Analysis of Methanol in the Presence of Ethanol, Using a Hybrid Capillary Electrophoresis Device with Electrochemical Derivatization and Conductivity Detection

Concurrently with ethanol, many other compounds can be formed during the fermentation of grains and fruits. Among those, methanol is particularly important (because of its toxicity) and is typically formed at concentrations much lower than ethanol, presenting a particular challenge that demands the implementation of separation techniques. Aiming to provide an alternative to traditional chromatographic approaches, a hybrid electrophoresis device with electrochemical preprocessing and contactless conductivity detection (hybrid EC-CE-C4D) is herein described. The device was applied to perform the electro-oxidation of primary alcohols, followed by the separation and detection of the respective carboxylates. According to the presented results, the optimum conditions were obtained when the sample was diluted with 2 mmol L-1 HNO3 and then electro-oxidized by applying a potential of 1.4 V for 60 s. The oxidation products were then electrokinetically injected by applying a potential of 3 kV for 4 s and separated using a potential of 3 kV and a background running electrolyte (BGE) consisting of 10 mmol L-1 N-cyclohexyl-2-aminoethanesulfonic acid (CHES) and 5 mmol L-1 sodium hydroxide (NaOH). n-Propanol was used as an internal standard and the three carboxylate peaks were resolved with baseline separation within <3 min, defining linear calibration curves in the range of 0.10-5.0 mmol L-1. Limits of detection (LODs) of 20, 40, and 50 μmol L-1 were obtained for ethanol, n-propanol, and methanol, respectively. To demonstrate the applicability of the proposed strategy, a laboratory-made sample (moonshine) was used. Aliquots collected along the beginning of the fractional distillation presented a decreasing methanol ratio (from 4% to <0.5%) and a growing ethanol ratio (from 80% to 100%) in the collected volume.