Stationary phase Carbon-coated phases

The distinguishing features of gas chromatography are a gaseous mobile phase and a solid or immobilized liquid stationary phase. Liquid stationary phases are available in packed or capillary columns. In the packed columns, the liquid phase is deposited on a finely divided, inert solid support, such as diatomaceous earth or porous polymer, which is packed into a column that typically has a 2- to 4-mm id and is 1 to 3 m long. In capillary columns, which contain no particles, the liquid phase is deposited on the inner surface of the fused silica column and may be chemically bonded to it. In gas-solid chromatography, the solid phase is an active adsorbent, such as alumina, silica, or carbon, packed into a column. Polyaromatic porous resins, which are sometimes used in packed columns, are not coated with a liquid phase. 

Other applications that utilize different types of reversed-phase columns in both dimensions have been advocated by Carr (Stoll et al., 2006) for metabolomics work in small-molecule separations. These stationary phases include a pentafluorophenyl-propyl stationary phase in the first dimension and a carbon-coated zirconia material stationary phase in the second dimension. A common mistake in 2D method development is to mismatch the solvent system the two solvent systems must be miscible as discussed below. 

Small stationary phase coated fiber exposed to sample, then transferred to inlet Inert gas bubbles through sample vaporized analytes collected on trap and desorbed into GC Supercritical carbon dioxide extracts analytes from liquid and solid samples... 

One disadvantage of all silica-based stationary phases is their instability against hydrolysis. At neutral pH and room temperature the saturation concentration of silicate in water amounts to lOOppm. Solubility increases with surface area, decreasing particle diameter drastically with pH above 7.5. This leads also to a reduction of the carbon content. Hydrolysis can be recognized during the use of columns by a loss in efficiency and/or loss of retention. Bulky silanes [32], polymer coating [33], or polymeric encapsulation [34] have been used in the preparation of bonded phases to reduce hydrolytic instability, but most of the RPs in use are prepared in the classical way, by surface silanization. Figure 2.3 schematically shows these different types of stationary phases. 

The experimental apparatus and technique has been described in detail elsewhere.(20>21) The retention factors of naphthalene and biphenyl under isothermal conditions at various pressures were obtained using capillary columns coated with a cross-linked phenyl polymethylphenyl-slloxane stationary phase with carbon dioxide as the fluid mobile phase. A Varian 8500 syringe pump was operated under computer control providing accurate, pulsefree control of the fluid pressure. 

Chambers, S.D. and Lucy C.A. Surfactant-coated graphitic carbon-based stationary phases for anion-exchange chromatography. J. Chromatogr. A. 2007, 1176, 178-184. 

Carbopak B and C are graphitised carbon black, having siuface areas of 12 and 1(X) mVg, respectively. They e usually modified with a light coating of a polar stationary phase. DifiGcult separations of the Cj to CjQ hydrocarbons can be achieved... 

In order to increase the selectivity and efficiency of extraction, novel stationary phases have been developed, particularly in trace analysis. Cyclodextrins, graphi-tized carbon blacks, and conductive polymers such as polypyrrole and poly aniline have been investigated [73]. Sol-gel polymers also seem to be interesting. A particularly important feature is the high thermal stability of these polymers [74]. Recent applications for analysis of biological samples have been described in several articles [75-81]. Review articles present recent developments in methodology, the SPME technique [82-84], and novel coatings [85, 86]. 

One of the most interesting thermally stable groups of stationary phase materials has been the polybutadiene, carbon and phenyl-coated zirconia... 

Several other stationary phases made from different proportions of typical phases (methyl, phenyl, cyanopropyl), or from special compounds such as polytrifluropropyl-siloxane, or different columns such as PLOT (porous layer open tubular), columns coated with a modified graphitized carbon or with a silicone based polymer with chiral groups incorporated into the polymeric chain, columns coated with derivatized cyclodextrins (for the separation of chiral compounds), etc. are also utilized. 

In RP-TLC, silica gel plates impregnated with a strong hydrophobic agent (paraffin oil or silicone oil, usually 5%) have been extensively used in the past as nonpolar stationary phases. Nowadays, plates covered with octa-decyl-silanized (ODS) silica gel are available. In this material, the silanol groups are etherified with alkyls containing 8 (Cg) or 18 (Cig) carbon atoms. The low wettability of HPTLC plates coated with highly etherified silica gel poses limitations in the water content of the mobile phase. This problem is circumvented by the use of RP-Cig plates with 50% etherification. However, the presence of free silanol groups may lead to undesirable silanophilic interactions, especially with low water content in the mobile phase. 

Supercritical pSFC applications can be defined as those in which the mobile phase is a single substance heated and pressurized above its critical point. Carbon dioxide has overwhelmingly been the compound of choice for these mobile phases. Stationary phases typically used for these applications have been polymeric materials or polymer-coated porous silica. Chromatography on uncoated silica-based stationary phases with CO2 has, in general, been unsuccessful. 

Fig. 1 Use of polymer-coated silica stationary phase with a neat carbon dioxide mobile phase. Column Deltabond SFC Methyl (150 X 2.0 mm) mobile phase CO2 pressure gradient 75 bar initial for 2 min, then 50 bar/min ramp to 180 bar, then 15 bar/min ramp to 300 bar flow 0.5 mL/min temperature 100°C injection 5 jjlL detection flame ionization detector sample solvent methylene chloride. Peak key 1 BHT 2 dimethyl azelate 3 triethyl citrate 4 tributyl phosphate 5 methyl palmitate 6 methyl stearate 7 diethylhexyl phthalate 8 Tinuvin 327 9 Spectra-Sorb UV531 10 tri(2- ethylhexyl) trimellitate 11 dilauryl thiodipropionate ...

H2. Halasz, I., and Horvath, C., Micro beads coated with a porous thin layer as column packing in gas chromatography. Some properties of graphited carbon black as stationary phase. Anal. Chem. 36, 1178-1186 (1964). 

By coating the support with a solid which will cover the adsorption sites. The example given under (1) of coating the support for amine separations with potassium hydroxide or carbonate should strictly be classified under this heading. A more troublesome procedure is silver-plating the support, and by this means improved separations of alcohols have been obtained from columns containing Apiezon stationary phase. 

---