LIQUID–SOLID EXTRACTION

  When a liquid is extracted by a solid, phase A of the Nernst distribution law [equation below] refers to the liquid sample, and phase B, the extracting phase, represents the solid (or solid-supported liquid) phase:

centrate semivolatile organic compounds from liquids into the solid phase.

The liquid sample was placed in contact with the flowable, bulk solid extracting phase, an equilibrium between the two phases was allowed to occur, followed by physical separation (by decanting or filtering) of the solid and liquid phases. During the past quarter century, different approaches to solidphase extractions of semivolatile organic compounds have emerged, including three described here: solid-phase extraction (SPE), solid-phase microextraction (SPME), and stir bar sorptive extraction (SBSE). Like LLE, SPE is designed to be a total, or exhaustive, extraction procedure for extracting the analyte completely from the entire sample volume via the sorbent.

Unlike LLE, SPE is a nonequilibrium or pseudoequilibrium procedure. Unlike SPE, SPME is an equilibrium procedure that is not intended to be an exhaustive extraction procedure. SPME is an analytical technique in its own right that is inherently different from SPE or LLE. SBSE is physically a scaled-up version of SPME, but in principle it is more closely related to LLE (as it has been applied to date), in that it is an equilibrium partitioning procedure that unlike SPME more easily presents the opportunity to achieve exhaustive extraction. Each variation on the theme of liquid–solid extraction is an important addition to the analyst’s arsenal of procedures for recovering semivolatile organics from liquids.