Winner: Prof. Caroline West
West focuses primarily on Supercritical Fluid chromatography and has been awarded the Jubilee medal (2021) for her splendid contributions to the field. Daniel Meston was able to get an opportunity to discuss her past and present in this ChromSoc interview.
What do you feel are the most important advances in SFC over the last decade?
The first and most important one is the improvement in instruments. They are now better in accordance with chromatographers’ expectations than older systems, especially in terms of sensitivity and robustness. Another advancement I am greatly interested in is the progressive change in habits concerning mobile phase composition. The users now feel free to use wider ranges of co-solvent gradients and a variety of mobile phase additives, which has opened the way for applications to more polar analytes than before.
You have collaborated widely with both academia and industry. In your experience what makes a good partnership and what advice would you provide to get the most out of these collaborations?
A good partnership should be based on open and frequent discussions. In particular, each part must be clear on their respective expectations related to the project, so that no one feels unsatisfied in the end. I am also deeply attached to science integrity and deontology in science. I try to make it clear from the start that I would never compromise on this point, whatever the partner is offering.
You have made significant progress in defining approaches to stationary phase characterisation in SFC. What are your broad findings on this and are there particular phase types you would recommend for method development?
The most important aspect of this research was to show that all stationary phase chemistries may be useful in SFC. Not all columns are useful to everyone, but considering all application areas, all columns may find some use. To achieve diverse separations in a most efficient development process, the best is to have a good selection of complementary phases. For instance, in a starter kit I would always have a bare silica gel, another polar stationary phase like a 2-ethylpyridine, something aromatic with mixed polarity like a pentafluorophenyl, and something hydrophobic like a C18 phase. They will not solve all your problems but with that, you can already do a lot.
Similarly, you have you have reported broadly on the action and impact of using additives in mobile phases for SFC. Can you discuss what additives you would recommend and when you would use them?
Ammonium salts are great favourites currently. I like ammonium hydroxide for basic compounds. Ammonium formate or acetate are also good (especially when SFC is hyphenated to ESI-MS) but the baseline drift can be problematic when working with UV detection. For acidic and hydroxylated species, I prefer acidic additives. For instance, we have had much success with methanesulfonic acid. In any case, a small concentration of water (2 to 5% in the co-solvent) is helpful in improving solubility for polar analytes and additives.
Due to the vast breadth of organic solvents amenable to SFC, sample preparation can be relatively straight forward for many compound types in SFC. However, for those with complex or highly aqueous matrices e.g. environmental samples or solid-oral pharmaceutical dosage forms, this can be more involved. Can you provide your thoughts on this and the impact on peak shapes?
Dilution solvents have varied effects on peak shapes, depending on analyte, mobile phase composition and stationary phase polarity. It is difficult to predict what will happen and general rules have not been established yet. For aqueous samples, moderate dilution with acetonitrile is often sufficient to achieve adequate peak shapes. For solid samples, the possibility to extract the sample with supercritical fluid extraction (SFE) hyphenated to SFC is emerging and deserves more attention. A fluid that is capable to extract your analytes should be the best one to avoid disturbance in the separation.
Following on from this – where do you feel SFC is best utilised?
SFC is excellent in many cases and very good in many others! Let me take that question from the reversed point of view: where is it not good? Although some research is emerging in this field, perhaps big polar molecules (polar proteins, RNA) are not suitable for SFC, but anything else is manageable.
In-silico retention modelling is a very important aspect of method development in liquid chromatography – what have been the barriers to implementation in SFC?
Modelling a compressible fluid is not like modelling a liquid. In HPLC, several variables are approximated with linear trends (mobile phase composition, temperature). In SFC, there are more parameters that can impact the separation and most effects are non-linear, sometimes not even monotonous! There are however progresses in this direction and I am confident that we will reach that point in the near future.
In 2018 you were involved in a cross-lab study for the determination of pharmaceutical impurity levels. Can you tell us a little more about that and the findings?
I am very grateful to Amandine Dispas and Davy Guillarme who directed this project. SFC definitely needed someone to organize such a study. The results were very good, as good as usually observed in LC actually! Somehow, SFC always needs to prove itself, more than other established techniques. I hope that such studies will finally give more confidence to potential users.
In what direction do you see the next advances in specifically SFC and in broader separation science?
I like to think that older barriers between the chromatographic techniques are falling down. The old dream of unified chromatography, permitting a more flexible use of separation techniques without strict reference to the fluid state or retention mechanism, is at hand… To achieve this point, we need more flexible instruments, and more flexible chromatographers who do not limit their education to a single technique!
