Temperature Programming for High-Speed GC

Fast temperature programming (20-50 °C/min) is used with relatively short separation columns to achieve high-speed separations of mixtures covering a wide boiling point range. A cryofocusing inlet is used to obtain narrow injection plugs. High-speed temperature-programmed chromatograms are evaluated by considering local peak capacity as a function of carbon number and boiling point for the normal alkanes in the range C(8)-C(19). The peak capacity generation rate (peaks per second) as a function of carbon number and the total cumulative peak capacity as a function of time are also considered for various column lengths and carrier gas flow rates. Column lengths in the range 3.6-25.4 m and average carrier gas velocity values in the range 50-200 cm/s are considered. For a 6.8-m-long, 0.25-mm-i.d. column operated at an average carrier gas velocity of about 100 cm/s and using a nominal programming rate of 50 °C/min, C(19) elutes in 178 s with a total peak capacity of 168 peaks. If the programming rate is reduced to 20 °C/min, the C(19) elution time more than doubles but the total peak capacity increases by only 20%. For a 25.4-m-long column using a nominal 50 °C/min programming rate, the C(19) retention time is 262 s with a peak capacity of 279 peaks. The use of average carrier gas flow rates greater than about 100 cm/s, which is common in isothermal high-speed GC, results in a considerable loss in total peak capacity with remarkably little reduction in analysis time.