COMPARISON OF SEVERAL METHODS FOR THE DETERMINATION OF TRACE AMOUNTS OF POLAR ALIPHATIC MONOCARBOXYLIC ACIDS BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

Reversed-phase high-performance liquid chromatography followed by direct detection by UV absorbance at 200 nm is a convenient method for the determination of polar aliphatic carboxylic acids. It can only be used for "clean" samples, except for formic and acetic acid, owing to a lack of retention. Up to butyric acid, ion-exclusion chromatography with conductimetric detection is generally preferred, allowing the detection of less than 120-mu-g/l. Each ion-exclusion column has its own selectivity and efficiency; interfering compounds are another problem. The precision at the 1 ppm level is about 2% (n = 10). Pre-column derivatization of carboxylic acids with 4-bromomethyl-6,7-dimethoxycoumarine has been fully automated but is not recommended; limitation of the water content, internal standardization and carry-over correction are necessary. Pre-column derivatization with 2-nitrophenylhydrazine is a better way to obtain detection limits of one nanogram (or 50-mu-g/l) of C4-C9 carboxylic acid. The analyses are fully automated, can be performed in an aqueous environment and give a precision better than 4% at levels of 1 mg/l of analyte.