Question 1

What makes supercritical fluid chromatography different from gas and liquid chromatography?

Accepted Answer

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.

Question 2

How are supercritical fluids created and what properties do they have?

Accepted Answer

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.

Question 3

What types of columns can be used in supercritical fluid chromatography?

Accepted Answer

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.

Question 4

What instrumentation is required for supercritical fluid chromatography?

Accepted Answer

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.

Question 5

How do organic modifiers expand the applications of supercritical fluid chromatography?

Accepted Answer

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.

Question 6

What compounds and materials can be analyzed using supercritical fluid chromatography?

Accepted Answer

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.

Question 7

Why is carbon dioxide preferred as the mobile phase in supercritical fluid chromatography?

Accepted Answer

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.

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