2Wb correlate well with the theoretical 2D injected width, given by 2Winj = (1F/2F) ·PM. However, the relevant 1F appears to be dictated by the 1D flow rate when no pressure is applied (during the pw interval), instead of 1F being the average flow rate on 1D (defined by the 1D dead time). The findings provide strong evidence for a differential flow modulation mechanism.