Quantification of methylamine in the headspace of ethanol of agricultural origin by selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry, SIFT-MS, has been used to investigate if absolute levels of trace compounds in the headspace of ethanol/water vapour mixture can be quantified. This case study was directed towards the analysis of methylamine in distilled ethanol of agricultural origin because of its relevance to quality control legislation in the distillery industry. This has required a detailed study of the ion chemistry occurring – initiated by H₃O⁺ precursor ions – when ethanol/water vapour mixtures are introduced into the H₃O⁺/helium carrier gas swarm and has resulted in the construction of a full scheme of the complex ionic reactions that occur. It has been found that under the SIFT-MS flow reactor conditions (He pressure 130 Pa and temperature 299 K) the terminating ions of the several parallel and sequential reactions that occur are the proton bound ethanol clusters ions, C₂H₅OH₂⁺(C₂H₅OH)_n, with n = 1,2,3, proton bound trimer (n = 2) being the dominant species. These ethanol cluster ions can be used as precursor (reagent) ions for the chemical ionisation of the methylamine present in the ethanol/water vapour, which produces two characteristic product ions CH₃NH₂H⁺(C₂H₅OH)_1,2 that are used for the methylamine analysis. The ratio of the product ion count rate to the precursor ion count rate is used in an analogous way to the routinely used for SIFT-MS analyses to quantify the methylamine concentration. The results of calibration experiments show that using SIFT-MS it is possible to quantify methylamine in liquid ethanol/water mixtures at levels of 0.1 mg/L or greater.