Flow regime at ambient outlet pressure and its influence in comprehensive two-dimensional gas chromatography

With method development in one-dimensional GC already being a tedious task, developing GC × GC methods is even more laborious. The majority of the present GC × GC applications are derived from previously optimised 1D-GC methods, from which especially the carrier gas flow settings are copied. However, in view of the high pressure inside the first-dimension column (high flow resistance of the narrow-bore second-dimension column), diffusion in the first column is much slower than in 1D-GC. Proper optimisation of the column combination and the carrier gas flow can considerably improve separations in GC × GC. To assist in the process of selecting column dimensions and flow rate optimization, we have developed a computer programme, based on Excel ®, that enables quick and simple calculation for all types of column combinations. The programme merely needs column dimensions and carrier gas type as input parameters and calculates all resolution and velocity parameters of the GC × GC separation by using flow rate and plate height equations. From the calculations a number of interesting conclusions can be drawn. As an example, the calculations clearly show that the majority of column combinations reported up till now have been operated at a far from optimal flow – and, consequently, a far from optimal resolution. Probably even more important is the conclusion that the majority of column combinations used so far, i.e. those with 100 μm I.D. second-dimension columns, are not necessarily the best choice for GC × GC.