Operating temperature, stationary phases

One of the important operational variables in CEC is the analyte—sorbent interaction. In reversed-phase separations (typical in CEC) the hydrophobicity of the stationary phase determines the selectivity of the separation, and retention can be controlled by adjusting the surface chemistry of the packing, composition of the mobile phase, and temperature. In contrast to HPEC, the CEC column plays a dual role in providing a flow driving force and separation unit at the same time hence electrophoretic and chromatographic processes are operational. The stationary phase chemistry is dealt with in detail in Section III on column technology. 

An ideal stationary liquid phase for GLC should exhibit selectivity and differential solubility of the components to be separated and a wide range of operating temperature. A phase should be chemically stable and have a low vapor pressure at elevated column temperatures (< C0.1 torr). A minimum temperature limit near ambient temperature, where the liquid phase still exists as a liquid and not as a solid, is desirable for separations at or near room temperature and eliminates a gas-solid adsorption mechanism (e.g., Carbowax 20M is a solid at temperatures below 60°C. In choosing a liquid phase, some fundamental criteria must be considered ... 

The relative retention is dependent on (1) the nature of the stationary and mobile phases and (2) the column operating temperature. 

An important problem with all liquid stationary phases is their tendency to bleed from the column. The temperature limits listed in Table 12.2 are those that minimize the loss of stationary phase. When operated above these limits, a column s useful lifetime is significantly shortened. Capillary columns with bonded or... 

Elution Chromatography The components of the mobile phase supphed to the cohimn ter feed introduction have less affinity for the stationary phase than any of the feed solutes. Under trace conditions, the feed solutes travel through the cohimn as bands or zones at different velocities that depend only on the composition of the mobile phase and the operating temperature and that exit from the cohimn at different times. 

It is clear that the separation ratio is simply the ratio of the distribution coefficients of the two solutes, which only depend on the operating temperature and the nature of the two phases. More importantly, they are independent of the mobile phase flow rate and the phase ratio of the column. This means, for example, that the same separation ratios will be obtained for two solutes chromatographed on either a packed column or a capillary column, providing the temperature is the same and the same phase system is employed. This does, however, assume that there are no exclusion effects from the support or stationary phase. If the support or stationary phase is porous, as, for example, silica gel or silica gel based materials, and a pair of solutes differ in size, then the stationary phase available to one solute may not be available to the other. In which case, unless both stationary phases have exactly the same pore distribution, if separated on another column, the separation ratios may not be the same, even if the same phase system and temperature are employed. This will become more evident when the measurement of dead volume is discussed and the importance of pore distribution is considered. 

The basis of chromatography is in the differential migration of chemicals injected into a column. The carrier fluid takes the solutes through the bed used for elution (mobile phase). The bed is the stationary phase. Based on mobility, the retention-time detectors identify the fast and slow-moving molecules. Based on internal or external standards with defined concentration, all unknown molecules are calculated in a developed method by software. GC columns are installed in an oven which operates at a specified temperature. A diagram of an oven with GC column is shown in Figure 7.16. 

In high temperature (HT)-GC-MS and PyGC-MS experiments, special attention should be given to the stability of the column. GC columns can lose some of the stationary phase ( bleeding ) when heated up to the maximum operating temperature the thicker the stationary phase, the more column bleed may be expected. When coupled on-line to a mass spectrometer, the stationary phase may foul the ion source, which leads to rapid decay in sensitivity and detection of usually siloxane-related mass peaks at m/z 207, 281, 355, etc. HTGC-MS coupling was discussed by ref. [218]. 

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. 

Stationary phase Operating temperature ro Typical applications... 

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