Automated in-syringe single-drop head-space micro-extraction applied to the determination of ethanol in wine samples

A novel approach of head-space single-drop micro-extraction applied to the determination of ethanol in wine is presented. For the first time, the syringe of an automated syringe pump was used as an extraction chamber of adaptable size for a volatile analyte. This approach enabled to apply negative pressure during the enrichment step, which favored the evaporation of the analyte. Placing a slowly spinning magnetic stirring bar inside the syringe, effective syringe cleaning as well as mixing of the sample with buffer solution to suppress the interference of acetic acid was achieved. Ethanol determination was based on the reduction of a single drop of 3mmol L(-1) potassium dichromate dissolved in 8mol L(-1) sulfuric acid. The drop was positioned in the syringe inlet in the head-space above the sample with posterior spectrophotometric quantification. The entire procedure was carried out automatically using a simple sequential injection analyzer system. One analysis required less than 5min including the washing step. A limit of detection of 0.025% (v/v) of ethanol and an average repeatability of less than 5.0% RSD were achieved. The consumption of dichromate reagent, buffer, and sample per analysis were only 20μL, 200μL, and 1mL, respectively. The results of real samples analysis did not differ significantly from those obtained with the references gas chromatography method.