van Deemter Equation. Gas chromatography is an analytical technique that permits the separation and quantitation of complex mixtures. The mixture flows through a chromatographic column in a stream of carrier gas (usually helium), where the components separate and are detected. In the analysis of a sample of gasoline, for example, the components are separated based on their volatility, the lowest-boiling emerging from the separation column first. The degree of separation can be treated mathematically in the same way as for fractional distillation: a column can be considered to have a number of theoretical plates, just as a distillation tower in a refinery has actual “plates” for the separation of different petroleum products (naphtha, gasoline, diesel fuel, etc.). For gas chromatography, separation efficiency is usually expressed in terms of HETP (Height Equivalent to a Theoretical Plate), the column length divided by the number of theoretical plates. Separation efficiency is a function of the carrier gas flow rate v, as shown in the following figure. There is an optimum flow rate that provides the smallest HETP; too fast and there is not sufficient time for equilibration, too slow and gaseous diffusion allows the components to re-mix. The van Deemter Equation describes the relationship between HETP and carrier gas flow rate:
where v = carrier gas flow velocity. The data in Table 14-5 (also on the CD) shows measurements of HETP for a gas chromatographic column, using different flow rates.
Table 14-5
