Constant holdup times in gas chromatography by programming of column temperature and inlet pressure.

The requirements for attaining constant holdup times in capillary gas chromatography (GC), while linearly programming both column temperature and inlet pressure, are illustrated. A scheme, which is based on a series of responses from isothermal and isobaric conditions, is proposed. The responses from two commonly used GC instruments, one with a mass-selective detector operating at near-vacuum pressure and another with a flame-ionization detector at ambient pressure, are analyzed. A significant deviation from Poiseuille flow is noted due to the use of helium as a carrier gas. Nonetheless, the experimental holdup times are approximately constant over a range of temperatures and pressures. Theoretical analysis reveals the spatial and temporal dependence of flow rates inside the column during the programmed runs.