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Liquid-liquid extraction (LLE) is considered by most residue analysts to be the best method for quantitative recovery of organic pesticides from aqueous solutions. Continuous liquid-liquid extraction is a logical extension of the process. This paper reviews the LLE process especially the alternative of continuous LLE for the isolation of organic pesticides from aqueous solutions. Trends and user problems are noted. A continuous liquid-liquid extraction apparatus with capabilities for continuous solvent recycling and sample enrichment is described. The principal parts of this design are a helix Teflon coil mixing chamber, a column phase-separator, and an evaporative concentrator. Initial experiments were made to evaluate the design. Extraction efficiencies ranging from 85 to 93 percent were obtained for five organophosphate pesticides extracted from water with benzene under the optimized conditions of coil length, helix diameter, flow rate, and water to solvent ratio. The apparatus is simple and economic to build. It is contamination-free as all parts in contact with water and solvent are made either of Teflon or glass. Complete automation of this apparatus is possible by adding liquid-level controllers, solenoid valves, and electronic relay systems.

The detection of organic pesticides in aquatic environments represents an unique analytical problem. It is unique in the context that specificity is required under extremely sensitive conditions as opposed to gross analysis for trace organics in water. First, there is the problem of pesticide identity. Second, direct measurement is not feasible because of sensitivity and specificity requirements. Extraction, concentration, and cleanup techniques must be employed prior to qualitative analysis and quantification. Third, natural waters may contain such organic interferences as aromatic and aliphatic compounds from industrial waste waters, and naturally occurring colored compounds. These impurities are often present in concentrations greater than the pesticide. Two objectives are offered herein: (1) a brief critique of extraction, cleanup, identification, and quantification techniques is given in order to highlight limitations fallacies, and pitfalls, and (2) a model system for the recovery, separation, and confirmation of organophosphate pesticides, oxons, and their hydrolysis products. This model system provides the basic needs of pesticide methodology; namely, simplicity and minimum sample handling. The problems of recovery from water by liquid-liquid extraction, separation by gas-liquid chromatography, and subsequent confirmation by micro-ultraviolet absorption spectrophotometry are discussed.