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

Abstract

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 (2Wb), 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 2Wb, 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 2Wb. 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.