The technique of liquid–liquid extraction (LLE) is still among the most popular in routine sample preparation (1). Most chemists
remember well their college experiments in the organic chemistry laboratory, continuously shaking their large separatory funnels
attempting to isolate a pure fraction from a synthesis. This age-old technique has seen few changes from its roots, which
date back at least a century, perhaps more. In 1996, I reviewed the basics of LLE and covered some newer variations that were
available at that time (2). Classical LLE uses copious amounts of solvent that are often hazardous and it is time consuming
to perform. Over the last 10 years, with the developing interest in miniaturization in analytical chemistry with resultant
solvent and sample savings, some newer miniaturized approaches to liquid extraction have been reported. Compared with classical
LLE, these approaches have resulted in more efficient sample enrichment, faster sample preparation, and easier automation.
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.
Single-Drop Microextraction
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.
Figure 1
|
SDME also has been referred to as solvent microextraction, liquid–phase microextraction, and liquid–liquid microextraction.
In the original experiments of Cantwell and Jeannot (6), the droplet size was 8 μL of an immiscible organic solvent (n-octane) contained in a rod-shaped PTFE probe hollowed out at one end. The probe was immersed in an aqueous sample contained
in a 1-mL vial that was stirred with a magnetic stirrer. Because the 8-μL volume was too large to inject directly into a GC
system, the authors took an aliquot, which limited sensitivity. However, in their next publication (9), as well as the similar
work of He and Lee (10), the droplet size was reduced to 1–2 μL by using the tip of a GC syringe needle as the drop holder.
The entire droplet was then injected into the GC. A schematic of the single-drop microextraction experiment is shown in Figure
1.
In SDME, there are a few experimental parameters that should be controlled precisely to have reproducible results. Similar
to SPME, the partition equilibrium is not reached in the experiments, so precise timing is essential for good precision. Cantwell
and Jeannot (9) found that they could achieve relative standard deviations of 1.5% even when the extraction was only 38% of
the equilibrium concentration. Note that enrichment factors are generally less than 100 in the SDME experiment.
Follow Us
RSS
Bookstore
