Containment liquid-phase retention systems

In GC, a sample containing the compounds of interest, which may be gas or liquid, is pushed through a column by an inert carrier gas (the mobile phase). The sample moves through a packed or capillary column at different speeds. The sample s components are separated and emerge at different retention times because they are distributed differently between the mobile and the stationary phases. The detection system responds to the presence and concentration of the separated compounds, thus making it possible to identify and quantify them. 

The use of chiral stationary phases (CSP) in liquid chromatography continues to grow at an impressive rate. These CSPs contain natural materials such as cellulose and starch as well as totally synthetic materials, utilizing enantioselective and retentive mechanisms ranging from inclusion complexation to Ti-electron interactions. The major structural features found in chiral stationary phases include cellulose, starch, cyclodextrins, synthetic polymers, proteins, crown ethers, metal complexes, and aromatic w-electron systems. 

Sample injection in gas chromatography often seems deceptively simple a microlitre aliquot is rapidly injected into an inlet system, and elution and detection follow. Samples containing substantial amounts of non-volatile material, however, require one or more preparation steps in order to isolate volatile analytes from non-volatiles that would otherwise contaminate the inlet system and column, eventually leading to impaired chromatographic performance. Examples of such procedures include liquid-liquid extraction, solid-phase extraction and filtration. The use of a pre-column (viz. a retention gap or a guard column) is often required, even if prepared samples are used. 

The procedure of Beroza and Bowman, Anal. Chem. 37,291, 1965, which follows, shows how it is done. Assume a hexane-acetonitrile system. A 5.0 mL aliquot of the upper phase containing a given pesticide is analyzed by gas-liquid chromatography. To a second 5.0 mL aliquot in a graduated, glass-stoppered, 10.0 mL centrifuge tube is added an equal volume of lower phase, the tube is shaken for about 1 minute, and the upper phase is analyzed exactly like the first 5.0 mL aliquot. The ratio of the quantitative results of the second analysis to those of the first (peak areas or heights) is the P-value. This is the amount of the pesticide in the upper phase (second analysis) divided by the total amount of pesticide (first analysis). Table 9-1, p. 104, shows how valuable this can be o,o -DDT and TDE have identical retention times. 

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