Chemically selective stationary phases compounds

A good example of highly selective stationary phases is those based on the complexing reaction between the separated volatile compounds and the non-volatile active component of the stationary phase. Complexing in the stationary phase is a particular case of reversible chemical interaction. The selectivity of such phases is extremely high it is sufficient for the separation of the isomers of some compounds, particularly nuclear isomers of unsaturated organic compounds whose molecules differ only in the content and position of deuterium atoms. 

Because coal-derived liquids and tars are usually complex mixtures of organic chemicals, the separation and identification of individual PACs in these samples have been best accomplished by using open-tubular-column (capillary-column) gas chromatography (GC) (3, 4). Selective detectors for nitrogen and sulfur, as well as mass spectrometry (MS), can be easily combined with GC. Especially when authentic standard reference compounds and selective stationary phases (5-10) are used, capillary-column GC is, by far, the easiest and most reliable technique available. 

A proper choice of the liquid stationary phase is exceedingly important to a successful chromatographic separation. A great number of chemically different stationary phases have been described in the scientific and commercial literature. Several requirements govern the choice of a chemical substance as a GC stationary phase. First of all, it should have adequate selectivity for the substances to be resolved. It must be chemically stable at the column temperatures used in a given separation problem. The stationary phase must easily adhere as a uniform film to the column support without running off the column if such mechanical instability occurs, the phase contaminates the detector and, naturally, the columns function properly for only a limited time. Finally, the stationary phase should be a well-defined chemical compound, so that the column preparation as well as the chromatographic process itself are reproducible. 

The Diacel columns can be used for the separation of a wide variety of compounds, including aromatic hydrocarbons having hydroxyl groups, carbonyls and sulfoxides, barbiturates, and P-blockers (35,36). There are presendy nine different cellulose derivative-based columns produced by Diacel Chemical Industries. The different columns each demonstrate unique selectivities so that a choice of stationary phases is available to accomplish a separation. 

Some selected applications (chemical class, mobile phase, stationary phase, column, detector) are as follows fatty acids, C02, PEG or CN-PS, OTC, FID organotin, C02, PhMe-PS, OTC, FPD [305], ThepSFC-ICP system is an effective tool for speciation of metal-containing compounds [362], even in the presence of modifiers to control the retention of components. 

Normal-phase (NP) and reversed-phase (RP) liquid chromatography are simple divisions of the LC techniques based on the relative polarities of the mobile and stationary phases (Figure 4.10). Both NPLC and RPLC analysis make use of either the isocratic or gradient elution modes of separation (i.e. constant or variable composition of the mobile phase, respectively). Selection from these four available separation techniques depends on many variables but basically on the number and chemical structure of the compounds to be separated and on the scope of the analysis. 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

Stationary phases can be made highly selective by adding compounds to them which have affinities for certain chemical species. For example, silver nitrate, incorporated into a polar liquid preferentially retards the elution of alkenes by formation of weak -complexes. A selection of stationary phases with their maximum operating temperatures and useful applications is given in Table 4.10. 

Gas-Liquid Chromatography. In gas-liquid chromatography (GLC) the stationary phase is a liquid. GLC capillary columns are coated internally with a liquid (WCOT columns) stationary phase. As discussed above, in GC the interaction of the sample molecules with the mobile phase is very weak. Therefore, the primary means of creating differential adsorption is through the choice of the particular liquid stationary phase to be used. The basic principle is that analytes selectively interact with stationary phases of similar chemical nature. For example, a mixture of nonpolar components of the same chemical type, such as hydrocarbons in most petroleum fractions, often separates well on a column with a nonpolar stationary phase, while samples with polar or polarizable compounds often resolve well on the more polar and/or polarizable stationary phases. Reference 7 is a metabolomics example of capillary GC-MS. 

Different stationary phases have different polarity and chemical composition and are designed for the separation of different classes of organic compounds. The proper selection of the stationary phase is a key to a successful separation. The selection process has been made easy due to the efforts of column manufacturers who conduct research in the area of environmental pollutant analysis and offer a range of columns specifically designed for dilferent applications. The EPA validates analytical methods using these state-of-the-art columns and recommends them as part of the approved methods. 

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