The application of two total transfer valve modulators for comprehensive two-dimensional gas chromatography of volatile organic compounds

Rotary and diaphragm 6-port 2-position valves have been evaluated as modulators for comprehensive GC. A total transfer methodology was used such that all analyte materials from the primary column were passed to the second separating column. The transfer methodology used repeated short periods of stop flow, but retained good primary column performance since primary carrier flow was partially maintained as pressure equilibrated along the length of the first column. Highly resolved separations of complex samples such as petrol and essential oils were achieved with equivalent performance (in terms of second column resolution) to thermal modulation with flame ionisation detection (FID). The valve modulators tested could outperform thermal modulators for very highly volatile organic species since the modulation process was insensitive to analyte vapour pressure. The diaphragm valve tested showed the best performance and was incorporated with a thermal desorption system to provide a high sensitivity separation of volatile organic compounds (VOCs) in air with LODs for individual VOCs of 2-4 pptV for a 1-L air sample.