Peak sweeping and gating using thermal gradient gas chromatography.

When axial temperature gradients are applied in gas chromatography (GC), i.e., "thermal gradient GC" (TGGC), the temperature changes both in time and position, T(t,L), along the column, allowing unique control of the movement and elution of sample components. One method of performing TGGC involves introducing a sample into a column with a preset decreasing temperature gradient along its length, waiting for a short time until the sample separates along the gradient, and then raising the temperature to sweep all of the compounds out of the column and into the detector (i.e., "peak sweeping"). This method of operation is demonstrated here using a simple laboratory apparatus based on simultaneous resistive heating and convective cooling. An experimental comparison between isothermal GC (ITGC), temperature programmed GC (TPGC) and TGGC shows that TGGC is essentially equivalent in performance to TPGC operation when using the same column length (peak capacity production rate of 106, 381 and 469 min(-1), respectively); however, narrower peaks and higher signal-to-noise are achieved in TGGC. Furthermore, TGGC helps to minimize band broadening and peak tailing that arise from column adsorption and less than perfect sample injection. The low thermal mass of the TGGC system allows rapid column heating (4000°C/min) and cooling (3500°C/min) for selective separation (i.e., "peak gating") of compounds in a mixture without sacrificing the resolution of earlier or later eluting compounds.