Dissolution stationary phase

Silica gel or alumina is usually used as the stationary phase. For these active phases a solvent of weak elution power is recommended for dissolution of the sample. A... 

When an amino-type column becomes contaminated by physically adsorbed, non-polar materials, it may be cleaned by washing with acetonitrile, hexane, and dichloromethane. If column failure is due to covalent interactions, or to dissolution of the stationary phase, there is very little that can be done to regenerate it. Samples should therefore always be cleaned up, and a pre-column and a saturator column should always be used with the analytical column. 

When working with capillary columns, the splitless mode is used for very dilute samples. In this mode, the injection is made very slowly, leaving valve no. 2 in the closed position (Fig. 2.5) for approximately 0.5 to 1 min. This allows vaporisation of the compounds and solvent in the first decimetre of the column by a complex mechanism of dissolution in the stationary phase, which is saturated with solvent. Compound discrimination is very weak using this method. The proper use of this injection mode, which demands some experience, requires a temperature program that starts with a colder temperature so that the solvent can precede the analytes in the column. This mode is typically used for trace analyses. The opening of valve no. 2 eliminates, from the injector, compounds which are less volatile and that can interfere with the analyses. 

Column temperature affects the relative retention of different compounds and elevated temperature permits high-speed chromatography to be conducted.25 Figure 25-28 suggests a systematic procedure for method development in which solvent composition and temperature are the two independent variables.1 For elevated temperature operation, pH should be below 6 to retard dissolution of silica. Alternatively, zirconia-based stationary phases work up to at least 200°C. 

Components of the sample are retained in the column for different lengths of time due to adsorption-desorption, solution-dissolution, chemical affinity, size exclusion, and other mechanisms of varying nature. Various components are continually washed from one part of the stationary phase and recaptured by another by the moving phase. Different components elute in groups from the column with respect to time from injection. Dispersion in the system causes the bands of components to emerge with... 

Silica based stationary phases are unstable to alkaline conditions due to loss of the alkyl chain and dissolution of the silica... 

Some problems associated with conventional LLC (e.g., the loss of the liquid stationary phase through dissolution in the mobile phase) may be obviated by chemically bonding the liquid stationary phase to the support medium. This type of liquid-liquid chromatography is designated bonded phase chromatography (BPC)(11). Since the properties of bonded phases may differ substantially from those of coated phases, BPC separation characteristics may differ from those of conventional LLC. Many phases have exhibited increased efficiency when bonded to the support medium. Most current reverse phase HPLC work involves the use of stationary phases bonded to microparticles. 

To avoid basic silica dissolution that leads to poor reproducibility, reduced efficiencies, poor peak shapes and high back pressure carbon-based stationary phases were also tested. Unlike silica-based reversed phases, they can be exposed to both highly acidic and alkaline environments (pH 1 to 14) and very high temperatures without degradation [15] they proved very useful in the analysis of organic acids [16]. 

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]. 

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