Bonded stationary phases porous carbon

More recently, Engel and Olesik have also concluded that formic acid is a good modifier for carbon dioxide in SFC. The results they obtained on porous glassy carbon stationary phases with 1.5% (w/w) formic acid in CO2 (16) showed that formic acid was effective because of its strong H-bond donor and very weak H-bond acceptor characteristics higher concentrations of formic acid (3%), however, were found to polymerize on the porous glassy carbon surface (17). 

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. 

Chemical stability of carbon over the entire pH range has led to considerable interest in the development of carbon-based stationary phases for RPC. Porous graphitised carbon with sufficient hardness, well-defined and stable pore structure without micropores, which ensures sufficient retention and fast mass transfer can be prepared by a complex approach consisting of impregnation of the silica gel with a mixture of phenol and formaldehyde followed by formation of phenol-formaldehyde resin in the pores of the silica gel, then thermal carbonisation and dissolution of the silica gel by hydrofluoric acid or a hot potassium hydroxide. solution [48. The retention and selectivity behaviour of carbon phases significantly differs from that of chemically bonded pha.ses for RPC. Carbon adsorbents have greater affinity for aromatic and polar substances so that compounds can be separated that are too hydrophilic for adequate retention on a Cix column. Fixed adsorption sites make these materials more selective for the separation of geometric isomers [49]. 

A wide variety of native and derivatized CyDs are available for use as mobile phase additives in HPLC. In liquid chromatography to study the interaction of CyDs with guest molecules, the information about cydodextrin adsorption on the stationary phase is very substantial. It should be noted that the separation ability of bonded CyDs and CyDs added to the mobile phase in HPLC is not always the same (see below). To assess the adsorption of CyDs on the stationary phase, the chromatographic properties of native and permethylated CyDs applied in RP18 and porous graphitic carbon (PGC) column have been studied [25-29]. On the RP18 column the adsorption of yS-CyD is much stronger than that of a- or y-CyDs. On the PGC, the order of elution is a- < < y-CyD, in accordance with the increase of... 

Packed column SFC stationary phases are very similar or identical to those used for HPLC. With neat CO2 mobile phases, polymer or polymer-coated silica stationary phases have typically been used. With modified-C02 mobile phases, bonded-phase silica columns are typically used. For structural separations, diol, amino, or cyano stationary phases are most often used. For stereochemical separations, derivatized polysaccharide-bonded sihca columns are most often the stationary phases of choice. A powerful feamre of modified-C02 pSFC is the ability to serially connect different stationary phases to obtain enhanced or multiple mechanism separations. With subcritical (super heated) water mobile phases, the use of polymer, porous graphitic carbon, and polymer-coated zirconia stationary phases has been described. 

Several HPLC stationary phases are used for the separation of glycans, including amide-bonded hydrophilic interaction (HILIC), reversed phase (RP), weak anion exchange (WAX), and porous graphitized carbon (PGC). Due to the heterogeneous nature and complexity of many glycan populations, analysis may require the use of several orthogonal methods [7],... 

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