Determination of carboxylic acids in water by gas chromatography–mass spectrometry after continuous extraction and derivatisation

This paper describes a new approach for the determination of monocarboxylic, dicarboxylic and tricarboxylic acids (35 compounds) in water. The analytes, in acid medium (pH ∼1.3), were sorbed on an 80 mg LiChrolut EN–Supelclean ENVI-18 (1:1) column and subsequently eluted with methanol. After evaporation of the extract to ∼10 μL, the analytes were spiked with 60 μL of the derivatising reagent and derivatised in a household microwave oven for 3 min. Among the reagents tested (BF3/1-butanol; acetyl chloride/1-butanol; isobutyl chloroformate/1-butanol; trimethylphenylammonium hydroxide, N,O-bis-(trimethylsilyl)acetamide, N,O-bis-(trimethylsilyl)trifluoroacetamide and trimethylchlorosilane), the best results in terms of reaction yield and stability of the derivatives were obtained with the mixture of 1% trimethylchlorosilane in N,O-bis-(trimethylsilyl)trifluoroacetamide. Microwave assisted derivatisation was used as an alternative heating approach for the rapid silylation of carboxylic acids. The proposed method proved to be a suitable analytical procedure for several types of carboxylic acids in water, with limits of detection within the range 0.6–15 ng L−1, precision values from 4.0 to 6.0% (as within-day relative standard deviation) and recoveries from 93 to 101% for all the target analytes.