ABSTRACT
Usually single wavelength UV detectors are used with preparative SFC. It is the intensity, or sometimes slope, of that detected signal which will likely be used for fraction collection. UV flow cells must be capable of tolerating the high pressures inherent to SFC conditions many rated up to 400 bar. Sensitivity is lower in SFC than HPLC, though this is not usually a problem with the sample sizes relevant to preparative operation. Since the UV signals are normally so large in prep-SFC, the higher noise resulting from the refractive index change of CO2 with an automated backpressure regulator fluctuation does not impact the ability to trigger fraction collection. Other detectors such as Evaporative light scattering detection (ELSD) and others have been used in prep-SFC but are much less prevalently utilized and will not be described in this review. Automated backpressure regulators are the devices used to fix post-column pressure in modern prep SFC and are critical to maintaining stable system performance. There are as many varieties as the instrument vendors that utilize them, but in general they involve an electronically controlled restriction point that generates a motor movement to compensate for pressure changes in order to maintain a desired set point. The flow from the backpressure regulator outlet is usually plumbed to waste container by default in which after decompression the mobile phase again separates into its liquid and gaseous constituents. CO2 is allowed to exhaust through a proper channel while the liquid component is collected into a waste container. For preparative SFC, however, when fraction collection is triggered by an appropriate UV signal, this flow to waste is diverted into collection vessels, in many instruments through cyclone separators. A valve opens to one of the cyclone separators and flow is directed at a downward direction against the interior wall of the cylinder in a vortex fashion. CO2 is vented through a pipe in the top of the cylinder while liquid containing the compound of interest is collected in the cyclone and drained through another valve at the base into the final collection container. A typical preparative SFC instrument is depicted in the block diagram shown in Fig. 7.1. Today commercial semi-preparative to preparative SFCs come in scales comprising flow rates between 10 g/min to 1 kg/min and are used to purify material quantities from a few milligrams up to kilograms. A variety of applications for chiral preparative SFC have been described and its prevalence continues to expand beyond the most common use for small-molecule chiral separations.
