جنبه های سبز، تحولات و چشم اندازهای روش های میکرو استخراج فاز مایع Green aspects, developments and perspectives of liquid phase microextraction techniques

Liquid–liquid extraction (LLE) is one of the oldest extraction techniques used most frequently in the case of aqueous samples with complex matrix composition. The technique is based on sequential treatment of the same sample with fresh portions of a solvent or with a series of solvents of increasing polarity. As a result, various extract fractions are obtained which are enriched with a different analyte or a group of analytes. However, multistage analytical procedures are highly time-consuming and labor intense which in consequence results in long exposure of laboratory personnel to harmful vapors from chemical reagents, particularly organic solvents. Moreover, the risk of losing analytes or of sample contamination increases with the increasing number of operations performed on the same sample. Therefore the application of alternative pro-ecological, automated [1], solvent-free extraction techniques or techniques employing a minimal amount of solvents (liquid-phase microextraction techniques, LPME), and those which use safe and non-toxic extractants (e.g. ionic liquids, supercritical liquids, surfactant solutions [2], and supramolecular solvents [3]) has become one of the most popular research topics in analytical chemistry in recent years [4–۱۶]. A definition of liquid microextraction is all modes of sample preparation technique used solvent in volumes of 100 μL or less for analytes extraction [17], allowed the integration of extraction and enrichment of analytes to the level above the method detection limit, as well as the analyte isolation from sample [18]. The use of alternative microextraction techniques for sample preparation reduces the number of errors that commonly result from multi-stage procedures, and limits the negative impact on the environment and the health of analytical chemists performing laboratory work. The reduction of the amount of organic solvents employed during the extraction process translates into lowered utilization costs of waste treatment and spent solvents, which in turn allows the cost reduction of analytical procedures as well as saving money on the purchase of high purity solvents. So, it is clear that the improvement of sample microextraction techniques could be of interest for environment ethic considerations and business opportunities. This new green approach is often described in literature as the three R’s, which stands for replace, reduce and recycle (replacement of toxic solvents with green solvents, reduction of solvent consumption and waste production, and solvent recycling) [19]. However, we cannot renounce to the use of high sensitive analytical techniques suitable to obtain multiparametric information about complex samples through the use of chromatography after an appropriate dissolution of samples and preconcentration of the target analytes and in this case, the techniques discussed in this paper offer a good alternative to the most commonly employed long and tedious procedures used for sample preparation and sample clean-up, including analyte preconcentration. The use of ultrasonic irradiation [20], microwaves [21], green extraction medium [22–۲۴], and use of electrochemical support and commercial available autosamplers, are opening alternatives for a fast, nondestructive and low cost processes and for improving the available methodologies, also opening the way for automation and integration of sample treatments and analytical measurements and not only in this way but also improving the extraction processes. In recent years, one of the most investigated topics in analytical chemistry has been the use of ionic liquids, due to their valuable characteristics: (i) very low vapor pressure, (ii) high viscosity, (iii) high thermal stability, (iv) non-flammability, (v) capable to dissolve a wide spectrum of organic and inorganic compounds and (vi) specific electrochemical characteristics [25–۲۸]. An attempt was also made to use ionic liquids as universal solvents in chromatographic, electrochemical and extraction techniques [29– ۳۳]. Due to their capacity to dissolve different organic compounds, ionic liquids are a real alternative to conventional solvents used in liquid–liquid extraction techniques [34]. Therefore investigations of the properties of ionic liquids in relation to their application as solvents are of utmost importance for elaborating efficient extraction procedures [35]. Toxic organic solvents used in liquid extraction techniques can be also substituted by surfactant-based coacervate [2,36] in micellar-mediated extraction techniques, e.g. cloud point extraction CPE [37–۴۱] and coacervative extraction (CAE) [42–۴۴]. Coacervates are large colloidal micelles (drop-shaped microscopic structures) that self-assemble in colloidal systems. Coacervate based on anionic or cationic surfactants is produced by cooling the solution below its cloud point, while in the case of non-ionic surfactants the solution has to be heated above its cloud point [45,46]. Thanks to the semipermeable barrier around the coacervates, the extraction of analytes can take place inside the micelles. Non-polar and low-solubility analytes in an aqueous micellar solution dissolve inside the micelles and aggregate into the surfactant-rich phase, while the remaining aqueous sample contains the diluted surfactant as monomers or dimers at a concentration that approximates its critical micelle concentration. Similarly, in organic solutions, the presence of reverse micelles increases the solubility of hydrophilic substances. After the extraction, the solution is centrifuged, cooled (to increase the micellar phase viscosity), and decanted to be later dispensed into a measuring instrument. Micelle-based extraction techniques are simple, inexpensive and eliminate the need for use of toxic solvents, the problem of the formation of emulsions and lack of sensitivity for more volatile analytes, appearing in the standard liquid–liquid extraction techniques. High capacity to concentrate a wide range of analytes makes the surfactant-rich phases are multipurpose solvents, allowing for a high recoveries and high concentration factors to obtained. In this paper, microextraction techniques and the application of alternative solvents are discussed in detail, with special emphasis on strategies for reducing, or even eliminating of use of organic solvent. This deep review based on the most relevant, representative, and the most latest references. This knowledge of the details will be very helpful in making a decision concerning the choice of a particular solution in order to use on the sample preparation step and to make analytical methods greener.