Advantages and disadvantages of supercritical fluid chromatography (SFC)
Even though high performance liquid chromatography (HPLC) is a widely used technique for extractions of analytes in many classes, SFC has clear advantages over it. In HPLC a substantial amount of organic solvent is generated with each extraction, which then needs to be disposed. However, the disposal of the organic solvents is expensive at $5 - $10 per gallon, whereas SFC uses considerably less or no organic solvent which leads to a decrease in analysis costs. In replacement of organic solvents an inert environmentally friendly mobile phase is used, often carbon dioxide, as it is energy efficient in the isolation of the desired products. Also without the use organic solvents the product is more concentrated compared to HPLC where the solvent must be evaporated, without the need to evaporate any solvent there is a reduction in energy and labour costs.
SFC is similar to gas chromatography (GC) in that it has a lower viscosity and higher diffusion coefficient than HPLC which allows for quicker, more efficient separations as it more effective at entering porous solid materials than liquid solvents. The separation time can be cut down from hours or days to a few tens of minutes. As seen in Table 1, supercritical fluids lie between liquids and gases, which allows for SFC to use features of both HPLC and GC.
Due to supercritical fluids having gas like and liquid like density it has a greater solvating power so SFC has a larger molecular range which includes non-volatile molecules which methods like GC do not include. Also, unlike GC which does not analyse thermally unstable compounds, SFC is able to due to the low critical temperatures of supercritical fluids such as carbon dioxide (31oC); an advantage of supercritical fluid carbon dioxide is that it has a varied solvating strength that allows for selective extractions. Along with this by altering the temperature and/or pressure it is possible to achieve higher selectivity.
The range of detectors is also wider for SFC compared to GC or HPLC this is because in SFC the mobile phase can be liquid or gas like, so GC and HPLC detectors can be used. For example SFC with flame ionization detection (FID) can provide quantification of resolved materials with a sensitivity of <= 0.1 ng. Due to the range of detectors available for SFC and the low critical temperature of the carbon dioxide mobile phase, the detection and analysis of thermally labile compounds has been successful.
Another advantage SFC has over HPLC is separation of chiral compounds, in HPLC the process is very time consuming, in SFC however, due to the lower viscosity of the supercritical fluids, the chiral separation can be run at a flow rate of up to 5 times faster than that of the HPLC all while avoiding pressure build up. The higher flow rate of SFC consequently means that the productivity is higher than HPLC methods.
When used in large scale extractions, fluid carbon dioxide can be recycled and then reused this minimize the amount of waste generated.
(0.6-2) x 10-3
Diffusion Coefficient cm2/s
(1-4) x 10-1
(0.2-2) x 10-5
Viscosity g cm-1 s-1
(1-3) x 10-4
(1-3) x 10-4
(0.2-3) x 10-2
Table 1: Comparison of Properties of Supercritical Fluids, Liquids and Gases
Due to the fact that SFC has features of both GC and HPLC, SFC has diversity in the columns that can be used which are either open tubular (GC) or packed (HPLC). The packed column SFC is often longer and more stacked which causes an increase in the number of theoretical plates (over 100,000).
Further advantage is SFC is very clean; mobile phase contaminants are usually trace quantities of other gases. The mobile phase is very free of dissolved oxygen and is not particularly reactive and the mobile phase is easily and rapidly removed.
However, there are some disadvantages of SFC these include that if molecules are highly polar they are not soluble in the mobile phase.
Usually SFC only moves a small amount of a large specimen onto the column
However, these limitations have been overcome through instrumental modifications that more appropriately address purifications of micro-scale and nano-scale quantities of physiological molecules. More sophisticated 2D systems (2D-SFC) allow for the interfacing of 2 SFC columns having different column coatings or packing and thus provide for orthogonal separation capabilities.
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