Peak Capacity and Selectivity in Gas Chromatography13 Apr 2016
I recently had the pleasure of hearing Professor Taduesz Gorecki from the University of Waterloo lecture on comprehensive two-dimensional gas chromatography (GCxGC). One of his chief areas of research is on the GCxGC modulator, which is essentially the "injector" for the second dimension column in this multidimensional chromatography approach. A twist on the usual process from Taduesz involved "stop-flow" modulation, where the column flow in the first dimension is halted while the injection and separation occur for the second dimension. Where the usual second dimension columns and separations in GCxGC are super short (sometimes L less than 1 m and time less than 2 sec) to preserve the first dimension separation, "stop-flow" modulation allows longer second dimension separations (and longer second dimension columns to improve them). The first and second dimension separations are more independent of each other than in traditional GCxGC.
GCxGC in general, and especially stop-flow GCxGC, generate very large peak capacities versus one-dimensional GC, up to an order-of-magnitude higher. But in his lecture Tadeusz reminded us that without good choices of stationary phase selectivity in both dimensions, that peak capacity won't be fully realized. I can easily demonstrate that with some recent work I did for an upcoming lecture at the 40th International Symposium on Capillary Chromatography and 13th GCxGC Symposium. Note in the figure below how the Stabilwax column in the second dimension pulls apart the diesel, placing the aromatic analytes further away (higher) from the aliphatic compounds (line of peaks at the bottom of the contour plot). However, the Rtx-200 (a trifluoropropylmethyl phase) does not have the necessary selectivity to allow full use of the 2D space. Never fear; I exploited that space later by analyzing an environmental sample containing PAHs, pesticides, PCBs, explosives, and priority pollutants. Sometimes it's all about having selectivity choices in GCxGC, and that's where Restek excels.