Enzymatic single-drop microextraction for the assay of ethanol in alcohol-free cosmetics using microvolume fluorospectrometry detection

A green assay based on the development of an enzymatic reaction in drop format under headspace single-drop microextraction conditions is described for the first time. An aqueous drop containing the enzyme alcohol dehydrogenase and the cofactor β-Nicotinamide adenine dinucleotide has been used as fluorescence probe for determining ethanol in alcohol-free cosmetics by microvolume fluorospectrometry. Experimental parameters affecting the microextraction performance were carefully optimized. Under the conditions employed, the contribution of other alcohols was found to be negligible. After 10 min of microextraction, a detection limit of 0.04 μg g−1 ethanol, a repeatability, expressed as relative standard deviation, of 5.3% for a 0.05 mM ethanol standard and a preconcentration factor of 391, were reached. Accuracy of the proposed methodology was evaluated by comparison of calibration slopes corresponding to external calibration with aqueous standards and standard addition calibration. The method was successfully applied to different alcohol-free cosmetics (external calibration was carried out in all cases). Additional advantages such as simplicity and high sample throughput can be highlighted. The greenness profile of proposed methodology was established using NEMI criteria (US National Environmental Methods Index).

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► An enzyme-based assay in drop format is performed for ethanol determination in alcohol-free cosmetics.
► An aqueous drop containing ADH and NAD is used as a probe to extract ethanol.
► Fluorescence changes are monitored by microvolume fluorospectrometry.
► Matrix effects from the cosmetic sample are not present using the headspace-SDME mode.
► LODs better than those obtained by other enzymatic-based methods are reached.