Greening Analysis with SFC

Supercritical Fluid Chromatography (SFC) was first applied in 1962 using exotic supercritical fluids for a very specific application. Back then it was certainly not green; it used fluoro-chlorinated organic solvents that are now banned because of their negative impact on the atmospheric ozone layer.

Today, the great majority of SFC applications use carbon dioxide (CO₂) as the supercritical fluid and most users implicitly mean SFC with CO₂ when they refer to the technique. There is also a debate about the correctness of the term SFC, as – in practice – conditions are probably often subcritical rather than supercritical (see Tea With Pat Sandra: tas.txp.to/0415/patsandra); however, this debate does not affect the green aspects of the technique.

Although CO₂-SFC has been used for decades, it is only recently being recognized as a real alternative to classical chromatography (normal or reversed phase). There are several reasons why the technique has not had a smooth ride over the last 30 years, among them: (i) the lack of robustness of the instruments, (ii) the cost of CO₂ delivery infrastructure, (iii) the limited interest in environmental considerations, and (iv) regulatory constraints in some countries.

Now that our society is becoming more aware of environmental concerns – global warming, wasting energy, the production of greenhouse gases – there is a consensus that we must reduce our reliance on combustible fuels of fossil origin and protect our environment. SFC can contribute to this objective even though it may seem paradoxical given that modern SFC does not seem green because it uses CO₂ as the mobile phase. But actually, SFC does not produce CO₂ – it makes use of available CO₂ which is a byproduct of various industrial chemical and biological processes.

The re-emergence of SFC was driven by the particular application of chiral separations, which are mostly performed under normal phase conditions. The switch was relatively easy; SFC is also a normal phase mode of chromatography and most solvents used in chiral HPLC are alkanes, which have similar chromatographic and physical properties to those of supercritical CO₂. The switch has allowed users to diminish the amount of organic solvent used for chromatographic chiral separations by about 60 percent, not only reducing the direct emission of organic solvent into the atmosphere but also reducing the amount of organic waste that has to be burnt, producing additional CO₂. Moreover, for preparative applications, in which the amount of organic solvent to be evaporated is considerably less, the energy for evaporation of the fractions is significantly reduced. In this respect, CO₂-SFC can be considered as a successful example of ‘green switching’.

The argument has motivated interest in the application of SFC as an alternative to reversed phase chromatography (RP-HPLC), which uses acetonitrile as the most common organic solvent component. In reality, for reversed phase chromatography, there were additional factors, including the high toxicity of acetonitrile, which rapidly metabolizes to cyanhydric acid after inhalation or skin penetration. It has now been demonstrated that SFC can also replace RP-HPLC in about 75 percent of the applications related to small molecule purification. The application range has dramatically expanded within the last two years and now covers drug molecules and intermediates, natural products, metabolites, small linear and cyclo-peptides, pesticides, lipids, fatty acids, carbohydrates, steroids, hormones...

The application diversity is rapidly growing for analytical purposes in classical small molecule analysis, such as drug analysis, bioanalysis, drug abuse, food and perfume industry. It goes without saying that this green switch has been made possible thanks to the high dedication of a number of instrument manufacturers.

SFC use at the preparative scale for purification is also attracting more and more attention. Even though the green impact of the switch is smaller for RP-HPLC compared with normal phase applications (the amount of organic solvent is only partially reduced because acetonitrile is replaced with methanol), it still consumes about 20 percent less organic solvents on average. Moreover, in RP-HPLC preparative applications, the evaporation of aqueous fractions by lyophilization is an energy consuming process that requires about seven times more energy compared to fractions produced by the SFC approach.

Times have changed and, wherever possible, SFC should be the preferred technique considering the incontestable environmental advantages and cost benefits. SFC is unlikely to become the universal separation technique, but where it does not yet fit the purpose, we should at least explore its potential.

Further use of CO₂-SFC at larger scale, pilot, production, or flash chromatography should be strongly encouraged. In short, CO₂-SFC can help make our world greener.