Microextraction Techniques

The main characteristics of on-line SPME-HPLC(-MS) are shown in Table 7.18. Most of the SPME fibres are compatible with HPLC solvents. SPME combined with HPLC provides a means by which simple, rapid concentration of analytes can be achieved together with a means of introduction of the concentrated analytes to the HPLC system. This eliminates the need for larger injection volumes, and avoids derivatisation if the analytes were to be detected by GC. An advantage of the SPME method over LLE methods is the absence of a solvent peak in chromatograms obtained after extraction by SPME. SPME is not suitable for organic solutions. As SPME is a microextraction technique, coupling to ft, HPLC may be envisaged. 

In the past decade, several novel solvent-based microextraction techniques have been developed and applied to environmental and biological analysis. Notable approaches are single-drop microextraction,147 small volume extraction in levitated drops,148 flow injection extraction,149 150 and microporous membrane- or supported liquid membrane-based two- or three-phase microextraction.125 151-153 The two- and three-phase microextraction techniques utilizing supported liquid membranes deposited in the pores of hollow fiber membranes are the most explored for analytes of wide ranging polarities in biomatrices. This discussion will be limited to these protocols. 

A variety of microextraction techniques have evolved in recent years to address the challenges inherent in the analysis of samples derived from... 

During the last 20 years or so, the SPME technique has probably reached the culmination of its development in terms of mode of operation, automation, miniaturization and interfacing to other instruments, innovation of new coating materials, calibration procedures, and fields of application. As a result of these developments, SPME has become the currently most commonly used microextraction technique in held and laboratory experiments of a multidisciplinary nature.29 Accordingly, the following sections will merely record progress in SPME from different perspectives. 

Hyotylainen, T. and M.-L. Riekkola. 2008. Sorbent- and liquid-phase microextraction techniques and membrane-assisted extraction in combination with gas chromatographic analysis A review. Anal. Chim. Acta 614 27-37. 

Bagheri, H. and A. Salemi. 2004. Coupling of a concentric in-two-tube solid phase microextraction technique with HPLC-fluorescence detection for the ultratrace determination of polycyclic hydrocarbons in water samples. Chromatographia 59 501-505. 

Basher, C., A. Parthiban, A. Jayaraman, et al. 2005. Determination of alkylphenols and bisphenol-A. A comparative investigation of functional polymer-coated membrane microextraction and solid-phase microextraction techniques. J. Chromatogr. A 1087 274—282. 

Micro liquid-liquid extraction (MLLE) and other microextraction techniques... 

The combination of these microextraction techniques that combine sampling, extraction and pre-concentration into a sigle step, with high sensitive gas chromatograph detectors such as mass spectrometer is the way to determine compounds at levels of ng L 1 that could be important for wine aroma characterization. 

Laaks, J., Jochmann, M.A., Schmidt, T.C. Solvent-free microextraction techniques in gas chromatography. Anal. Bioanal. Chem. 402, 565-571 (2012)... 

Fang, Y. and Qian, M.C. (2005) Sensitive quantification of sulfur compounds in wine by headspace solid-phase microextraction technique, J. Chromatogr. A, 1080(2),... 

AguUera-Herrador E, Lucena R, Cardenas S et al (2010) The roles of ionic liquids in sorptive microextraction techniques. Trends Anal Chem 29 602-616... 

Wirth EF, Chandler GT, Dipinto LM, et al. 1994. Assay of polychlorinated biphenyl bioaccumulation from sediments by marine benthic copepods using a novel microextraction technique. Environ Sci Technol 28 1609-1614. 

Augusto F, Carasek E, Costa Silva RG, Rivelhno SR, Batista AD, Martendal E, New sorbents for extraction and microextraction techniques, J. Chromatogr. A 2010 1217 2533-2542. 

Recent developments in microextraction techniques based on crown ethers 13JIP(76)253. 

Contarini, G. and Povolo, M. Volatile fraction of mUk Comparison between purge and trap and solid phase microextraction techniques. J. Agric. Food Chem. 50,7350-7355 (2002). 

For the determination of the Hg species methylmercury, phenylmercury and Hg(ll), ozone was used successfully in a batch cold-vapor system [126]. The preconcentration and speciation of Cr(lll) and Cr(VI) in water sample can be performed using solid-phase extraction (SPE) [105]. An SPME (solid-phase microextraction) technique has been used as the sample preparation system for the innovative simultaneous multielement/multi-species determination of six different mercury, tin. and lead species in waters and urine with GC/MS-MS [141], [142], The determination of arsenic species [As(III), As(V), MMA. DMA, arsenocholine, arsenobetaine)] has also been shown to work with an on-line digestion step prior to hydride generation AAS [125] (see Fig. 7). 

The dispersive liquid—liquid microextraction technique (DLLME) [16] has attracted much attention due to its simplicity and to the high enrichment factors that can be achieved. DLLME is a fast micro-extraction technique based on the use of a ternary mixture, composed by an aqueous phase, an organic phase (extractant) and an additional organic solvent also named as disperser solvent, which is miscible in both phases. The disperser is initially mixed with the extractant and then rapidly injected into the sample. By the fast dissolution of the disperser into the aqueous phase, the extractant is disrupted into small droplets enhancing the effective surface area of extraction. The separated extractant droplets are then sedimented at the bottom of the vial or float upon the aqueous sample-disperser phase, depending on the density of the extractant used. 

New sorbents for extraction and microextraction techniques. Journal of Chromatography A, 1217,2533-2542. 

At present, LPME is still a noncommercial microextraction technique and many different SLM compositions have been used. Generally, for nonionized compounds, 1-octanol and toluene have been mostly used in two-phase LPME, while 1-octanol and dihexyl ether have been mostly used in three-phase LPME. In some cases, carriers or ion pair reagents are added to the sample phase to extract charged analytes. An example of this is the addition of aliphatic sulfonic add to the donor phase at pH 7. At this pH, it will form ion pairs with positively charged basic compounds. The neutral ion pair is extracted into the SLM. At the interface between the SLM and the acceptor phase, which exhibits a low pH, the basic analytes are released through protonation of the aliphatic suFonic acid (Figure 9.12). 

G. Edwards, P. Halley, G. Kerven, and D. Martin, Thermal stability analysis of organo-silicates, using solid phase microextraction techniques. Ihermochimica Acta, 429 (2005), 13-18. 

The concept of the equilibrium sampler is analogous to that of the octanol-water equilibrium partition coefficient (fQ,w) used since the 1970s to predict the potential for persistent nonpolar contaminants to concentrate in aquatic organisms [71]. The use of equilibrium-t) e passive samplers in the aquatic environment depends on the development of a sampler-water partition coefficient (fCs ) defined as the ratio of sampler to water concentration of the compound of interest at thermod)mamic equilibrium. The other key parameter determining the utility of an equilibrium-type passive sampler is the time taken to reach an approximate equilibrium condition. A range of approaches applied in developing equilibrium-t)q)e passive samplers include polyethylene or silicon sheets of various volume to surface area ratio [72] and solid-phase microextraction techniques [73]. 

C. Basheer, J.P. Obbard, and H.K. Lee, Analysis of persistent organic pollutants in marine sediments using a novel microwave-assisted solvent extraction and liquid-phase microextraction technique, /. Chromatogr. A 1068 (2005) 221-228. 

Jeleh, H.H., Majcher, M., and Dziadas, M. (2012) Microextraction techniques in the analysis of food flavor compounds a review. Anal. Chim. Acta, 738,13-26. 

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