When a supercritical fluid is used as a chromatographic mobile phase, the resulting process, supercritical-fluid chromatography (SFC), has characteristics of both high-performance liquid chromatography (HPLC) and gas chromatography (GC)14. For example, mobile phase solvation in SFC allows relatively low-temperature elution of materials, but the gas-like viscosity of the mobile phase results in higher optimum velocities and shorter analysis times than when using liquids in the same columns.
Increased analysis speed is important, but is overshadowed by genuine, new capability. With COZ, NzO, NH3 and several other SFC mobile phases, it is possible to use a flame-ionization detector4-'. This provides the long-sought combination of a solvating mobile phase and sensitive, universal detection.
Many analysis problems fall between the capabilities of GC and HPLC. With GC, both volatility and thermal stability are required for a successful analysis. Although these are not required for HPLC, sensitive, universal detection is not possible today with liquid mobile phases. Unit1 the combination of SFC-flame-ionization detection (FID), there was no convenient, sensitive capability for determining low volatility or thermally labile analytes that do not strongly absorb light.
Since the mobile-phase pressure must be reduced at the SFC-column-flameionization detector interface, solutes can condense, often resulting in noisy, spiked speaks4+. This has limited the application of FID and other GC detection methods to SFC. Fjeldsted and co-workers 4,5, have been successful at reducing the consequences of spiking in their straight-walled, capillary restrictors by the use of electronic filtering. Rawdon has adapted a flame-ionization detector for packed-column SFC which is reported free of spiking to solute molecular weights of 2000'. Our attempts at producing similar, pinched restrictors for capillary SFC have not been successful. However, we have been successful in significantly reducing spiking with capillary restrictors. This has allowed an increase in the molecular-weight range of capillary SFC-FID.