Instrumentation for SFC

Implementation of SFC has initially been hampered by instrumental problems, such as back-pressure regulation, need for syringe pumps, consistent flow-rates, pressure and density gradient control, modifier gradient elution, small volume injection (nL), poor reproducibility of injection, and miniaturised detection. These difficulties, which limited sensitivity, precision or reproducibility in industrial applications, were eventually overcome. Because instrumentation for SFC is quite complex and expensive, the technique is still not widely accepted. At the present time few SFC instrument manufacturers are active. Berger and Wilson [239] have described packed SFC instrumentation equipped with FID, UV/VIS and NPD, which can also be employed for open-tubular SFC in a pressure-control mode. Column technology has been largely borrowed from GC (for the open-tubular format) or from HPLC (for the packed format). Open-tubular coated capillaries (50-100 irn i.d.), packed capillaries (100-500 p,m i.d.), and packed columns (1 -4.6 mm i.d.) have been used for SFC (Table 4.27). 

For descriptions of several commercial instruments for SFC, see F. Wach, Anal. Chem.. 1994, 66. 369A B. Erikson. Anal. Chem.. 1997,69. 683A. 

The SFC systems are grouped into two categories analytical SFC for chemical analysis and preparative SFC for scale-up chemical synthesis and purification. On a fundamental level, the instrumentation for SFC consists of the following (1) a fluid delivery system with high-pressure pumps to transport the sample in a mobile phase and to control the pressure (2) the column in a thermostat-controlled oven where the separation process occurs (3) a restrictor to maintain the high pressure in the column (4) a detection system and (5) a computer to control the system as well as to record the results (see Figure 9.5 as an example). In SFC the mobile phase... 

The instrumentation for SFC represents hybrid assemblies of GC and HPLC instruments (Figures 6.3 and 6.4). To assure the flow rate of the supercritical fluid a syringe pump or a reciprocal pump is used, which is maintained below the critical temperature by means of a cryostat regulated at around 0 C. In instances where an organic modifier is added, either a second pump or a tandem pump is utilized which has two chambers, one for the supercritical fluid and the other for the modifier. The liquid then passes through a coil maintained above the critical temperature in order to convert it to a supercritical fluid. 

As shown in Figure 29-1. in a commercial instrument for SFC, such instrument variables as pumping pressure, oven temperature, and detector performance arc computer controlled. 

The instrumentation for SFC (Figure 5.3) consists of a fluid delivery unit (pump), injector, column, a heated column compartment, detector, and restrictor (to maintain pressure). 

As mentioned earlier, the pressures and temperatures required for creating supercritical fluids derived from several common gases and liquids lie well within the operating limits of ordinary HPLC equipment. Thus, as shown in Figure 29-1, instruments for SFC are similar in most aspects to the instruments for HPLC described in Section 28C. There are two important differences between the two techniques, however. First, a thermoslatted column oven, similar to that used in GC (Section 27B-3), is required to provide precise temperature control of the mobile phase second, a restrictor. 

SFC has been performed with either open capillary columns similar to those used in GC or packed columns transferred from LC, and the instrumentation requirements differ for these two approaches [12]. This chapter will focus on the use of packed column technology because of its dominance in the area of pharmaceutical compound separations. Current commercial instrumentation for packed column SFC utilizes many of the same components as traditional LC instruments, including pumps, injection valves, and detectors. In fact, most modem packed column SFC instm-ments can also be used to perform LC separations, and many of the same stationary phases can be used in both LC and SFC [9]. 

A schematic diagram of a chromatograph for SFC is shown in Figure 6.10. In general, the instrument components are a hybrid of components developed for gas and liquid chromatography that have been subsequently modified for use with supercritical fluids. Thus, the. fluid delivery system is a pump modified for pressure control and the injection system a rotary valve similar to components used in liquid chromatography. The column oven and... 

The use of SCF as extracting media and mobile phases for chromatography is now commonplace, and SFE and SFC occupy established niches in analytical chemistry. Commercially available instrumentation for SFE and SFC have been available since the mid 1980s. Basic requirements for analytical scale SFE equipment that can perform selective removal or solubilisation of target analytes or analyte classes consist of a... 

In the mid-to-late 1990s, SFC became an established technique, although only holding a niche position in the analytical laboratory. The lack of robust instruments and the inflexibility of such systems has led to the gradual decline of SFE-SFC. Only a small group of industrial SFE-SFC practitioners is still active. Also the application area for SFC is not as clearly defined as for GC or HPLC. Nevertheless, polymer additives represent a group of compounds which has met most success in SFE-SFC. The major drawbacks of SFE-SFC are the need for an optimisation procedure for analyte recovery by SFE (Section 3.4.2), and the fair chance of incompatibility with the requirements of the chromatographic column. The mutual interference of SFE and SFC denotes non-ideal hyphenation. 

Enantiomeric separations have proven to be one of the most successful applications of packed column SFC. Despite initial reluctance, many analysts now use SFC routinely for both analytical and preparative chiral separations. Additional studies of chiral recognition in SFC and continued improvements in instrumentation will ensure a prominent role for SFC in chiral separations methodology in the future. 

Thermodynamic behaviors and retention mechanisms for SFC are unique. Low temperatures and high pressures or high densities usually favor fast separation of enantiomers in SFC. In the case that the isoelution temperature is below the working temperature, the selectivity increases as temperature increases and higher temperatures are favorable for chiral separation. Future development in SFC will likely include new chiral column technologies and instrumentation refinement. A greater variety of chiral columns packed with smaller particles will open up more areas of application for fast chiral separations. In addition, improvement in signal-to-noise ratio of... 

Another technique is supercritical fluid chromatography (SFC), which is a chromatographic technique that in many ways is a hybrid of GC and HPLC. It is recognized as a valuable technique for the analysis of thermolabile compounds, which would not be amenable to analysis by GC or HPLC. Few applications have been reported for SFC in the field of OCP and OPP determination (16). The advantages reported for SFC are versatility in separation (by the addition of modifier or the choice of stationary phase) and detection (with LC or GC detectors). However, SFC is a little-used technique because it still presents a wide range of instrumental problems (14-16). 

For this work, a 5 meter x 50 micron ID fused silica column, coated with a 0.25 micron polydimethylsiloxane film was introduced directly into the source chamber through the transfer line normally used for GC/FTMS. A restrictor was created at the end of the column by using a microflame to draw out the end of a 1 meter portion of deactivated but uncoated column to an inside diameter of approximately one micron. Details of the instrumentation used for SFC have been described elsewhere [19]. With the SFC interface in place, pressures in the source chamber were approximately 5 x 10 5 torr. Despite this high source cell pressure, we were able to obtain relatively high quality mass spectral data with analyzer side detection at 5 x 10"7 torr. 

The choice of possible mobile phases is more limited. The critical properties (critical pressure pc and temperature Tc) should be within practical reach. Moreover, stable compounds are required, which do not show disintegration at elevated temperatures and pressures. Also, the mobile phase must not be too agressive towards the materials used in the column (usually silica-based phases) and the instrumentation (mainly stainless steel). Therefore, mobile phases that are extremely interesting from a chemical point of view, such as supercritical ammonia and, especially, supercritical water, have found little use so far. Table 3.9 lists some possible mobile phases for SFC together with their chemical properties. 

During the last 15 years, the applications of supercritical fluids (SFC and SFE) have shown a fast advance among others, from a historical perspective, SFC was developed after GC was well established and when HPLC was starting. The interest for SFC has grown with the GC and HPLC development and technological innovations that had occurred independently of SFC research, but surely allowed that commercial SFC instruments could be introduced in the 1980s. 

One might be surprised to find a full section on gas delivery in the instrumentation section, but the lack of reasonable solutions for delivery of pure CO2 held back the more widespread use of SFC for quite some time. Reasonably priced commercial gas delivery systems designed specifically for SFC have only recently become available. There are subtle problems that make the carbon dioxide delivery more difficult than it appears it should be. 

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