EVALUATION OF FLOW RESTRICTORS FOR OPEN-TUBULAR SUPERCRITICAL-FLUID CHROMATOGRAPHY AT PRESSURES UP TO 560 ATM

Proper performance of the flow restrictor is of crucial importance in supercritical fluid chromatography. This is especially the case when t he restrictor is the interface between the chromatographic column and a detector operating at lower pressure than the column. The performanc e of three types of fixed flow restrictor, the ''tapered'', the ''inte gral'', and the ''frit'', was examined. The test mixtures chosen cover ed a range of polarities and molecular weights. Polydimethylsiloxane w ith an average molecular weight of 10,000 was used as a low polarity p robe.Two polyethylene glycol mixtures were chosen as higher polarity p robes. One consisted of a mixture of polyethylene glycol standards wit h average molecular weights ranging from 200 to 1450. The other polyet hylene glycol mixture had an average molecular weight of 2000. The pol ydimethylsiloxane standard did not elute when the frit restrictor was used. Satisfactory results were obtained when the tapered and integral restrictors were employed. All three restrictors provided satisfactor y results for polyethylene glycol oligomers eluting up to approximatel y 40.5 MPa (400 atm). However, for oligomers eluting above this pressu re, the peaks merged into a broad hump with the frit restrictor, while the tapered and integral restrictors provided satisfactory separation s.The integral restrictor produced more evenly spaced, more well resol ved peaks at the high-pressure end of the chromatograms than did the t apered restrictor. This is shown to be due to differences in the rates of increase in mobile-phase velocity with increasing pressure for the two restrictors.