Modulation in comprehensive two-dimensional gas chromatography: 20 years of innovation

With almost 20 years having passed since John B. Phillips described the first comprehensive two-dimensional gas chromatography (GC × GC) separation, much has occurred in this ever-expanding field of separation science. GC × GC is currently one of the most effective techniques for the separation and analysis of complex mixtures, offering significantly greater peak capacities than conventional chromatographic methods. The technique is generally based upon separations performed on two chromatographic columns characterized by considerably different selectivities, joined together through a modulating interface. The modulator periodically traps or samples the primary column effluent, usually refocuses it into a narrow chromatographic band and injects the focused fraction into the secondary column. The modulator is often referred to as the 'heart' of the instrument, since a GC × GC separation is impossible without its use. This article reviews major innovations in GC × GC modulator development since its first use by Phillips in 1991. Emphasis has been placed on modulator design and function.