Supercritical fluid chromatography (SFC) uses a supercritical fluid — a substance above its critical temperature and critical pressure — as the mobile phase. Carbon dioxide (CO₂) is the most widely used fluid (T_c = 31.1°C, P_c = 73.8 bar) because it is non-toxic, non-flammable, chemically inert, and readily available. As a supercritical fluid, CO₂ possesses liquid-like density (providing solvating power) and gas-like viscosity and diffusivity (enabling high flow rates with low backpressure and fast mass transfer), resulting in separations that are both faster and more efficient than HPLC.
SFC instrumentation includes a pump capable of delivering CO₂ in the liquid state (usually cooled), a modifier pump for adding organic co-solvents, an injector, a column oven, a back-pressure regulator (BPR) to maintain system pressure, and a detector. The BPR must keep the mobile phase supercritical throughout the column and then allow depressurization before detection. Modifiers such as methanol, ethanol, or acetonitrile (typically 1-40%) are added to increase the solvent strength of CO₂, which alone is non-polar and insufficient for eluting polar analytes. The addition of water or acidic/basic additives can further extend the polarity range.
A key advantage of SFC is its compatibility with a wide range of detectors. UV/Vis detection is the most common, but SFC interfaces naturally with mass spectrometry (MS) because the CO₂ mobile phase is volatile and easily removed, reducing background compared to HPLC-MS. Flame ionization detection (FID) is possible when CO₂ is used without organic modifiers (unlike HPLC), making SFC-FID a powerful tool for quantifying non-UV-absorbing compounds such as lipids and hydrocarbons. Chiral separations are particularly successful in SFC because the low-viscosity mobile phase allows use of long chiral columns with high efficiency, and the solvation properties of CO₂ often enhance chiral recognition.
Compared to HPLC, SFC offers shorter analysis times, lower solvent consumption (reducing cost and environmental impact), and faster column equilibration. Compared to GC, SFC can analyze thermally labile and non-volatile compounds without derivatization. SFC is widely used in the pharmaceutical industry for chiral separations, purity analysis, and flash chromatography purification. In the natural products field, SFC separates lipids, terpenes, and essential oils. The technique has become the method of choice for preparative chiral separations, where the ease of solvent removal from CO₂ greatly simplifies product isolation.
