Supercritical Fluids as Mobile Phases

More rarely used are nitrous oxide (N2O, = 36°C, = lOOkPa) or 

The density and therefore the solvating power of supercritical fluids varies according to the pressures to which they are submitted. As a consequence, a pressure gradient in SFC is equivalent to an elution gradient in HPLC, or a temperature gradient in GC. 

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The use of supercritical fluids as mobile phases in chromatography can offer several advantages because their properties are between those of liquids and those of gases. In particular, the viscosity of a supercritical fluid is almost that of a gas (50 times lower than that of a solvent) while its solvation properties (governed by the distribution coefficients K) are similar to those of a nonpolar solvent such as benzene. 

In addition to conventional liquid chromatography, supercritical fluid chromatography (SFC), using a supercritical fluid as mobile phase (mostly scf-C02), has attracted attention in the last decades [58, 164, 168, 169]. Supercritical fluids provide a favourable medium for the transport of solutes through a chromatographic column because they resemble a gas in terms of viscosity, a liquid in terms of density, and are intermediate between these two phases in terms of diffusivity. For some physieal properties of supercritical solvents, see Section 3.2. 

Depending upon the choice of temperature and pressure, the behaviour of a supercritical fluid can sometimes looks like a dense gas and sometimes like a liquid. For these reasons, use of supercritical fluids as mobile phases in chromatography presents certain advantages. 

Klesper introduced supercritical fluids as mobile phase for column chromatography but limited development took place until the early 1980s when Lee introduced open tubular columns. Most supercritical fluid separations today use packed columns of small internal diameter. 

The first use of supercritical fluids as mobile phases in chromatography was reported in 1962 by Klesper et al. (2). The interest in the use of supercritical fluids grew slowly at first, probably due in large measure to the concurrent rapid development of high performance liquid chromatography (HPLC) as well as some technical difficulties which impeded the introduction of commercial SFC instrumentation. Interest began to grow rapidly in... 

Chromatography with a supercritical fluid as mobile phase was first reported more than twenty years ago [1], although only in recent years have its advantages been realized. These are especially relevant to oligomers, polymers, and polymer additives, and supercritical fluid chromatography (SFC) is finding increasing use in analyses in polymer chemistry. 

J. O. Frazier, Investigation into the Quantitative Aspects of Supercritical Fluids as Mobile Phases for Both Chromatography and Extraction, PhD dissertation, Virginia Tech, Blacksburg, VA, 1990. 

This Chapter relates the problems associated with scaling-up the technique for use at the laboratory preparative level and above and pays special attention to the technical demands on materials and apparatus to ensure complete safety in the operation of the chromatograph. The ideas and designs of others will be reviewed and compared with the authors own approach. The advantages and disadvantages offered by the various systems will be explained in sufficient detail for the reader to adopt the system most suited to his/her needs. The non-specialist reader may not be familiar with the use of supercritical fluids as mobile phases in chromatography so the Chapter is introduced with a brief explanation of the nature of the supercritical state. 

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

The use of both sub- and supercritical fluids as eluents yields mobile phases with increased diffusivity and decreased viscosity relative to liquid eluents [23]. These properties enhance chromatographic efficiency and improve resolution. Higher efficiency in SFC shifts the optimum flowrate to higher values so that analysis time can be reduced without compromising resolution [12]. The low viscosity of the eluent also reduces the pressure-drop across the chromatographic column and facilitates the... 

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. 

Beside the use of MIPs in conventional HPLC, Mi-polymers may also be established in supercritical fluid chromatography, which is characterized by faster equilibration times combined with the use of the environmental friendly C02 as mobile phase. Although preliminary results show relatively broad peaks, chiral separation could be performed based on polymers imprinted with an enantiomer. However, the long-term stability of the photochemically generated polymers seems to be a problem [89]. 

The utilization of supercritical fluids in conjunction with adsorbents and active solids is well documented in the technical literature. The most frequently cited applications involve the use of dense gases for the regeneration of adsorbents (1) and as mobile phases in supercritical fluid chromatography (2). Numerous... 

It was suggested in the late 50 s that supercritical fluids could be used as mobile phases. (1J Supercritical Fluid Chromatography (SFC) was introduced in 1961 when Klesper( ) demonstrated the use of a supercritical fluid as a mobile phase. These "dense gases" or supercritical fluids have the ability to solubilize nonvolatile compounds, and thus cause them to migrate down a column in partition chromatography. ( 3)... 

In chromatography, one phase is held immobile or stationary, and the other one is passed over it (the mobile phase). The designations GC and LC refer to the physical state of the mobile phase. Further classifications can be made by naming the mobile and stationary phases thus we have gas-solid (GS), gas-liquid (GL), liquid-liquid (LL), and liquid-solid (LS) chromatography. More recently, supercritical fluids have been used as mobile phases, and these techniques have been named supercritical fluid chromatography (SFC) irrespective of the state of the stationary phase. Other names have also become popular, and Table 1 shows a complete classification scheme. Included in the classification scheme are not only the states of the two phases but also the configuration of the chromato-... 

Over the past twenty years, fluids above their critical temperatures and pressures have been used for a variety of analytical purposes first as mobile phases in chromatography [1,2], thus giving rise to supercritical fluid chromatography, and then as solvents for the selective extraction of species — from solid samples in most cases [3-5]. 

Among the works of supercritical fluid separations of PCBs, UV has been the most popular detector. A Microbore Cig column was used to separate individual PCB congeners in Aroclor mixtures. Density and temperature programming was also utilized for separation of PCBs. Both packed (with phenyl and Cig) and capillary (Sphery-5 cyanopropyl) columns were used in this work. Carbon dioxide, nitrous oxide, and sulfur hexafluoride were tested as mobile phases for the separation of PCBs. 

Supercritical fluid chromatography Chromatography involving a supercritical fluid as the mobile phase. 

Supercritical fluid chromatography (SFC) with ELD, using CO2 or C02-MeOH as mobile phase, was applied to simultaneous determination of 11 priority phenols and 13 polycyclic aromatic hydrocarbons. Voltammetric measurements allow low-nanogram detection limits of reducible and oxidizable analytes, even if they elute simultaneously from the chromatographic column . SFC with MeOH-modified CO2 was performed under isobaric and pressure-programmed conditions, combined with ELD. LOD was 250 p,g of 2,6-dimethylphenol for oxidative ELD and 100 pg of 1,3-dinitrobenzene for reductive ELD . Various sorbents were investigated for SPE preconcentration prior to SFC 20 . 

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