Selection of Stationary Phases

Reversed Cjg phases with high silanol activity Hypersil ODS, Platinum Qg EPS 

Cjg phases with lower hydrophobictty (separation of substances with different polarities, separation of acids) YMC Pro C18, Ultrasep ES RP18e Pharm, Luna Cjg, YMC ( sphere ODS-H80. MP Gd Cjg 

Special phases for fast separations Chromolith Performance RP18e (fast separations with high flow rates) 

The dimensions of the columns are selected in relation to the goal of the method development and the number of the relevant peaks that are to be expected. When the goal is a method purely for assay purposes, with a maximum sample throughput, one installs columns of length 50 mm into the thermostats. For purity testing and the goal of optimum resolution of aU peaks, then columns of length 250 mm should be used. If, however, one expects only a small number of peaks [n 7), it is recommended that the shorter columns are used. 

A set of columns that has been systematically set up according to Table 4 has the advantage that one can easily determine correlations between the peak patterns and the peculiarities of the stationary phases and can use them beneficially in optimizing the method. 

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The stationary phase in LC is a fine granular solid such as silica gel. It can be used as such (mainly for nonpolar compounds), or the granules can be modified by a surface-bonded coating that changes (reverses) the polarity of the gel. A very small selection of stationary phases is listed in Table 35.2. 

Several hundred types of Hquid phases are commercially available. These have been used individually or in combination with other Hquid phases, inorganic salts, acids, or bases. The selection of stationary phases for a particular appHcation is beyond the scope of this article, however, it is one of the most important chromatographic tasks. Stationary phase selection is discussed at length in books, journal articles, and catalogs from vendors. See General References for examples. 

The last tvo approaches represent promising beginnings for new methods to characterize stationary phase selectivity. The methods are evolutionary and not fully developed at present. Their future prospects are quite good and should eventually evolve into a standardized protocol for phase characterization. This is urgently required to make both the selection of stationary phases from those currently available and the rationale synthesis of new phases a logical process. 

Solvent selectivity is a measure of the relative capacity of a solvent to enter into specific solute-solvent interactions, characterized as dispersion, induction, orientation and coaplexation interactions, unfortunately, fundamental aiq>roaches have not advanced to the point where an exact model can be put forward to describe the principal intermolecular forces between complex molecules. Chromatograidters, therefore, have come to rely on empirical models to estimate the solvent selectivity of stationary phases. The Rohrschneider/McReynolds system of phase constants [6,15,318,327,328,380,397,401-403], solubility... 

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. 

Selection of stationary phase should provide 100% sample recovery. 

Preparation, Testing, and Selectivity of Stationary Phase Materials... 

The selectivity of stationary phase materials can be understood if the method of their synthesis is understood. Differences in the same type of stationary phase material from different manufacturers or even from the same manufacturer depend on the synthetic methods and the quality control that has been employed. Details of the individual synthetic processes from different manufacturers have not been published, but are basically the same.1,2... 

An improvement in the resolution can also be achieved by increasing the separation of the peak (A/R) or narrowing the peak width (increased N). An increase in AtR depends on the selection of stationary phase materials and eluent a reduction of the peak width depends on improving the column... 

SRM 869a Column Selectivity Test Mixture for Liquid Chromatography [44] is composed of three shape-constrained PAHs (phenanthro[3,4-c]phenanthrene, PhPh l,2 3,4 5,6 7,8-tetrabenzonaphthalene, TBN and benzo[a]pyrene, BaP) and is routinely employed to evaluate the shape selectivity of stationary phases. The retention differences between the nonplanar TBN and planar BaP solutes (expressed as a selectivity factor axEN/BaP = provide a numerical assessment of... 

Smith, N., and Evans, M. B. (1999). Comparison of the electroosmotic flow profiles and selectivity of stationary phases used in capillary electrochromatography.. Chromatogr. A 832, 41-54. 

Kollie, T.O., Poole, C.F., Abraham, M.H., and Whiting, G.S., Comparison of two free energy of solvation models for characterizing selectivity of stationary phases used in gas-liquid chromatography. Anal. Chim. Acta, 259,1-13, 1992. 

Open tubular columns enable the chromatographer to effect separations impossible by other techniques, for example, separation of isomers. Their use permits a smaller inventory of columns and a smaller selection of stationary phases. Their main advantage is their high efficiency and their disadvantage is their small load capacity. 

Sivtsova, E. V., Kogan, V. B., Ogorodnikov, S. K. Application of gas-liquid chromatography for the selection of stationary phase for use in analysing methyl-chlorosilane mixtures. Zh. Prikl. Khim. (Leningrad) 38, 2609 (1965). — Anal. 

Chromatography is a very versatile technique offering a wide range of solid phase materials and detector types which can deal with very complex mixtures. In practice all materials and conditions used in the instrument are carefully chosen to match the type of sample mixture involved. This includes selection of stationary phase (chemical and physical properties) column type and length sample pretreatment, operational temperatures, pressures, and flow rates physical and chemical nature of mobile phase detector type and so forth. Detection to nanogram level is quite common and some systems can detect to picogram level using very small volumes of sample. 

Almost any polar solid can be used, the most common choices being silica gel or alumina. A selection of stationary phases is listed in table 4.7 in order of decreasing adsorptive power. Silica gel and alumina are highly polar materials that adsorb molecules strongly. They are said to be active adsorbents. Activity is determined by the overall polarity and by the number... 

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