Development of an Ion-Pairing Liquid Chromatography Method for the Determination of Phenoxyacetic Herbicides and Their Main Metabolites: Application to the Analysis of Soil Samples

A modified ion-pairing liquid chromatography method is developed for the analysis of 2,4-dichlorophenoxyacetic acid (2,4-D), 4-chloro-2-methylphenoxyacetic acid (MCPA) and their phenolic metabolites in water and soil samples. The separation and determination of all compounds of interest can be performed with a mobile phase containing 30% acetonitrile and 10 mM tetrabutylammonium hydroxide as the ion-pairing reagent. Separation of the two phenoxyacids is strongly affected by the pH of the mobile phase, which has to be buffered to pH 7.2 with phosphoric acid. The increase in the hydrophobicity of the sample solvent results in wider peaks for the two phenoxyacids, which reduces the resolution in their separation. The evaluation of different methanol:water solvent mixtures shows that no baseline separation for the two phenoxyacids is obtained when the methanol content in the mixture is higher than 60%.