Ronald E. Majors

The purpose of this installment of "Sample Prep Perspectives" is to review some of the modern and perhaps novel miniaturized LLE techniques to give readers an idea where these techniques can be used to solve sample preparation challenges. Here I will focus on techniques that duplicate the classical LLE experiment, in which users can choose the same two immiscible phases that would be used in separatory funnel extractions. Techniques such as stir-bar coated extractions (3) and in-tube solid-phase microextraction (SPME) (4) will not be discussed because they involve the use of polymeric materials such as polydimethylsiloxane or polypyrrole as the organic phase rather than the more conventional water-immiscible organic solvents.

The simplicity and low cost of SPME, developed by Pawliszyn and coworkers (5) in 1990, has made it into a popular sampling and sample preparation technique for gas chromatography (GC) and to a lesser extent for liquid chromatography (LC). In SPME, a fiber coated with a stationary phase is placed into a solution or headspace and analytes diffuse or are moved by convection into the stationary phase. The concentrated analytes are transferred to a chromatography column by thermal desorption (GC) or liquid extraction (LC). The popularity of the technique has spurred the development of similar technologies.

One such technology, termed single-drop microextraction (SDME), describes a configuration in which a droplet of solvent contained at the end of a PTFE rod or GC syringe needle replaces the coated fiber. The analytes diffuse into this droplet in a similar manner as into the SPME fiber. The original work first described by Cantwell and Jeannot (6) was based upon the experiments of Liu and Dasgupta (7). The latter investigated gas molecules partitioning into liquid droplets. Wood and coauthors (8) recently reviewed the technique of headspace SDME.