Liquid Liquid Extraction

Liquid-liquid extraction is generally reserved for more complex samples because it offers poorer precision than other techniques. It is most commonly used for the preparation of biological samples in which less precise methods can be tolerated. Occasionally, however, an extraction is necessary for the determination of a water-insoluble compound in a water-soluble matrix, such as the analysis of fat-soluble vitamins in tablets or menthol in pharmaceutical lozenges. In these cases, the water-soluble matrix must be treated with water to gain access to the analytes, but the solvent cannot be made sufficiently nonpolar to dissolve the analytes by adding a water-miscible solvent.

As discussed previously, in partitioning systems, there are some critical parameters that must be controlled. Selectivity and recovery can be manipulated by solvent selection (see Table 5). For ionizable drugs, the pH must be controlled to optimize the recovery. Generally, when the drug is in the ionized form, it is more soluble in the aqueous phase. Likewise, when the drug is in the uncharged state, it will generally partition into the organic layer, which is often exploited in sample clean-up steps called back-extractions. After the analyte is transferred to the organic solvent, a back-extraction technique can be used to take the sample back to an aqueous phase by adjusting the pH of the aqueous phase. By using sequential extractions and back-extractions, a high degree of selectivity can be achieved. However, care must be taken to maintain reproducible, high recovery.

To overcome poor extraction efficiencies, organic fraction from sequential extractions can be pooled (26). Liquid extractions are also used for trace enrichment of the analyte by evaporating the organic layer to dryness and then reconstituting into a smaller volume of mobile phase.

The addition of specific enhancers has been used to effect liquid-liquid extractions. Valenta et al. (79) have improved the extraction of phenobarbitol by using specific binding compounds to enhance the partition coefficient. By adding

Table 5 Typical Liquid-Liquid Extraction Solvents (in order of polarity)
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