Supercritical fluid chromatography is a relatively new technique as an instrumental method. It is a hybrid of liquid chromatography and gas chromatography. This method is used to analyze classes of compounds that are not readily amenable to either GC or LC. These compounds include non volatile and thermally labile and contains no chromophore that can be used for spectrophoto detection.
Fatty acid content of natural products is typically determined by GC or HPLC technique which both involve chemical modification of the compounds. Here GC requires the derivatization of the fatty acids to methy esters , leaving doubt as the whether the esters found are naturally occuring or formed through the derivatization process. While SFC is capable of analyzing fatty acids without derivatization as well as the natural occuring methyl esters.
Critical temperature is the temperature above which a distinct liquid phase does not exist regardless of pressure, where liquid and vapour have same density and no phase change occurs above this point.
Definition
When a substance is brought above a particular temperature and pressure both liquid and gas phase disappears and a new state generated which is called supercritical fluid. In SFC the mobile phase is supercritical fluid which is above its critical temperature and critical pressure. It has three properties - density, viscosity and refractive index which vary with temperature and pressure.
Mobile phase should have critical parameters that are easily reached using chromatographic pumps and ovens common to currently used instrumentation.
Advantages over carrier gases and liquid mobile phases:
- Solubility properties
- Physical properties
- Detector compatibility
In SFC mobile phase affinity for the analyte is a function of mobile phase density. Density is controlled by controlling system pressure/temperature. Highly polar solvents are not easy to handle (high critical parameters and high reactivity)
Advantages against HPLC:
- Super critical fluids have low viscosity and hence faster analysis and less pressure drop across the column.
- Column length of 10 to 20 m are used.
- Can be used with a wide range of sensitivity detectors.
- Resolving power is 5X that of HPLC
- SCFs have solvating powers similar to liquid organic solvents, but with higher diffusivities, lower viscosity and lower surface tension.
- Since the solvating power can be adjusted by changing temperature and pressure separation of analytes from solvent is fast and easy.
- By adding modifiers to SCF its polarity can be changed for having more selective separation power.
- In industrial process involving food or pharmaceuticals, on does have to worry about solvent residuals as you would if a "typical" organic solvent were used.
- SCFs are generally cheap, simple and many are safe. Disposal cost are much less and in industrial processes the fluids can be simple to recycle.
Important properties:
- Remarkable ability to dissolve large, non-volatile molecules.
- Dissolved analytes are easily recovered.
- No need for organic solvents and hence environment friendly.
- Inexpensive, Innocuous and non-toxic.
- Higher diffusion coefficient and lower viscosity relative to liquids faster and higher resolution separation.
Main Components of SFC.
- Column
- Mobile Phase
- Injection
- Pump
- Detector
- Column : Two types of column are used here First is Capillary column of fused silica coated with cross-linked chemical bonded stationary phase that are used in GC. Second is Packed column developed for high pressure liquid chromatography are being used with SFC.
- CO2 is the mobile phase used in SFC.
- Injection: For packed SFC a typical LC injection valve is commonly used and for capillary SFC pneumatically driven are used.
- SFC Pump: For packed column reciprocating pumps are generally used while in capillary SFC syringe pumps are used.
- SFC Detectors: Optical detectors , flame detectors and spectroscopic detectors can be used.
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