Liquid chromatography of carboxylic acids with potentiometric detection using a tungsten oxide electrode

A tungsten oxide electrode was evaluated for potentiometric detection of carboxylic acids separated using various liquid Chromatographic methods, such as reversed-phase, ion-interaction and ion-exclusion. The separation of formic, acetic, propionic and butyric acids was performed using reversed-phase chromatography with 10% methanol/water solution at pH 6.0 as the eluent. The calibration plots were linear in the range 0.5–3.0 mmol, and the detection limits ranged ca. 0.01–0.05 mmol for the test acids. Potentiometric detection was also applied to ion-interaction chromatography, and the separation of carboxylic acids was improved by the use of 0.3 mmol decylammonium salicylate at pH 5.5 as an eluent. In this method, detection limits for formic, acetic, and propionic acids obtained were 0.01, 0.05, and 0.1 mmol, respectively. Comparison between the reversed-phase and ion-exclusion methods showed that the tungsten oxide electrode was more sensitive to the test acids for the ion-exclusion method, but gave a poorer separation for carboxylic acids using dilute sulphuric acid as the eluent. The potentiometric method of detection was compared with the UV method, and was shown to have advantages in terms of low cost, simplicity of construction and operation, and sensitivity when used in conjunction with the reversed-phase method of separation.