Rapid analysis of carboxylic acids and esters with a direct analysis in real time ion source

Coal oxidation produces carboxylic acids (CAs), including aliphatic acids, benzoic acids, and benzenepolycarboxylic acids, which are important fine chemicals which could be used to understand the structural features of coals. However, detecting CAs usually presents great challenges due to extremely troublesome pretreatments. Therefore, it is essential to develop an analytical method for the rapid detection of CAs from coal oxidation.

A series of model compounds (MCs) of oxidation products and two practical samples were investigated by direct analysis in real time time-of-flight mass spectrometry (DART-TOFMS) under three different analytical conditions (ionizing gas temperature, organic solvent, and MC concentration).

Ionizing methyl benzoate, dimethyl phthalate, and dimethyl adipate produces typical ions of methyl esters, including [M - OCH₃ ]⁺ , [M + H]⁺ , and [M + NH₄ ]⁺ . In contrast, the characteristic ions generated from CAs are polymer ions, such as [2 M + NH₄ ]⁺ , [3 M + NH₄ ]⁺ , [4 M + NH₄ ]⁺ , and [5 M + NH₄ ]⁺ , indicating the strong intermolecular hydrogen-bond interaction among CAs.

Results suggest that DART-TOFMS could rapidly analyze CAs or esters in coal oxidation products according to their typical ions to further gain deep insights into the coal structure.