Miniaturized liquid-phase extraction techniques

Trends in miniaturization of liquid-phase extraction procedures have been spawned and given many different names they refer to the same principle but with different, or sometimes the same, 

The work described by Jeannot and Cantwell implied utilizing two different apparatuses for two different steps a Teflon rod supporting the drop (for extraction) and a microsyringe (for injection of the extract into the analytical instrument). The work was further developed by incorporating only a microsyringe for extraction (as solvent drop holder) and extract injection (as sample injector) into the analytical instrument.7 

To enhance extraction efficiency in the SME system (also called single-drop microextraction [SDME]), He and Lee developed a procedure termed liquid-phase microextraction (LPME),89 which, in its static mode, resembles the SME system. 

---

Compared with conventional particle-packed SPE cartridges, FIT-SPE provides an increased surface area for the extraction medium and a reduced pressure drop during extraction and desorption. Also, the undesirable plugging effect from insoluble materials in real samples can be very much diminished. Utilization of FIT-SPE has been discussed in a few review articles concerning the on-line coupling of miniaturized SPE to microcolumn liquid-phase separation techniques.24,25... 

During the last few years, miniaturization has become a dominant trend in the analysis of low-level contaminants in food and environmental samples. This has resulted in a significant reduction in the volume of hazardous and expensive solvents. Typical examples of miniaturization in sample preparation techniques are micro liquid/liquid extractions (in-vial) and solvent-free techniques such as solid-phase microextraction (SPME). Combined with state-of-the-art analytical instrumentation, this trend has resulted in faster analyses, higher sample throughputs and lower solvent consumption, whilst maintaining or even increasing assay sensitivity. 

Microporous membrane liquid-liquid extraction (MMLLE) is a two-phase extraction setup. In MMLLE procedures, the membrane material and format (FS and HF), extraction units, and system configurations are identical to those described in SLM (Section 4.4.1.2).63 The two-phase HF-MMLLE system is identical to that used in Section 4.4.3, although sometimes with minor differences. In contrast to three-phase SLM extraction, MMLLE employs a microporous membrane as a miniaturized barrier between two different phases (aqueous and organic). One of the phases is organic, filling both the membrane pores (thus making the membrane nonporous) and the compartment on one side of the membrane (acceptor side). The other phase is the aqueous sample on the other side of the membrane (donor side). In this way, the two-phase MMLLE system is highly suited to the extraction of hydrophobic compounds (log Ko/w > 4) and can thus be considered a technique complimentary to SLM in which polar analytes (2 < log Ko/w < 4) can be extracted. 

One more trend that is worth mentioning is the miniaturization of sample preparation techniques. Solid phase microextraction is one good example of where very small samples are consumed and very small extracts are produced. Solid phase extractions can also be scaled down by reducing the bed volume or by use of coated membranes. Likewise liquid-liquid extractions can be scaled down conserving both sample and solvent. 

As a corollary to this, more direct sample preparation procedures have been the pursuit of many scientists, who believe that miniaturization of analytical techniques can be a key solution to many of the unwanted drawbacks of LLE and SPE. Currently, several miniaturized extraction systems have been investigated, which are based primarily on utilizing downsized liquid, solid, or membrane extraction phases. 

---