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The drive for alternative microextraction techniques continues. Solid-phase microextraction (SPME) and liquid-phase microextraction (LPME) led to an avalanche of related approaches to the extraction of analytes from solution in a form suitable for analysis. Hollow-fibre LPME, single drop microextraction and liquid-liquid-liquid microextraction are just some of the newer methods. Although, by definition, these "micro" techniques use far less solvent than traditional liquid-liquid and solid-phase extractions, they can demand long extraction times for good recoveries. This weakness was overcome by dispersive liquid-liquid microextraction, in which the extraction solvent was dispersed into the aqueous sample with the help of a dispersive solvent. However, it was one step forward and another step sideways because the final extract is not water-rich, limiting the choice of mobile phase for subsequent HPLC analysis. A further novel technique developed by two scientists in Taiwan aims to solve the aforementioned problems. Shang-Da Huang and Wan-Chun Tsai from the National Tsing Hua University, Hsinchu, have introduced dispersive liquid-liquid-liquid extraction (DLLLME) which produces an HPLC-compatible final extract in a short time. They demonstrated the method for the extraction of four common chlorophenoxy acid herbicides from water. Standard solutions of o- and p-chlorophenoxyacetic acid, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,3,5-trichlorophenoxyacetic acid (2,4,5-T) were used initially to optimise the procedure. Samples were acidified, mixed with sodium chloride, then shaken with 1,1,2,2-tetrachloroethane (TCEE). The organic phase was separated at the bottom of the tube by centrifugation and removed in a microsyringe to a second tube. Here, it was topped up with a solution of sodium hydroxide containing 5% methanol, the acceptor phase, but the two solutions do not mix. This is where the "dispersive" element of the procedure comes in. The liquids were agitated with a microsyringe, pumping them repeatedly in and out to create a cloudy mixture. This had the effect of increasing the interfacial area between the two solutions to give good extraction efficiency and short extraction times. The two phases were separated again by centrifugation and the acceptor phase was collected for HPLC analysis with UV detection without any further manipulation. The whole extraction was carried out in about two minutes. During optimization, dichloromethane had a 1.3-1.5-fold better extraction efficiency than TCEE but was not selected due to its poor repeatability, at 8.6-12.2% compared with 4.4-5.6% for TCEE. The other parameters were also optimised, including extraction volume and time, the acidity and basicity of the respective phases and the microsyringe pumping time. The procedure led to detection limits in the range 0.11-0.95 µg/L for the four herbicides and calibration curves with good linearity over 0.4-1000 µg/L for o- and p-chlorophenoxyacetic acid and 0.16-400 µg/L for 2,4-D and 2,4,5-T. The optimised method was applied to spiked tap water and river water, giving recoveries >95% and >80%, respectively. Recoveries for p-chlorophenoxyacetic acid were over-estimated at 116.9 and 115.2%, respectively, due to interference from an unidentified compound. The chromatograms of natural (unspiked) tap and river water revealed that concentrations of all four herbicides were below the detection limits. The researchers declared that DLLLME is better than other LPME techniques due to the rapid extraction times, comparable or better detection limits, and low volume of organic solvent, making it more environmentally friendly. The detection limits are at least as good, providing a method that should be easy and convenient for the extraction of chlorophenoxyacetic acid herbicides from aqueous samples. Related links:
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