Hybrid microextraction using microporous silica as support of a thin solvent film

A new microextraction approach which stands midway between solid phase microextraction (SPME) and single drop microextraction (SDME) has been recently reported by researchers of the University of Isfahan at Iran. The extraction device is quite similar to this employed in SPME, although a bare stainless steel wire is used instead of the classic fiber. The wire is chemically treated in order to anchor a nanolayer of silica on its surface. This synthesis is achieved by a hydrothermal reaction which simply consists on the immersion of the bare wire in a sodium hydroxide solution in close contact with a glass precursor. The reaction, which develops at high temperature, involves the dissolution of the glass precursor in the alkaline solution and its final deposition on the wire. As a result of the process, a microporous layer of silica with a thickness of ca. 14 µm is obtained in the surface of the metallic wire. For analytes extraction, the treated wire is immersed in an appropriate organic solvent which is immobilized by adsorption on the porous surface. This immobilized solvent is the final responsible of the extraction following the general principles of SDME.

This hybrid microextraction combines some of the strengths of SPME and SDME, avoiding their main shortcomings (see Figure 1). Carry over between samples is avoided since a new layer of solvent is used for each extraction. The selection of the extracting phase is more versatile in contrast with SPME where the type of commercially available phases is limited while the procedures for the lab-synthesis of special phases are complicated. Moreover, the hybrid microextraction requires very low volumes of solvent, being therefore a cheaper approach. Furthermore, the new proposal presents some advantages over SDME. On the one hand, the microdrop of solvent is mechanically stable on the wire and its surface to volume ratio is more favorable improving the kinetic of the extraction. The low amount of solvent used as extractant (0.18 µL, while the usual volume in SDME is 2 µL) provides also higher enrichment factors

Advantages of the hybrid microextraction over classic SPME and SDME

The proposed microextraction technique has been evaluated for the determination of some organophosphorous compounds in water by gas chromatography/nitrogen phosphor detection. The obtained results shows an excellent extraction efficacy with preconcentration factors as high as 3210. Moreover, the precision of the extraction was lower than 8.4 % (expressed as relative standard deviation) even when different devices are employed.

Readers are referred to the article, recently published in Analytica Chimica Acta, for further details. In the article they will find the detailed synthesis procedure of the modified wire as well as the optimization process. Special attention should be given to the final comparison between the proposal and conventional microextraction techniques like SPME, SDME or hollow fiber liquid phase microextraction. This final combination highlights the usefulness of the proposal.

References

Link to the article. Microporous silica with nanolayer structure coated with renewable organic solvent film as a novel extracting phase: A combination of solid- and liquid-phase microextraction.