Stationary phases temperature operating rang

The thermal stability of the stationary phase is also important. Because each stationary phase has a range of thermal stability, it is important to control column temperature within the specified range. For the nonpolar phases, the temperature limit is determined by the stability of the poly-imide coating. The introduction of aluminum clad columns notably broadens the useable temperature range. Oxidation at higher temperatures limits the operating temperature of intermediate to polar phases. 

Rough guide to the temperature operating range for bonded poly(siloxane) stationary phases in open tubular columns... 

Over the years thousands of substances have been used as stationary phases. For several reasons most of these have been abandoned in favor of a small number of liquids and adsorbents with favorable thermal stability and kinetic properties, complementary selectivity, reasonably well-defined and reproducible chemical composition, and if used in WCOT columns, the possibility of immobilization. Practical considerations dictate that liquid stationary phases should be inert, of low vapor pressure, have good coating characteristics, and have reasonable solubility in some common volatile organic solvent. The desirability of a wide temperature operating range tends to dictate that most common stationary phases are polymeric materials, although polymers are more likely to show greater composition variation than stoichiometric compounds. 

The range of stationary phases which is now available commercially is very extensive and offers an adequate choice of polar character and thermal stability. Table 2.15 lists some of the more generally useful stationary phases, their maximum operating temperatures and the classes of compounds for which satisfactory separations have most usually been found. 

Chromatography is a very versatile technique offering a wide range of solid phase materials and detector types which can deal with very complex mixtures. In practice all materials and conditions used in the instrument are carefully chosen to match the type of sample mixture involved. This includes selection of stationary phase (chemical and physical properties) column type and length sample pretreatment, operational temperatures, pressures, and flow rates physical and chemical nature of mobile phase detector type and so forth. Detection to nanogram level is quite common and some systems can detect to picogram level using very small volumes of sample. 

A liquid-liquid system can be created by coating a particulate matter with a thin layer of a liquid phase, similar to the way packed columns are used in GLC. To maintain such an LLC column, the stationary phase should be insoluble in the mobile phase, just as GLC phases need to be involatile at the temperature of operation. Unfortunately, insolubility is an absolute demand that can at best be approximated in practice. The solubility of the stationary phase in the mobile phase becomes even more critical once some flexibility is desired with regard to the choice of the mobile phase. For example, mixtures of several pure solvents are usually required in order to adapt the eluotropic strength (polarity) of the mobile phase such that the capacity factors fall in the optimum range. 

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