Greening SFC
Supercritical fluid chromatography (SFC) is often proclaimed to be a green technology. How well does the reality match up to the claims?
Green chemistry is defined as chemical processes that reduce or eliminate negative environmental impacts. SFC is perceived to be “green”, as compared to classical high-performance liquid chromatography (HPLC), based on two features that appear to match the definition. First, CO2 is nontoxic and safe to use. Second, SFC reduces solvent consumption and waste. The favorable impression of SFC is further supported by the economical aspect of reduced solvent consumption and waste disposal, and the comparatively low cost of CO2 – environmental and economic arguments are readily confused. These factors, alongside the technological improvements now available on the market, have prompted a re-birth of SFC.
How compelling is the supposed greenness of the method? Having practiced SFC for ten years, I am willing to question the point.
Greening SFC
Supercritical fluid chromatography (SFC) is often proclaimed to be a green technology. How well does the reality match up to the claims?
Green chemistry is defined as chemical processes that reduce or eliminate negative environmental impacts. SFC is perceived to be “green”, as compared to classical high-performance liquid chromatography (HPLC), based on two features that appear to match the definition. First, CO2 is nontoxic and safe to use. Second, SFC reduces solvent consumption and waste. The favorable impression of SFC is further supported by the economical aspect of reduced solvent consumption and waste disposal, and the comparatively low cost of CO2 – environmental and economic arguments are readily confused. These factors, alongside the technological improvements now available on the market, have prompted a re-birth of SFC.
How compelling is the supposed greenness of the method? Having practiced SFC for ten years, I am willing to question the point.
Typically, when a new SFC method is developed to replace an older HPLC method, performance is the sole criterion. Performance is measured via resolution or via quantitative information, such as sensitivity and linearity. If high-throughput is required, the short cycle time afforded by SFC’s high flow rates and short equilibration is a plus point, while for preparative separations, the system’s productivity is the selling point. Environmental aspects are hardly ever considered. SFC does have inherent features that make it environmentally friendly. However, in everyday practice these are rarely considered, and much effort is needed to make SFC the green technology that it is proclaimed to be.
Greening Guidelines
Since CO2 is essentially a by-product of the fermentation industry, SFC systems are often presented as “good recyclers” of CO2: SFC chromatography, the argument goes, uses CO2 produced by others. While this is true, it does not change the fact that this very same CO2 still ends up being released into the atmosphere. Should it be recycled? To put things into perspective, one analytical SFC system operating for one working day only releases about the same amount of CO2 that an eco-friendly car produces in 20 km. This we can overlook. However, the amount of CO2 released by preparative-scale SFC systems is much larger and certainly worth recycling or capturing, something that is still far from being systematic practice.
Beyond carbon dioxide, the use of eco-friendly solvents and additives, such as ethanol and water, should be encouraged. In achiral SFC, the effects of changing the co-solvent are minor, so ethanol could replace methanol and acetonitrile for a large proportion of analytical applications. And water has been proven to be a very interesting additive (or even co-solvent, in ternary mixtures with carbon dioxide and ethanol) to enhance solubility and peak shapes of polar compounds.
Method development strategies should be revised to include only orthogonal chromatographic systems, ensuring that the final method will be developed in the shortest time possible. Most users are still relying on essentially redundant systems that make screening processes unnecessarily long. Besides, I have often found that several solutions exist for one separation problem, and one of them has significantly advantages over the others, in terms of solvent consumption and analysis time.
Green chemistry applies innovative scientific solutions to real-world environmental situations. For SFC, innovative solutions are still required to:
- Limit the energy required for the production and recycling of CO2.
- Limit the energy required to cool down CO2 for pumping.
- Improve sample transfer between the back-pressure regulator and the fraction collector without the need for a large make-up flow rate.
- Allow for multi-detection of several samples analyzed in a parallel or simultaneous fashion.
- Ensure good mass spectrometric sensitivity without the need for additional liquids.
SFC does have the potential to be a very green separation and purification method. It is in the hands of manufacturers and users to make it such.
Caroline West, now an associate professor in analytical chemistry, ICOA, University of Orléans, France, only heard derisive comments when she first start working with supercritical fluid chromatography: “Why the hell are you wasting your time with this non-existent technique?” Now, the tide has turned. West discovered SFC when she was only a “baby” analytical chemist, so she can no longer live without it. “Unravelling its surprising phenomena and projecting the quasi-unlimited possibilities of this separation method is keeping my neurons 150% busy. I’ve no time to fall into the boredom of using long and fastidious methods with unsatisfactory resolution.”
