Supercritical fluid extraction (SFE) for determination of metalworking fluid aerosols

A common methodology for analyzing metalworking fluid (MWF) aerosols in workplace air is based on gravimetry before and after organic solvent extraction of the MWFs from a suitable collection filter. Because MWFs have different chemical and physical properties, various mixtures of organic solvents have been used to extract the MWFs from their collection device. An alternative to organic solvents, used in the work presented in this article, is the use of a supercritical fluid. The efficiency of supercritical fluid extraction (SFE) was investigated by weighing conditioned filters before and after extraction of samples spiked with MWFs at different concentrations using the American Society for Testing and Materials method. For three common straight oil MWFs spiked on filters, supercritical carbon dioxide gave recoveries of 92-101% with a low standard deviation (0.2-1.9%). For semisynthetic MWFs, carbon dioxide had to be mixed with methanol to obtain recoveries above 80%. With the optimized method using 7% methanol in carbon dioxide, the 10 investigated MWFs could be extracted in 30 min with a recovery of 90-98%. The amount of MWFs spiked on the filters varied between 0.10-1.65 mg. In Sweden, the limit value for MWFs is 1 mg/m3. Thus, our spike level is in the range of 10-200% of the limit value if sampling for 8 hours with 2 L/min. The use of SFE methodology results in small volume extracts (3 mL) with concentrations at such high levels that analysis of chemical components in the MWF can be carried out without further volume reduction.