Comprehensive two-dimensional gas chromatography using direct flow modulation to extend the secondary dimension...

A simple direct flow modulator has been constructed and evaluated. This modulator employs a tee to connect the primary and secondary columns so that a negative pulsed flow can be introduced through the third leg of the connector into the joint between the columns. The modulation gas pressure can be adjusted to allow a longer recovery time for the primary column flow after the reversal initiated at the beginning of a modulation period, resulting in a longer secondary separation time. In this study, a 120 s secondary separation time has been achieved. That is long enough for use with a longer secondary column and secondary temperature ramping. Factors which affect the secondary peak width at base (²W_b), including modulation pulse width, modulation period, and inlet/modulation pressure differential, have also been investigated. Our results indicate that all three of these factors have a large impact on ²W_b, since they all affect the amount of primary effluent loaded onto the secondary dimension. Various sizes of tee connectors - 360 µm, 1/32″, and 1/16″ - have been investigated as well. Out of those three, the 360 µm tee gives the best performance in terms of less peak tailing and narrower ²W_b. The time needed to switch from one dimension GC to GC×GC for this method is discussed, and several methods to shorten the waiting time have been proposed. Applications of this method on gasoline and reference gas oil standard have been demonstrated.