Derivatives of organic acids

Abstract

The high polarity, low volatility and thermal instability of organic acids make them unsuitable for direct injection into the gas chromatograph. The carboxylic acids are among the most polar organic compounds known and this polarity is often enhanced in naturally occurring acids by other functional groups containing oxygen, nitrogen or sulphur. Their low volatility is reflected in high latent heats of vaporization and sublimation (Chalmers and Watts, 1972b), and even among the lower-molecular-weight acids, intermolecular hydrogen-bonding reduces volatility. As a result it is necessary to convert the carboxylic acid group into a non-polar volatile thermally stable derivative, such as an ester, prior to gas chromatography. A possible advantage of liquid chromatography is that this derivatization should not be required, although the inadequacy of detection methods for organic acids in these systems has resulted in the introduction of special derivatives for liquid chromatography. These are covered briefly in the chapter on liquid chromatography (Section 4.2), the present chapter being confined to the preparation of derivatives, suitable for gas chromatography, of all functional groups commonly occurring in physiological and pathological organic acids.