Microwave-Assisted Extraction as an Alternative to Soxhlet, Sonication, and Supercritical Fluid Extraction

Microwave-assisted extraction (MAE), Soxhlet, soni cation, and supercritical fluid extraction (SFE) were compared in their ability to extract 95 compounds listed in U.S. Environmental Protection Agency Method 8250. Freshly spiked soil samples and 2 standard reference materials were extracted with hexane-acetone (1 + 1) by MAE and Soxhlet extraction, with methylene chloride-acetone (1 + 1) by sonication extraction, and with supercritical carbondioxide modified with 10% methanol by SFE. Results indicate that MAE is a promising extraction technique. Of 94 compounds for which we are reporting data, 51 compounds gave MAE recoveries of >80%; 33,50-79%; 8,20-49%; and 2, <19%. Soxhlet extraction gave very similar results: 50 recoveries were >80%; 32,50-79%; 8,20-49%; and 4, <19%. Sonica tion recoveries were slightly higher: 63 values were >80%; 25,50-79%; 4,20-49%; and 2, <19%. SFE recoveries were the lowest: 37 values were >80%; 37, 50-79%; 12,20-49%, and 8, <19%. MAE gave the best precision: relative standard deviations (RSDs) were <10% for 90 of 94 compounds. Soxhlet extraction gave the worst precision: only 52 of 94 samples gave RSDs ≤10%. Although MAE looks promising, 15 polar basic compounds extracted at 115°C with hexane acetone for 10 min (1000 W power) gave poor recoveries. Because no technique gave acceptable recoveries for these polar compounds, we investigated their extraction with MAE using acetonitrile at 50° and 115°C. Ten of the 15 compounds were quantitatively recovered (>70%) with acetonitrile at 115°C.