A revival of supercritical fluid chromatography (SFC) has been observed recently. SFC has repeatedly enabled fast and efficient separations, and in some cases has even outperformed high performance liquid chromatography (HPLC). This article provides an overview of the most recent advances in the field of chiral and achiral separations in SFC. This involves research focused on the most critical parameters in SFC separations, but also on practical issues such as the serial coupling of columns. The recent evolution from classic SFC to ultrahighperformance supercritical fluid chromatography (UPSFC) is also discussed.
In 1962 Klesper et al. (1) were the first to report supercritical fluids as eluents in chromatographic separations. A supercritical fluid is obtained by elevating the temperature and pressure above the characteristic critical values of the substance concerned. This reversible physical state possesses unique and interesting properties that can be applied in chromatography. Because the viscosity and diffusivity of this phase are comparable to that of gaseous mobile phases, lower pressures are generated over the system and column. The density and solvating power, on the other hand, approach that of liquid mobile phases, creating a broad application range.
However, having to compete with other popular techniques, such as high performance liquid chromatography (HPLC) and gas chromatography (GC), the interest in supercritical fluid chromatography (SFC) was rather limited in the past and applications were relatively scarce. Additionally, SFC users pushed against the limitations of the available instrumentation to apply this technique. A strict control of the mobile phase density was vital to allow accurate and reproducible analyses, but often quite tedious to achieve. In addition, conventional ultraviolet (UV) detection was complicated by the supercritical state of the mobile phase. The refractive index of this phase is directly proportional to its density. Consequently, small changes in mobile phase density can drastically influence the refractive index and thus impede the UV-detection. Inspired by the technological evolution of HPLC, SFC has undergone major instrumental improvements over recent years. This has led to a revival of this technology, which is now routinely applied in a number of pharmaceutical laboratories. Inspired by the technological evolution of HPLC, SFC has undergone major instrumental improvements over recent years. The mechanisms of the back pressure regulator and oven have been improved, allowing a much stricter control of the mobile phase density. The system's void volume has been reduced, resulting in higher separation efficiencies, and adjustments to the flow path of the UV detector has resulted in an improvement of the baseline noise. This has led to a revival of this technology, which is now routinely applied in a number of laboratories. In this overview, the most recent advances and applications in SFC will be highlighted, with a special focus on its application in pharmaceutical separations.