22


May/June 2013


How Good is SFC for Polar Analytes?


by Caroline West, Institut de Chimie Organique et Analytique (ICOA), (1) CNRS UMR 7311, (2) University of Orléans, Pôle de Chimie, rue de Chartres, BP 6759, 45067 Orléans, France Email: caroline.west@univ-orleans.fr


Supercritical fluid chromatography (SFC) is hardly a new separation method, although it is emerging again after some years in the wilderness. However, while the technology is now fully mature, some important fundamentals are still poorly understood, as is the case for the polarity range of analytes amenable to SFC. This paper presents a non-exhaustive critical review of the analysis of polar compounds using SFC.


Packed column SFC is now gaining popularity again and progressively becoming the first choice of purification technique in many pharmaceutical companies. While it has long been a favoured technique for chiral separations, it is also being used for achiral separations. This is due to the recent introduction of specialised apparatus from different manufacturers, together with a slowly increasing awareness of the significant benefits of the technique. Indeed, SFC provides highly efficient and fast separations thanks to the large flow rates that can be reached with a mobile phase of low viscosity. In addition, the carbon dioxide-based mobile phase is an attractive feature allowing a significant reduction of solvent consumption in the chromatography laboratory.


A long history of misunderstandings


SFC was first introduced in 1962, but did not attract much attention at that time. A little later, during its first rise in the 1980’s and early 1990’s, there was great hope placed in this emerging technique. Unfortunately, much of this hope was misplaced: it was believed that supercritical fluids would provide greater


elution strength than gases, thus allowing for the elution of more polar compounds, without the need for the high temperatures required in gas chromatography, or without derivatisation. Several different fluids were explored initially, but carbon dioxide soon emerged as a favourite, thanks to its mild critical parameters, availability at a low cost, compatibility to most GC and HPLC detectors, relative inertness and absence of toxicity. However, for most compounds aside hydrocarbons the elution strength of pure carbon dioxide is not sufficient. It can be increased with high pressure, but not to a very large extent.


Users at the time were reluctant to use co- solvents, because the compatibility to the preferred detection mode, flame ionisation detection, was impaired and it was noted that column efficiency decreased significantly upon addition of a solvent. In some cases it was also reported that the mobile phase separated into two phases (carbon dioxide and a liquid solvent). Unfortunately, all these elements caused a relative drop in the interest of the academic community during several years.


It is now well known that phase separation can be avoided if backpressure is maintained


significantly above the critical pressure [1] or if


temperature is maintained below the critical one. However, it is now fully acknowledged that elution strength towards polar compounds can be greatly improved upon addition of a co-solvent [2]


, and that column


efficiency is actually improved upon addition of a polar co-solvent [3]


. Industry users then re-


discovered the advantages of prep-scale SFC in the mid-2000’s, particularly for chiral separations. To this particular application, SFC provides a high productivity compared to preparative HPLC, thanks to high flow rates, limited solvent consumption and concentrated fractions requiring less energy to evaporate the solvent.


Nowadays, SFC practice is much closer to HPLC than GC: HPLC detectors are favoured (UV-visible, evaporative light-scattering detection or mass spectrometry with liquid- compatible ionisation sources); the columns used are essentially HPLC-type packed columns; mobile phase composition comprises a significant proportion (typically 5 to 50%) of co-solvent named modifier (most commonly methanol, ethanol or isopropanol) and often small proportions (0.1 to 2%) of an additive (acid, base, or salt). Finally method


Figure 1: SFC separation 17 polar pesticides. Cosmosil 5CN-MS, gradient elution with methanol + 0.1% ammonium formate, 35°C, 3 ml/min. Reprinted from [53], with permission from Elsevier.


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