Supercritical Fluid Chromatography

Supercritical fluid chromatography uses a supercritical fluid mobile phase with properties between gas and liquid. Its viscosity resembles a gas while its density approaches a liquid, giving SFC faster separation than liquid chromatography but less longitudinal diffusion than gas chromatography. SFC also handles analytes difficult to volatilize, combining advantages of both techniques.

Supercritical fluids form by heating and pressurizing a substance above its critical temperature and pressure. Carbon dioxide is commonly used because its low critical conditions are easily achieved and maintained. The resulting fluid has gas-like viscosity, liquid-like density, and a diffusion coefficient intermediate between gases and liquids.

SFC employs both open tubular and packed columns. Packed columns, typically made of stainless steel, offer greater theoretical plates and handle larger sample volumes. Open tubular columns resemble fused-silica wall-coated columns. The low viscosity of supercritical media allows these columns to be longer than those used in high performance liquid chromatography.

SFC instruments resemble standard HPLC instruments but include a gas chromatography-like thermostatted column oven and a special pressure restrictor. The pressure restrictor maintains the mobile phase in its supercritical state. This hybrid design allows SFC to employ detectors from both GC and HPLC, and couples easily with mass spectrometry and FTIR.

Initially, SFC used only polar stationary phases, but organic modifiers like methanol have expanded its versatility. Modifiers allow adjustment of retention mechanisms by changing stationary and mobile phase compositions, enabling integration of numerous HPLC stationary phases into SFC. This flexibility makes SFC suitable for analyzing both nonpolar organics and polar solutes.

SFC analyzes compounds that gas chromatography or liquid chromatography cannot easily manage, including nonvolatile and thermally unstable substances. Applications include polymers, fossil fuels, waxes, drugs, and food products. SFC separates large molecules at relatively lower temperatures, increasing efficiency and enabling simplified coupling with analytical detectors.

Carbon dioxide is the most common supercritical fluid mobile phase because its critical temperature and pressure are low and easily achieved and maintained in laboratory instruments. CO2 provides the density and diffusion properties needed for efficient separation while remaining cost-effective and environmentally benign compared to other supercritical fluids.