Choice of Column

The column has an essential role in the separation process. Optimization of the separation process by suitable choice of chromatographic column, therefore, is the main starting point of any gas chromato-graphic analysis. Selection of a column is often made on the basis of the nature of the samples and the number of components to be analysed.

Capillary columns have been little used, and mainly for liquid anaesthetic analysis without gas components. The reason for this is the unfavourable retention factors of low boiling compounds on capillary columns operated at room temperature.

Packed columns may be subdivided as liquid partition and solid adsorbent columns. Almost all the anaesthetic gas analysis reported so far has been performed on packed columns of various lengths, either single or combined, commonly with 1/8 in and 1/4 in o.d. Liquid partition columns are generally employed to separate the high boiling or heavier components such as liquid anaesthetics, while solid absorbent columns are used for the permanent gases (CO2, O2 and N2).

Synthetic porous polymer beads, which have been in widespread use as solid adsorbent packing material, are available commercially under a variety of trade names (Chromosorb Century Series, Porapak). Columns packed with porous polymer beads are more versatile and capable of separating each of the individual groups of components such as light gases and liquid anaesthetics at different temperatures as well as their complex mixtures with suitable temperature and column arrangements. No special treatment is required to obtain symmetrical peaks as they are chemically inert to the anaesthetic substances under the chromatographic conditions employed (usually 20-220°C). The combined effects of increasing viscosity of the carrier gas and expansion of the stationary phase as the temperature rises result in a very marked decrease in the carrier flow (Figure 3), e.g. a temperature rise from ambient to 200°C decreases the flow of the carrier from around 50mLmin~1 to 20mLmin~1 at 40 psig (2.7 bar) He inlet pressure, with a 2 m, 80-100 mesh Chromosorb 101 column. Nevertheless, the chromatography remains adequate and gives peaks for the liquid anaesthetics which are easily integrated. The size of the particles, expressed in mesh size, is very important in the column efficiency as the separation is provided by the surface and structure characteristics of the packing material. When the size of the particles is reduced, the column efficiency is increased and so is the inlet pressure because of the high pressure resistance of the column. At the present time, 80/100 mesh is the most widely used fraction; however, in instances where higher efficiency is needed, 100/120 mesh is frequently used.

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Solar Panel Basics

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