Other Stationary Phase Materials

Hydroxyapatite and titania are also useful stationary phase materials for biological samples. Hydroxyapatite, Cai0(PO4)6(OH)2, as a hexagonal column 

Preparation, Testing, and Selectivity of Stationary Phase Materials 

Unger (ed.), Porous Silica , Journal of Chromatography Library, Vol. 16, Elsevier, Amsterdam, 1979. 

in Encyclopedia of Analytical Science , Academic Press, London, 1995, pp. 2558-2567. 

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Common standard compounds for reversed phase columns are toluene and naphthalene, which have retention factors, k, of about 3. The eluent modifier is methanol or acetonitrile at a concentration of 50-80%, depending on the hydrophobicity of the stationary phase material. For other stationary phase materials, corresponding analytes, with k = 3-5, can be used. 

Other stationary phase materials are also being investigated, such as coatings of dimethylpolysiloxane (PDMS) (with or without 5% the diphenyl doping), carbosilanes and Nanoglass, provided by Ohio Valley, SeacoastSciences, Dow Chemical and Honeywell SM (Nanoglass), respectively, which may or may not feature nanopores. 

The TLC process is an off-line process. A number of samples are chromatographed simultaneously, side-by-side. HPTLC is fast (5 min), allows simultaneous separation and can be carried out with the same carrier materials as HPLC. Silica gel and chemically bonded silica gel sorbents are used predominantly in HPTLC other stationary phases are cellulose-based [393]. Separation mechanisms are either NPC (normal-phase chromatography), RPC (reversed-phase chromatography) or IEC (ion-exchange chromatography). RPC on hydrophobic layers is not as widely used in TLC as it is in column chromatography. The resolution capabilities of TLC using silica gel absorbent as compared to C S reversed-phase absorbent have been compared for 18 commercially available plasticisers, and 52 amine and 36 phenolic AOs [394]. 

The packing materials described above separate chemical entities by exploiting chemical differences, e.g., hydrophobicity. Another class of stationary phases separates molecules based on chirality this is accomplished using a silica particle derivatized with a chiral moiety. There are several classes of chiral stationary phases including helical polymers, brushlike functional groups, protein/peptides, and inclusion complexes. Each of these is described in more detail below. Some manufacturers produce chiral stationary phases that operate either in reversed-phase or normal-phase mode, and some chiral stationary phases can be used in both modes. As with other stationary phases, there are numerous manufacturers of chiral stationary phases. However, contrary to Cl 8 and other achiral packing materials, each manufacturer of chiral stationary phases typically offers unique phases with completely separate selectivities. 

Gas chromatography is often divided into categories based on the type of stationary phase used. Gas-liquid chromatography (GLC) implements a porous, inert solid support that is coated with a viscous, nonvolatile liquid phase. On the other hand, gas-solid chromatography (GSC) uses a solid adsorbent as the stationary phase. Klee offers these general rules-of-thumb for selection of stationary phase materials use solid adsorbents to separate room-temperature gases, liquid stationary phases to separate room-temperature liquid and solid mixtures, polar phases for polar solutes, and nonpolar phases for nonpolar solutes. Table 1 lists common liquid- and solid-stationary phase materials available for use in capillary columns. Barry cross-refers numerous column materials from nine different manufacturers. ... 

Gas chromatography achieves separation based on solute partitioning into a thin liquid coating or adsorption onto a solid stationary phase material, whereas LC uses these and other mechanisms to achieve separation. The type of stationary phase and, in some cases, mobile phase determines the mechanism to achieve separation. Descriptions that follow are of the mechanisms and materials used most often. 

A common method to prepare bonded-phase materials is to use silica gel as the solid support and to react functionalized organosilanes onto the surface hydroxyl groups of the silica gel. Of course, other solid support materials can be used provided that they present surface functional groups (e.g., OH, COOH, and NH2) that can participate in modification reactions. The flexibility offered by this approach leads to stationary phase materials with myriad functionalities. Fig. 3 illustrates commonly used organosilane reactants and two example reaction paths that lead to bonded-phase silica. For more discussion on bonded-phase materials, see Ref.. ... 

One of the biggest problems in the early development of GC-MS was interfacing the column outlet to the mass spectrometer. Packed columns were used, and the high volumes of both sample and carrier gas overwhelmed the MS system, which operates under low pressure, and special interfaces had to be built. The advent of fused silica capillary columns meant that the GC-MS interface could be dispensed with, and the column eluent is introduced directly into the ion source. It is essential that column bleeding be minimized since the mass spectrometer will detect the stationary-phase materials. Bleeding is prevented by chemically bonding alkylsiloxanes to the column wall. Other low bleeding stationary phases are mentioned above. 

Generally, it is recommended to use two different supports for clean up and subsequent analysis to take advantage of different selectivities. The use of C-18 cartridges in sample clean up procedures followed by analytical HPLC on the same stationary phase material is therefore probably not the best choice. However, the availability of C-18 in prepacked cartridges is a major advantage compared to the other supports that are used. The packing of columns is usually too slow to be included in routine analysis and gives rise to additional variabilities between laboratories. 

Of the other stationary phases, the amino phases have been used extensively to separate sugars and peptides the nitrile phase has found application in the separation of porphyrins. An important consideration in the use of polar bonded-phase materials is an awareness of the reactivity of the terminal functional group, for example, aminoalkyl bonded phase should not be used for the chromatography of carbonyl compounds due to possible condensation reactions and formation of Schiff s bases. 

There are stationary-phase material names that give no information, such as INTERCHROM and ASAHIPAK, and others that do tell you something about themselves, such as LiChrospher and Inertsil. This subject cannot be dealt with in detail, but in the Table below you should find some help. Listed are numbers, letters, prefixes and suffixes from names that give an indication of properties of column materials. 

Diamide Chiral Separations. The first chiral stationary phase for gas chromatography was reported by GH-Av and co-workers in 1966 (113) and was based on A/-trifluoroacetyl (A/-TFA) L-isoleucine lauryl ester coated on an inert packing material. It was used to resolve the tritiuoroacetylated derivatives of amino acids. Related chiral selectors used by other workers included -dodecanoyl-L-valine-/-butylamide and... 

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