Analytical Microextraction Techniques

In the present chapter, the microextraction techniques have been considered from the perspective of the Green Analytical Chemistry and an attempt has been made in order to propose a modified eco-scale suitable to provide a green certification of the extraction steps in the frame of the consideration of reagents and energy consumption and waste generation. Especially important is the evaluation of the intrinsic toxicity and risks associated to the use of reagents and the need of an accurate evaluation of the amounts of reagents and wastes in order to avoid misclassifications. It has been proposed a new eco-scale, called “Green Certificate”, based on the application of a color code associated to a letter; from A to G, being A the greenest one, that gives an idea of how much green an analytical methodology is. In short, it is evident that the microextraction enhances the method greenness providing class A steps in front of classical analytical preconcentration and matrix removal which changes the method category to class D.

Despite remarkable advances in modern state-of-the-art analytical instruments featured with enhanced sensitivity and automated operation in recent years, the success of an analytical investigation still largely depends on the sampling and sample preparation techniques. The growing concerns about environmental pollution and public safety, solvent-free/solvent-minimized sorptive microextraction techniques have gained enormous popularity among practicing scientists over solvent intensive sample preparation techniques. A major share of the increasing popularity of sorptive microextraction techniques definitely goes to sol-gel coating technology. The enormous potential of sol-gel coating technology as a viable approach for creating hybrid organic-inorganic advanced material systems has been intuitively materialized, resulting in a large number of microextraction sorbents possessing unique selectivity, enhanced extraction efficiency, higher thermal, mechanical, chemical and solvent stability. The current chapter explains the basic principle of sol-gel coating technology as well as the step-by-step procedure to fabricate the coating, classifies and describes different sorptive microextraction sorbents, and presents the most recent developments in the field. Selected and representative applications of different sol-gel microextraction sorbents are also tabulated.

The structural versatility of ionic liquids (ILs), the wide range of interest properties that can present: low to negligible vapor pressure, from water-soluble to water-insoluble, from medium viscosity to high viscosity…, their synthetic tuneability, together with their impressive solvation abilities for analytes of quite different nature, make their use in microextraction techniques an obvious approach of great interest.

Thus, ILs and a group of interesting derivatives, such as polymeric ionic liquids (PILs) and IL-based surfactants, pure, mixed, or combined with other materials forming hybrid sorbents, have been used in all variants of liquid-phase and solid-based microextraction strategies.

This chapter will give an exhaustive overview of existing approaches that combine ILs and derivatives in liquid-phase microextraction (LPME) modes such as single-drop microextraction (SDME), hollow fiber liquid-phase microextraction (HF-LPME) and dispersive liquid-liquid microextraction (DLLME), including all their variants; and also in solid-based microextraction methods, such as micro-solid-phase extraction (μ-SPE), solid-phase microextraction (SPME), stir-bar sorptive extraction (SBSE), and stir-cake sorptive extraction (SCSE).

Nanomaterials (NMs) have attracted great attention in sample preparation. In particular, because of their high surface-to-volume ratios, NMs facilitate the implementation of microextraction techniques. Additional advantages derived from NMs are the possibility of increasing the selectivity through the functionalization of their surfaces, and the improvement of mechanical and thermal stability of the extraction devices. This chapter summarizes the main uses of NMs in solid and liquid microextraction techniques, and representative examples of applications are presented.

The fabrication of coatings for fiber solid-phase microextraction (SPME) is the main objective in many of the scientific research developed in the area. For this purpose, a variety of NMs have been used such as carbon-based NMs, especially carbon nanotubes (CNTs) and graphene, metallic and silica-based NMs, and more recently, metal-organic frameworks (MOFs). NMs have also been used to prepare new sorbents for other microextraction techniques such as in-tube solid-phase microextraction(ITSPME) and stir bar sorptive extraction (SBSE). The employment of magnetic nanoparticles (MNPs) has recently been introduced in microextraction, which leads to a new technique termed magnetic IT-SPME. Although the number of applications is still low, NMs are also receiving increasing attention in the main techniques of liquid phase microextraction namely, singe drop microextraction (SDME), hollow-fiber liquidphase microextraction (HF-LPME) and dispersive liquid-liquid microextraction (DLLME).

Sample preparation has been commonly considered a critical step of the analytical process. In this sense, remarkable efforts have been made to develop efficient sample preparation techniques which could overcome the limitations of conventional approaches. Since its inception in the early 1990’s, solid-phase microextraction (SPME) has become a widespread miniaturized sample preparation technique for extraction and preconcentration of target analytes from a large variety of matrices. Interestingly, sampling, extraction, enrichment and sample introduction can be integrated into a single step in SPME. This book chapter focuses on the basic principles and current state of the art of SPME. Specifically, both thermodynamic and kinetic aspects of the SPME technique are discussed in detail. In addition, those experimental variables that show a paramount role in the extraction process, and should therefore be optimized and controlled for optimal performance, are considered. Valuable contributions that enabled the development of this solventless technique and current challenges are identified. Other related SPME devices, such as internally cooled SPME, in-tube SPME and membrane SPME, are also described.

From the sorption-based methods available nowadays, stir bar sorptive extraction (SBSE) became a well-established analytical technique for sample preparation, in which hundreds of applications in almost all scientific areas have already been proposed in the literature. This remarkable analytical tool shows great capacity for static microextraction and outstanding performance to operate at the ultratrace level, in particular for the analysis of complex systems. Furthermore, is very effective, present easy manipulation in comparison to other alternative techniques and great reproducibility for the analysis of priority and emerging organic compounds. Recently, related static microextraction techniques were introduced, with particular emphasis to bar adsorptive microextraction (BAμE) that operates under the floating sampling technology, in which has demonstrated high analytical capacity and remarkable performance. This novel concept has also proved great effectiveness for ultra-trace analysis of organic compounds with polar characteristics, in particular from complex systems. The present contribution describes the fundamental principles, the experimental methodology, the main applications, as well as, the analytical potential of these novel microextraction techniques.

Sample preparation is a critical issue in bioanalysis since the matrix is often complex. A good sample preparation technique will remove the possible interferences from the sample matrix pre-concentrate the analyte and to be reproducible independent of the sample matrix. Recent developments of sample preparation techniques are directed toward automation, the smaller sample volumes and online coupling of sample preparation units and detection systems. Microextraction in packed syringe (MEPS) is a new type of solid-phase extraction (SPE) technique that is miniaturized and can be fully automated. In MEPS the solid bed is integrated in the injection syringe. The MEPS syringe can be used online for both extraction and injection of samples. The present chapter gives an overview of MEPS technique, including the MEPS description, formats, sorbents, experimental and protocols and factors that affect the MEPS performance. We also summarize MEPS recent applications in bioanalysis, environmental and food analysis.

The usefulness of dispersive extraction techniques leans on their ability to maximize the interaction between the sample and the extractant phase, thus increasing the extraction efficiency. As far as dispersive solid phase extraction is concerned, it was initially developed to increase the selectivity of the analytical process because the solid was added to retain the potential interferents from the sample matrix. In spite of its efficient sample clean-up, the sensitivity is its Achilles' heel as no preconcentration is usually achieved. Recently, the use of few milligrams of sorbent which is dispersed in a liquid sample for analytes isolation has raised a new miniaturized extraction technique, the so-called dispersive micro solid-phase extraction. This alternative is mainly focused on sensitivity enhancement. The chapter begins with a short contextualization of this extraction technique, followed by a brief description of the first approach in this context, viz, dispersive solid phase extraction. Next, the main contributions in the field of dispersive micro solid-phase extraction in this context will be described on the basis of the nature of the solid used. Also, the combination with dispersive liquid phase microextraction and the expected evolution of this miniaturized extraction technique are included.

Magnetic nanoparticles (MNPs) are attracting great interest for developing solid phase extraction (SPE). Nanoparticles involved in SPE processes present clear advantages with respect conventional SPE materials because of their high stability, simple preparation, and the short time involved in the sample preparation; avoiding time-consuming column passing and filtration, as it is in conventional SPE methods. In this e-book chapter, the progress involving MNPs used for SPE is presented and discussed. Taken into account the different types of MNPs used in this field, the different MNPs obtained by attaching inorganic components (e.g., metal oxides, carbon, noble and semiconductor metals), or organic molecules (e.g., surfactants and magnetic molecularly imprinted polymers, MMIP), are described through recent reported analytical applications of MNPs in SPE. The potential transfer of these analytical tools for daily work in routine laboratories is also pointed out.

Thin film microextraction (TFME) is considered as a type of solid-phase microextraction (SPME). There has been a growing interest in TFME as a novel sample preparation technique, which was originally introduced to address the limiting uptake rate and capacity sometimes observed with fiber microextraction. The inherent properties of TFME technique such as the excellent sample clean-up and the larger surface to volume ratio, enhance the sensitivity and the extraction rates. This e-book article is mainly focused on the fundamental principles behind and in the diverse existing TFME configurations, paying particular attention to cotter pin supported format, copper mesh holder and 96-blade format.

The number of applications of liquid phase microextraction (LPME) techniques has undergone a dramatic increase during the last years. Miniaturisation of sample preparation encompasses several advantages, i.e., low consumption of extractants, integrated operation, ease of clean-up, large preconcentration factors, apart from an increased greenness, as compared to classical solvent extraction. Among LPME, single-drop microextration (SDME) approaches have deserved much interest. Versatile and adaptable procedures to every each analyte and matrix have been reported. Extractants are not only limited to organic solvents, but ionic liquids (ILs) and even aqueous solvents can be also employed. Whereas, analytical techniques based on chromatographic/electrophoretic separations and some modes of atomic spectrometry were applied in early applications, SDME has also been exploited in combination with other detection approaches such as UV-vis spectrophotometry, fluorospectrometry, chemiluminescence, etc. This has spread the use of SDME to almost every application area. In this chapter, the state of art of SDME and its main modes is reviewed.

Various membrane-based extraction techniques are used in analytical chemistry mainly for pretreatment before analyte determination using chromatographic or other techniques. Membrane extraction can also be applied for the extraction of various metal ions and is then followed by atomic absorption or similar detection techniques. They allow high selectively for a number of analytes from chemically more or less complex samples and high concentration enrichment, easily thousands of times. Currently, the most common format for membrane extraction utilizes hollow-fiber membranes, which permit easy and versatile operation with a minimum of cost. A number of applications have been presented involving determination of polar and medium-polar compounds as acids and bases in samples of environmental and biological origin, usually in combination with liquid chromatography and mass spectrometry. For other analytes in which membrane extractions are various metal ions, the membrane extraction is followed by techniques like atomic absorption and similar. Also applications to non-polar compounds in mainly environmental samples, followed by gas chromatography, are described in the literature.

In this chapter, the basic theoretical principles for the common variants of membrane extraction are described. Guidelines for the selection of operational parameters as well as concrete advice for the practical implementation are provided.