Supercritical Fluid Instrumentation

Numerous applications of SEE were published during the 1980s soon after the availability of commercial instrumentation. Supercritical fluids (SFs) have useful characteristics for the extraction of trace analytes from solid samples, most notably... 

Prior to the commercialisation of LC-MS instruments, supercritical fluid chromatography (SFC) was mainly used for the analysis of oligomers. As the range of LC-MS instruments can be extended up to 4000 dal tons this capability makes them ideal to characterise oligomers. For example, it has been shown that silicone oligomers can be detected by LC-MS in food simulants. ... 

Mixtures can be identified with the help of computer software that subtracts the spectra of pure compounds from that of the sample. For complex mixtures, fractionation may be needed as part of the analysis. Commercial instmments are available that combine ftir, as a detector, with a separation technique such as gas chromatography (gc), high performance Hquid chromatography (hplc), or supercritical fluid chromatography (96,97). Instmments such as gc/ftir are often termed hyphenated instmments (98). Pyrolyzer (99) and thermogravimetric analysis (tga) instmmentation can also be combined with ftir for monitoring pyrolysis and oxidation processes (100) (see Analytical methods, hyphenated instruments). 

Supercritical fluid chromatography (SEC) was first reported in 1962, and applications of the technique rapidly increased following the introduction of commercially available instrumentation in the early 1980s due to the ability to determine thermally labile compounds using detection systems more commonly employed with GC. However, few applications of SEC have been published with regard to the determination of triazines. Recently, a chemiluminescence nitrogen detector was used with packed-column SEC and a methanol-modified CO2 mobile phase for the determination of atrazine, simazine, and propazine. Pressure and mobile phase gradients were used to demonstrate the efficacy of fhe fechnique. 

SEE is an instrumental approach not unlike PLE except that a supercritical fluid rather than a liquid is used as the extraction solvent. SFE and PLE employ the same procedures for preparing samples and loading extraction vessels, and the same concepts of static and dynamic extractions are also pertinent. SFE typically requires higher pressure than PLE to maintain supercritical conditions and, for this reason, SFE usually requires a restrictor to control better the flow and pressure of the extraction fluid. CO2 is by far the most common solvent used in SFE owing to its relatively low critical point (78 atm and 31 °C), extraction properties, availability, gaseous natural state, and safety. 

The instrumental requirements for supercritical fluid extraction are quite simple. A pump is essential to generate the extraction pressure in a themostated extraction vessel. The soluble sample components are then swept from the vessel through a flow restrictor into a collection device that is normally at ambient pressure. The fluid used for supercritical fluid... 

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... 

Principles and Characteristics In an attempt to develop a unified sample preparation system for extraction of various matrix/analyte combinations Ashraf-Khorassani et al. [498] have described a hybrid supercritical fluid extraction/enhanced solvent extraction (SFE/ESE ) system to remove both polar and nonpolar analytes from various matrices. The idea is that a single instrument that can perform extractions via pure C02 solvent, and all gradients thereof affords... 

SFC has been widely reviewed [272-283], with particular emphasis on pSFC [284-286] and cSFC [287], SFC principles and techniques [246], instrumentation [272,288,289], columns and phases [283,290], detection [236,291], past and future [292]. Various recent monographs deal with supercritical fluids, SFE and/or SFC [293-303], some specifically with pSFC [26,304]. 

On-line SFE-SFC modes present several distinct advantages that are beyond reach of either technique when used separately (Table 7.13). An obvious advantage of SFE is that it is an ideal way to introduce a sample into an SFC system. Because the injection-solvent is the same as the mobile phase, in this respect the criteria for a successful coupling of different techniques are fulfilled [94], i.e. the output characteristics from the first instrument and the input characteristics of the second instrument are compatible. Supercritical fluid techniques can separate high-MW compounds are significantly faster than classical Soxhlet extractions and require less heat and solvent. SFE-SFC techniques are versatile,... 

Packed columns, 10 769-772, 773 band broadening, 6 412 for distillzation, 5 768-776 gas chromatography, 6 377, 379 instrumentation, 6 423-424 Packed column supercritical fluid chromatography (pSFC), 19 567 Packed fiber-bed mist eliminators, 23 781 Packed towers, 25 810, 811 Packing(s)... 

Snyder JL, Grob RL, McNally ME, OostdykTS.The effect of instrumental parameters and soil matrix on the recovery of organochlorine and organophosphate pesticides from soils using supercritical fluid extraction. J. Chromatogr. Sci. 1993 31 183-191. 

In supercritical fluid chromatography, fluids above their critical point are used as mobile phases. This chapter discusses the principles of operation, mobile phase considerations, parameters that can be adjusted in method development as well as an overview of instrumentation required and a few pertinent examples from current literature. Not everything can be illustrated, but the advantages of this diverse technology will be highlighted. 

In terms of chromatography, the first individual credited with the use of supercritical fluids as the mobile phase is Ernst Klesper when in 1962 he reported on the separation of metal porphyrins using dense-gas chromatography (GC) or SFC [3]. But it was not until the 1980s that the analytical community took hold of the abilities and advantages of the technique with the advent of several commercial instrumentation ventures. 

A variety of modern instrumental analytical techniques have attracted considerable attention in the last decades as alternative separation and analysis methods with respect to HPLC. This includes, in particular, supercritical fluid chromatography (SFC), which utilizes condensed carbon dioxide (above or near its critical temperature of... 

Off-line SFE is conceptually a simple experiment to perform and requires only relatively basic instrumentation. The instrumental components necessary include a source of fluid, most often CO2 or CO2 with an organic modifier, a means of pressurizing the fluid, an extraction cell, a method of controlling the extraction cell temperature, a device to depressurize the supercritical fluid (flow restrictor), and a device for collecting the extracted analytes. 

On-line supercritical fluid extraction/GC methods combine the ability of liquid solvent extraction to extract efficiently a broad range of analytes with the ability of gas-phase extraction methods to rapidly and efficiently transfer the extracted analytes to the gas chromatograph. The characteristics of supercritical fluids make them ideal for the development of on-line sample extraction/gas chromatographic (SFE-GQ techniques. SFE has the ability to extract many analytes from a variety of matrices with recoveries that rival liquid solvent extraction, but with much shorter extraction times. Additionally, since most supercritical fluids are converted to the gas phase upon depressurization to ambient conditions, SFE has the potential to introduce extracted analytes to the GC in the gas phase. As shown in Fig. 13.8, the required instrumentation to perform direct coupling SFE-GC includes suitable transfer lines and a conventional gas chromatograph [162,163]. 

In the first section, the mechanisms involved in size exclusion chromatography are discussed this is an area where additional understanding and clarification still are needed. Data treatment with respect to statistical reliability of the data along with corrections for instrumental broadening is still a valid concern. Instrumental advances in the automation of multiple detectors and the developm.ent of a pressure-programmed, controlled-flow supercritical fluid chromatograph are presented. 

---