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Benzene represents aromatics and olefins. Pentanone represents keto compounds and esters. Butanol represents alcohols and weak acids. Nitropropane represents nitro- and nitrile compounds. Pyridine represents N-heterocyles.

Benzene represents aromatics and olefins. Pentanone represents keto compounds and esters. Butanol represents alcohols and weak acids. Nitropropane represents nitro- and nitrile compounds. Pyridine represents N-heterocyles.

>■ McReynolds constants constitute a classification system for GLC stationary phases based on their polarity.

Column conditions

The efficiency of a column can be assessed in a similar manner to that described for HPLC and values for the resolution index of two solutes, the number of theoretical plates and the height equivalent to a theoretical plate may also be calculated. Although it is easier to measure gas pressure, it is the actual gas flow, which is affected by the particle size and compression of the packing, that should be used in column assessment investigations.

In many instances, samples contain components with a wide range of volatilities and it may be difficult to separate them quickly and effectively at a fixed temperature; a temperature gradient may be used. The separation is initiated at the lower temperature for a specified period of time depending upon the retention times of the more volatile components. Subsequently the temperature of the column is raised at a specified rate to speed up the elution of the less volatile components. Such temperature programming can cause some problems but these are usually solved in the design of the instrument. As the temperature rises so the gas flow will fall owing to an increase in the backpressure within the column, and it is usual to have a system that regulates the gas pressure in order to maintain the flow rate. In addition, the increase in temperature may also cause loss of the solvent (bleeding) from the column, resulting in a baseline increase which must be taken into account in peak height measurements. Temperatures above the specified maximum for the stationary phase must not be used as the column will rapidly deteriorate.

Derivatives

Many substances are not initially appropriate for gas chromatography because of their relatively high boiling points or insolubility. In such cases it is often possible to modify the compound chemically and render it more amenable to separation. In some instances, the chemical modification is used to enable easier detection of the compound, e.g. the introduction of a halogen for use with electron capture detectors.

There are a wide variety of derivatization reactions but the most frequently used are indicated in Table 3.9.

Table 3.9 Derivatives for gas chromatography

Reaction

Derivatives

Reagents

Compounds

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