EPA Methods and Performance-based Analyses

Water samples are conveniently extracted using water-immiscible solvents in a separatory funnel. Polar solvents such as ethylacetate or dimethylether are used to extract organic acids. Nonpolar solvents such as hexane, n-heptane, cyclohexane, or dichloromethane are used to extract neutral lipids, for example, triglycerides, and other nonpolar constituents such as aliphatic hydrocarbons and organopesti-cides. Solid-phase extraction is increasingly used to separate and preconcentrate trace organics from water samples. 

Moist samples such as soils are usually dried in a vacuum oven and then ground. Before extraction, the samples are rehydrated by adding an aqueous buffer, which helps the transfer of analytes into a water-miscible extracting solvent such as acetone. 

The solid-solvent mixture is stirred or shaken and then filtered or centrifuged to separate the matrix. Typically three extractions, using small portions of solvent, are needed to assure quantitative extraction. Heating may help in some cases. Or ultrasonic energy (ultrasonic batch or ultrasonic probe) can be used to increase efficiency. Continuous Soxhlet extraction is often used. A Soxhlet extractor has a round-bottom distillation pot, with a porous thimble above it in a siphon chamber, into which the sample is placed. Solvent is continuously cycled through the sample by distilling the solvent to a condenser centered over the thimble. The condensed solvent permeates through the matrix and the thimble, and is siphoned back into the pot, where it is recycled. The extracted material is concentrated in the pot. 

Analytes extracted from environmental samples are concentrated prior to measurement by evaporating the solvent, using low temperature and reduced pressure for rapid solvent removal. Volatiles in soils and sediments may be sampled directly using headspace analysis for gas chromatography. 

Once you have the extracted and preconcentrated organic analytes, you may have to perform additional cleanup, for example, by running them through a column of adsorbent packing material such as silica or alumina. Then chromatography is most often used for measurement. Pesticides are commonly determined by gas chromatography with electron capture detection or GC-MS. A nonpolar capillary GC column is used. Trace PCB and polycyclic aromatic hydrocarbon (PAH) determinations can be done using HPLC with UV detection. 

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