Mobile phase in chromatography

Mobile phase in chromatography, the phase (gaseous or liquid) responsible for moving an introduced sample through a porous medium to separate components of interest. 

Mobile phases in chromatography and buffer systems in electrophoresis are examples of frequently used solvent mixtures. In a mixture of p components, only p— can be varied independently, which means that maximally p— mixture-related variables can be examined in the type of experimental designs typically used in robusmess testing. The value of the pth variable is determined by those of the other and used as adjusting component to complete the mixture. If one of the mixture components has an important effect on a response, then the composition of the whole mixture is important and should be strictly controlled. ... 

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. To be useful as a mobile phase in chromatography, a supercritical fluid must have a relatively low critical temperature and pressure, and a relatively high density/solvating power at experimentally accessible pressures and temperatures. The former criterion excludes water and most common organic solvents, whereas the latter excludes such low-boiling substances as helium, hydrogen, and methane. Commonly used fluids are listed in Table I. 

The use of hydrophobic stationary phases and aqueous mobile phases in chromatography on so-called reversed phase (RP) materials is widely established. Reversed phase materials typically consist of alkyl chains (mainly C18 in length) immobilised on silica material. The chromatographic retention of a solute on such a system directly depends on its partition between the aqueous mobile and the hydrophobic solid phase (Berthod et al. 2004). Therefore lipophilicity (expressed as partition coefficient logP between stationary and mobile phase, can be estimated from the retention behaviour of the solute of interest on reversed phase material as reviewed by Nasal et al. (2003). 

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]. 

The advantages of using supercritical mobile phases in chromatography were recognized in the 1950s by Kles-per et al. [1], among others. Carbon dioxide is the most frequently used supercritical mobile phase, due to its moderate critical temperature and pressure, almost... 

Eluent A mobile phase in chromatography that is used to carry solutes through a stationary phase. 

Mobile phase In chromatography, a liquid or a gas that carries analytes through a liquid or solid stationary phase. 

Mixture-Related Factors. Mobile phases in chromatography and electrolyte systems in electrophoresis are examples of frequently used solvent... 

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. 

Giddings et al. go on to relate that the chemical effect of the mobile phase in chromatography is manifested in the eluotropic series, which orders the elution strength of various liquids. They compared the elution power of a number of liquids with their solubility parameters, and they found a striking parallel between the two, i.e., Snyder s values correlated very well with Hildebrand s 8. Therefore, Giddings et al. suggested that the elution power of compressed gases may be related to the solubility parameter. But they stated explicitly... 

Supercritical fluids can also be used as mobile phases in chromatography [20, 21). Stationary phases used in both GC and LC can be employed. The sample is normally injected into a mobile phase which is in the subcritical liquid state. Subsequently it is converted into a supercritical fluid by raising the temperature above the critical point. 

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... 

Above the critical point (3 PC and 73 bar for carbon dioxide) a substance has remarkable properties it has a high dissolving power, which can be varied by changing the density the viscosity is similar to that of a gas and solute diffusion coefficients are intermediate between those in a gas and those in a liquid. These properties are, of course, particularly relevant to chromatography, and the properties of some of the fluids which have been used as mobile phases in chromatography are listed in Table 9.1. 

Chapter 2 discussed microemulsion structure. These organized media are stable and transparent. They are possible candidates for mobile phases in chromatography. Bile salt solutions are another kind of special micelles with chiral properties that can be used in MLC as well. Supercritical fluids (SF) were also used as surfactant solvents to perform micellar SFC, a variation of MLC. 

As in the case of MS-RTP, CD-RTP has been used as detector in LC. Unless the problems of low efficiency with CD mobile phases in chromatography are resolved, CD-RTP will not be a competitive alternative to the MS-RTP detection approach. 

In the present chapter we describe the relation between three classes of mobile phases in chromatography gas, supercritical fluid and liquid. Further, the relation between packed and open tubular columns is discussed. Mobile and stationary phases relevant to lipid separations are reviewed as well as instrumental aspects such as injection and detection. SFE of lipids for analytical and semi-preparative purposes is discussed. 

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. 

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