Supercritical fluid chromatography restrictors

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... 

Good reproducibility has been reported for capillary supercritical fluid chromatography using a direct injection method without a split restrictor. This method (Fig. 1.2(b)) utilises a rapidly rotating internal-loop injector (Valeo Inst. Switzerland) which remains in-line with the column for only a short period of time. This then gives a reproducible method of injecting a small fraction of the loop into the column. For this method to be reproducible the valve must be able to switch very rapidly to put a small slug of sample into the column. To attain this a method called timed-split injection was developed (Lee Scientific). For timed split to operate it is essential that helium is used to... 

Microwave induced plasma mass spectrometry has also been used as a detector for supercritical fluid chromatography (SFC) [113] for the separation of halogenated hydrocarbons. The design of an SFC-MIP interface must ensure that the frit restrictor temperature remains at a high temperature to prevent condensation of analytes. Stainless steel transfer lines may be used. The frit restrictor should be connected to a length of deactivated fused silica capillary, inserted through the transfer line, and positioned flush with the aluminum MIP torch inset (Fig. 10.21). 

Supercritical fluid chromatography (SFC) with open-tubular columns was first demonstrated in 1981 by Novotny and co-workers [1]. This technique, known as capillary SFC, was made available to the analytical community through the introduction of several commercial instruments in 1986. Initially difficult to use, improvements in instrumentation and hardware, coupled with a wider array of columns and restrictor options designed specifically for the technique, becoming available, have led to a general acceptance of the method in many laboratories. Not only useful as a research tool, capillary SFC is firmly established as an essential analytical method for production support and quality control in many industries. Some of these include chemical and petroleum manufacturing, pharmaceuticals, polymers, and environmental monitoring. 

In supercritical fluid chromatography (SFC), a supercritical fluid, usually carbon dioxide, is used. An SFC column resembles a GC column or a packed LC column. The operating temperature is lower than that in GC, which makes it more suitable for the analysis of thermally labile compounds. Another advantage is that water is not present and this is particularly valuable for the analysis of phosphatic antioxidants. In addition, high molecular mass antioxidants can be analyzed with SFC. SFC and FTIR have been used for the analysis of light stabilizers (UV absorbers) and antioxidants [43]. The eluted compounds were deposited on a KBr window after they had passed through a capillary restrictor. The hmit of detection was aroimd 100 ng. 

Further applications of M IPs for speciation include the use of supercritical-fluid chromatography for the determination of P, Cl, N, Si, Fe, and Zn species as car lubricant additives [531]. Hyphenation of packed-column supercritical fluid chromatography (SFC) to microwave-induced plasma optical emission spectrometry (MIP-OES) by flow-splitting via a restrictor to enable multi-detection, including UV or flame-ionization detection, has also been described. 

J. T. B. Strode and L. T. Taylor, Supercritical fluid extraction employing a variable restrictor coupled to gas chromatography via a sample pre-concentration trap , J. High Resolut. Chromatogr. 19 651-654 (1996). 

By changing the density of the snpercritical flnid, different fractions may be selectively extracted from the complex mixtnre or simple matrix. On decompression, the extracted solntes are precipitated and may be collected from injection into a gas chromatography or SFC for analysis. Figure 7.2 shows a simple apparatus for on-line supercritical fluid extraction (SFE)/SFC solutes extracted from the sample matrix are deposited from the end of a restrictor into the internal loop of the microinjection valve of the capillary SFC. The valve loop contents are subsequently switched into the SFC column by means of liquid or supercritical carbon dioxide. 

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