Application of Experimental Design for Determination of Methanol and Ethanol in Transformer Industrial Oils Using Headspace Single-Drop Microextraction

A simple, inexpensive, nearly solvent-free headspace single-drop microextraction (HS-SDME) followed by gas chromatography and flame ionization detection (GC-FID) was developed for determining methanol and ethanol in samples of transformer insulation oils. Five effective parameters in HS-SDME process, including extraction solvent volume, sample content, extraction time, extraction temperature, and agitation speed were screened and subsequently optimized using fractional factorial screening methodology (FFSM) and response surface methodology using Box-Behnken design (BBD). The extraction was carried out with dimethyl sulfoxide (DMSO) as micro drop, with 1µl of extraction phase injected into the GC-FID system. The calibration curve for the analytes was found to be linear within the range of 0.05 µg/g to 10.00 µg/g, and the limits of detection for methanol and ethanol were 0.02 µg/g and 0.03 µg/g, respectively. The relative standard deviation obtained within 1.8 - 5.2% and a good recovery achieved within the range of 95.0 to 102.6% in the analysis of oil samples revealed that methanol and ethanol could be determined as a chemical marker of the transformer life using this method.