Amines: Gas Chromatography

Aliphatic and aromatic mono-, diand polyamines are naturally occurring compounds formed as metabolic products in microorganisms, plants and animals, in which the principal routes of amine formation include the decarboxylation of amino acids, amination of carbonyl compounds and degradation of nitrogen-containing compounds. Accordingly, amines are important indicators of a wide variety of biochemical, clinical, toxicological and fermentation processes. Amines are also widely used as raw materials or as intermediates in the manufacture of industrial chemicals, e.g. pesticides, medicines, dyestuffs, rubbers, polymers, surfactants, cosmetics and corrosion inhibitors. Many of them are discharged into the atmosphere and water from anthropogenic sources such as foods, cattle feeds, livestock buildings, waste incineration, sewage treatment, automobile exhaust, cigarette smoke and various industries. Furthermore, many amines have an unpleasant smell and are hazardous to health as sensitizers and irritants to the skin, eye, mucous membranes and respiratory tract. Some amines are also suspected to be allergenic, mutagenic or carcinogenic substances due to their adsorption in living tissue. Amines are not only toxic of themselves but can also become toxicN-nitrosamines through chemical reactions with nitrosating agents such as nitrite or nitrate. Gas chromatography (GC) has been widely used for amine analysis because of its inherent advantages of simplicity, high resolving power, high sensitivity, short analysis time and low cost. In addition, a wide variety of detectors can be used: nitrogen}phosphorus (NPD), electrolytic conductivity (ELCD) and chemiluminescent (CLD) detectors offer increased selectivity for speciRc amines. Furthermore, the combined technique of GC-mass spectrometry (MS) can provide structural information for the unequivocal identiRcation of amines. Sub-nanogram detection limits can be achieved using these detectors. However, GC separation of free amines at very low concentrations generally has inherent problems related to the difRculty in handling low molecular mass amines because of their high water solubility, high volatility and ready oxidation under chromatographic conditions. Furthermore, amines tend to be strongly adsorbed and decomposed on the columns and give tailing peaks, ghosting phenomena and low detector response. The adsorption tendency in the analytical system, i.e. in sample vessels, injector, glass wool and GC column, is in the order primary' secondary'tertiary amines, and tailing becomes increasingly severe as the basicity of the amines increases. In addition, it is generally more difRcult to chromatograph aliphatic than aromatic amines. A common method of overcoming these problems is to convert such polar compounds to relatively nonpolar derivatives more suitable for GC analysis. A number of derivatives such as acyl, silyl, dinitrophenyl, permethyl, Schiff base, carbamate, sulfonamide and phosphonamide compounds have been used for this purpose. Another successful approach has been to employ less reactive column packing materials to reduce the interaction with solutes, for example, the use of porous polymers and the deactivation of supports by treatment with alkali. Wall-coated (WCOT), support-coated (SCOT) and porous-layer (PLOT) open tubular capillary columns, which minimize column}solute interactions, have also been used for this purpose. Free amines can be analysed after addition of alkali, either by direct injection or by headspace sampling, or they can be extracted into an organic solvent before analysis. Direct or headspace analysis of samples minimizes sample preparation, thereby reducing the possibility of contamination. Solidphase microextraction (SPME), with integrated sampling, extraction, concentration and sample introduction in a single step, has recently been used for amine analysis by coupling with GC. This article is concerned with the general aspects of direct GC separation of underivatized aliphatic and aromatic amines, and various characteristics with respect to columns are considered in more detail below.