Mobile phase in supercritical fluid

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... 

Pyo, D. Ju, D. Simple method for the preparation of water-modified or methanol-modified carbon dioxide as the mobile phase in supercritical fluid chromatography. Anal. Sci. 1994, 0(1), 171-174. 

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

Supercritical fluid chromatography (SFC) is a column chromatographic technique in which a supercritical fluid is used as a mobile phase. A supercritical fluid is a gas or liquid brought to a temperature and a pressure above its critical point. The first report of SFC dates back to 1962 when Kesper et al. [1] used supercritical fluid chlorofluorocarbons as a mobile phase for the separation of metal porphyrins. It was not until the early 1980s that an important breakthrough of the technique occurred. This was the introduction of capillary SFC and the availability of commercial instrumentation. These became major factors in the recent rise in popularity of SFC. According to the latest estimation, approximately 100 SFC articles are published in major journals every year. 

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)... 

S. Rokushika, K. P. Naikwadi, A. L. Jadhav, and H. Hatano. Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase. Chromatographia, 22 209-212,1986. 

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). 

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,... 

In supercritical fluid chromatography (SFC) the mobile phase is a supercritical fluid, such as carbon dioxide [15]. A supercritical fluid can be created either by heating a gas above its critical temperature or compressing a liquid above its critical pressure. Generally, an SFC system typically has chromatographic equipment similar to a HPLC, but uses GC columns. Both GC and LC detectors are used, thus allowing analysis of samples that cannot be vaporized for analysis by GC, yet cannot be detected with the usual LC detectors, to be both separated and detected using SFC. SFC is also in other... 

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. 

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]. 

Supercritical fluid chromatography is the name for all chromatographic methods in which the mobile phase is supercritical under the conditions of analysis and the solvating properties of the fluid have a measurable effect on the separation. SFC has some advantages over GC and HPLC it extends the molecular weight range of GC, thermally labile compounds can be separated at lower temperatures, compounds without chromophores can be sensitively detected, and the use of open-tubular and packed columns is feasible. SFC can be employed in both the analysis of natural pigments and synthetic dyes, however it has not been frequently applied in up-to-date analytical practice. 

In this technique, the mobile phase is a fluid in its supercritical state, such as carbon dioxide at about 50 °C and at more than 150 bars (15 MPa). The stationary phase can be a liquid or a solid. This approach combines the advantages of the techniques explained in sections 1.2.2 and 1.2.3. 

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. 

Supercritical fluid extraction conditions were investigated in terms of mobile phase modifier, pressure, temperature and flow rate to improve extraction efficiency (104). High extraction efficiencies, up to 100%, in short times were reported. Relationships between extraction efficiency in supercritical fluid extraction and chromatographic retention in SFC were proposed. The effects of pressure and temperature as well as the advantages of static versus dynamic extraction were explored for PCB extraction in environmental analysis (105). High resolution GC was coupled with SFE in these experiments. 

Cosolvent-modifled supercritical fluids are also used routinely in supercritical fluid chromatography (SFC) to modify solute retention times (11-20). In these reports, cosolvents are used to alter the mobile and stationary phase chemistries (16t17t20). However, distinguishing between such effects in a chromatography... 

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