Capillary column SFC

Two types of injectors are frequently employed. For packed column SFC, a standard six port rotary valve with an external sample loop of 1-10 pL has proven to be quite reliable. For capillary column SFC, a similar rotary valve with an internal "loop of 0.2 to 0.5 pL is typically employed. Frequently the rotor is pneumatically actuated in a very rapid fashion to allow only a small fraction of sample to be introduced ("time-split ) this is done to avoid column overload. Alternatively, the flow from the injector is split off in the same fashion as in GC. A disadvantage of the latter mode is the potential for sample discrimination. 

Goates SR, Sin CH, Simons JK, et al. 1989. Supercritical-fluid chromatography - supersonic-jet spectroscopy Part II. Capillary-column SFC with a sheath-flow nozzle. Microcolumn Sep 1(4) 207-211. 

Van Deempter curves for packed column HPLC ( 1), packed column SFC ( 2), capillary column SFC ( 3, 6, 7). and capillary column GC ( 4, 5). Curves 1 and 2, experimental data (5) Curves 3-7. calculated (see text). 

The statement that supercritical fluids are a hybrid of gases and liquids as we normally encounter them is graphically represented by the series of theoretical capillary column SFC... 

Table I. Order-of-Magnitude Comparison of Calculations for Capillary Column GC, Capillary Column SFC, Packed Column SFC, and Packed Column LC for Typical High Resolution Dimensions (d, = 250 ym, dp = 5 ym) and Best-Case Dimensions (d = 50 ym, dp = 3 ym). (Experimental values In Italics.)...

SFC is competitive with other chromatographies in terms of chromatographic efficiency and resolution. The packed column SFC results should be applicable to capillary column SFC. The results of these preliminary experiments in establishing a modifier selection framework are gratifying in that dramatic differences in the chromatographic behavior were seen from vertex to vertex on the various modifier selectivity triangles. However, there are many studies to be undertaken to make the transition from preliminary results to a coherent, useful framework for routine modifier selection ... 

A number of groups have shown how enantiomeric resolution of amino acids derivatized with non-chiral reagents is possible in SFC with chiral stationary phases. N-Acetylamino acid t-butyl ester racemates were rapidly resolved [17] on (N-formyl-L-valylamino)propyl silica with CO2 modified with methanol, acetonitrile and diethyl ether. A similar stationary phase allowed (18 rapid (< 5 min) separation of racemic N-4-nitrobenzoyl-amino acid isopropyl esto-s with methanol-modified CO2 the enantioselectivity in SFC was comparable with that in HPLC with isopropanol/n-hexane as mobile phase. Capillary column SFC on polysiloxane stationary phases containing chiral side chains has been employed... 

Supercritical fluid chromatography is developing along two lines that differ in the nature of the column used packed-column SFC and capillary-column SFC. 

Biicherl and co-workers [48] has described an integral restrictive interface with jet separation for coupling capillary column SFC with carbon dioxide mobile phase with high resolution mass spectrometry. 

The introduction of capillary columns in SFC by Novotny and Lee in 1981 [141] led to a revival of this technique, which was followed by a large number of papers on new applications. At the same time (1981) the first commercial packed-column SFC instrument came on the market. Four years later, in 1985, the first capillary column SFC instrument was available commercially. 

The oven of an SFC system should meet the same requirements as a normal GC oven. A constant temperature (variation 0.1 °C) must prevail in the entire oven at any time of a positive or negative temperature gradient. This is very important for reproducible capillary column SFC analysis. These columns are very sensitive to even slight variations in temperature, which can result in peak shape deformation, peak splitting, or irre-producible retention times. 

Figure 48. Capillary column SFC separation of Triton X-100 (surfactant)...

Giorgetti, A., N. Pericles, H. M. Widmer, K. Anton, P. Datwyler, Mixed mobile phases and pressure programming in packed and capillary column SFC unified approach, X Chromatogr. ScL, 1989,27,318-324. 

Figure 12.18 LC-SFC analysis of mono- and di-laurates of poly (ethylene glycol) ( = 10) in a surfactant sample (a) normal phase HPLC trace (b) chromatogram obtained without prior fractionation (c) chromatogram of fraction 1 (FI) (d) chromatogram of fraction 2 (F2). LC conditions column (20 cm X 0.25 cm i.d.) packed with Shimpak diol mobile phase, w-hexane/methylene chloride/ethanol (75/25/1) flow rate, 4 p.L/min UV detection at 220 nm. SFC conditions fused-silica capillary column (15 m X 0.1 mm i.d.) with OV-17 (0.25 p.m film thickness) Pressure-programmed at a rate of 10 atm/min from 80 atm to 150 atm, and then at arate of 5 atm/min FID detection. Reprinted with permission from Ref. (23).

An on-line supercritical fluid chromatography-capillary gas chromatography (SFC-GC) technique has been demonstrated for the direct transfer of SFC fractions from a packed column SFC system to a GC system. This technique has been applied in the analysis of industrial samples such as aviation fuel (24). This type of coupled technique is sometimes more advantageous than the traditional LC-GC coupled technique since SFC is compatible with GC, because most supercritical fluids decompress into gases at GC conditions and are not detected by flame-ionization detection. The use of solvent evaporation techniques are not necessary. SFC, in the same way as LC, can be used to preseparate a sample into classes of compounds where the individual components can then be analyzed and quantified by GC. The supercritical fluid sample effluent is decompressed through a restrictor directly into a capillary GC injection port. In addition, this technique allows selective or multi-step heart-cutting of various sample peaks as they elute from the supercritical fluid... 

Figure 12.24 Schematic diagram of the multidimensional packed capillary to open tubular column SFC-SFC system. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al., Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switching interface , pp. 1384-1388, copyright 1990, with permission from the American Chemical Society.

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]. 

The selection of the column type is mainly determined by the composition of the sample. In general open-tubular (capillary) columns are preferred for low-density (gas-like) SFC, whereas packed columns are most useful for high-density (liquid-like) SFC. Open-tubular columns can provide a much larger number of theoretical plates than packed columns for the same pressure drop. Volumetric flow-rates are much higher in packed column SFC (pSFC) than in open-tubular column SFC (cSFC), which makes injection and flow control less problematic. 

SFE can be combined with several forms of SFC, i.e. with conventional packed columns (l-4.6mm i.d. packed-column SFC or pSFC), with capillary columns (10-250 xm i.d. capillary SFC or cSFC), and recently with packed capillary columns (200-530 p,m i.d., 3-10 xm particles packed capillary SFC or pc-SFC). 

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