Extraction techniques fundamentals

Dean, J. R. and Cresswell, S. L., Extraction Techniques for Solid Samples, In Sampling and Sample Preparation for Field and Laboratory Fundamentals and New Directions in Sample Preparation, Pawliszyn,]., Ed., Vol. XXXVII, Elsevier Science, Amsterdam, Netherlands, pp. 559—586, 2002. 

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

Another popular and selective extraction technique widely used in bioanalysis is solid phase extraction (SPE). SPE is a separation process utilizing the affinity of the analytes to a solid stationary phase. By manipulating the polarity and pH of the mobile phase, the analytes of interest or undesired impurities pass through stationary phase sequentially according to their physical and chemical properties. For a SPE procedure, a wash step refers to the elution of the unwanted impurities which are discarded and the elution step refers to the elution of the analytes of interest which are collected. While the fundamental remains the same in decades, the continuing invention and introduction of new commercial stationary phases and accessory devices have boosted the application of SPE in bioanalysis and many other fields. 

The use of supercritical-fluid-extraction techniques in the fractionation of polysiloxanes has been demonstrated by the data presented. The poly-dispersities of the fractions were comparable with those generally attainable only by anionic-polymerization techniques, with which the incorporation of two functional groups is often difficult to attain. The ability to isolate these well-defined fractions will lead to important fundamental studies on structure-property relationships in multiphase copolymer systems. 

Before we begin the discussion of specific sample preparation techniques, it is necessary to review some of the fundamental theories that control these separation techniques (see Table 4). Phase equilibrium theories, phase contact, and countercurrent distributions provide the basis for the extraction techniques, e.g., liquid-liquid extractions as well as the various solid-phase extraction techniques. Solubility theories provide the basis for the preparation and dissolution of solid samples. Finally, understanding of the basic physicochemical theories that control intermolecular interactions is critical for successful development of sample preparation methods. 

In extraction techniques (LLC, SPE, etc.), normal-phase liquid chromatography (NPLC), and thin-layer chromatography, aliphatic hydrocarbons (e.g., -hexane, -heptane) are usually used. The elution strength of these solvents is often modified by addition of more polar solvents. The fundamental problem with the eluents in NPLC is dissolved water and trace amounts of olefins. These contaminations can induce a change in the wavelength cut-off values (UV detection, spectrophotometry), baseline perturbation, and poor reproducibility of retention data. Halogenated solvents such as dichloromethane can react with some organic solvents (e.g., acetonitrile) to form crystalline products. 

It has been more than fifty years since the discovery of the transuranium elements. The initial activities in this field established the fundamental solution and solid-state chemistry of the first two of these elements and their compounds under the auspices of the Manhattan Project. New separation methods including solvent extraction techniques and uranium isotope separation played a leading role in these programs. Tracer techniques were widely used to determine solubilities (or solubility liinits) of transuranium compounds as well as to obtain information about the coorination chemistry in aqueous solution. A little later, special solvent extraction and ion-exchange techniques were developed to isolate pure transplutonium elements on the milligram and smaller scale. The second edition of The Chemistry of the Actinide Elements, published in 1986 (i), covers most of these topics. A detailed overview of the history of transuranium chemistry is given in Transuranium Elements A Half Century (2). 

Liquid-phase equilibrium data of aqueous mixtures with organic solvents play an important role in the design and development of separation processes. In particular, liq-uid-hquid equihbria (LLE) investigations for ternary mixtures are important in the evaluation of industrial units for solvent extraction processes. The accurate interpretation of phase equilibria of different ternary mixtures is a fundamental and important key to improving solvent extraction techniques (Arce et al., 1995 SenoL 2006 Wu et al., 2003), which can be obtained from direct measuremertt of LLE data or by the use of different thermodynamic methods (Si-Moussa et al., 2008). 

Liquid-liquid extraction is perhaps the most classical of all sample preparation techniques, as it is taught as the early stages of most chemistry students careers (1). The fundamentals of liquid-liquid extraction provide a background for all other extraction techniques described in the literature. In liquid-liquid extraction, dissolved components are transferred from one liquid phase to another. Most commonly in GC, this is performed to dansfer analytes from an aqueous phase to an organic phase that is more amenable to gas chromatographic analysis. The main requirement is that the two liquid phases be completely immiscible. 

Additive invariants were first studied by Pomeau [pomeau84] and Goles and Vich-niac [golesSb]. Although, as we shall see below, there are some techniques that can be used to extract a few invariants from jjarticular systems, no general methodology currently exists. A fundamental obstacle appears to be that there is no purely discrete analogue of Noether s Theorem. 

The problem of carpet recycling is considered and the different methods being proposed or commercially utilised are discussed. The main component of the carpet waste is fibres of nylon-6 and nylon-66. The review of the literature includes a limited amount of journal publications, which focus primarily on fundamental aspects, and a large number of patents, which describe the available technologies. The most promising recycling techniques (depolymerisation, extraction, melt blending and mechanical separation) are described. 48 refs. 

As usually viewed by the reference material producers, a fundamental philosophy of certification rests on the concept of independent methodology, which is the application of theoretically and experimentally different measurement techniques and procedures to generate concordant results leading to one reliable assigned value for the property. Such assigned values are thus method-independent. Extractable concentrations are generated by specific procedures and are thus method-dependent, an idea that has to be rationalized with the fundamental method-independent concept in reference material certification work. 

MAP makes use of physical phenomena that are fundamentally different compared to those applied in current sample preparation techniques. Previously, application of microwave energy as a heat source, as opposed to a resistive source of heating, was based upon the ability to heat selectively an extractant over a matrix. The fundamental principle behind MAP is just the opposite. It is based upon the fact that different chemical substances absorb microwave energy... 

Today, analytical chemistry has such a wide variety of methods and techniques at its disposal that the search for general fundamentals seems to be very difficult. But independent from the concrete chemical, physical and technical basis on which analytical methods work, all the methods do have one principle in common, namely the extraction of information from samples by the generation, processing, calibration, and evaluation of signals according to the logical steps of the analytical process. 

None of the authors of this book is an expert in all the aspects of solvent extraction, nor do we believe that any of our readers will try to become one. This book is, therefore, written by authors from various disciplines of chemistry and by chemical engineers. The scientific level of the text only requires basic chemistry training, but not on a Ph.D. level, though the text may be quite useful for extra reading even at that level. The text is divided in two parts. The first part covers the fundamental chemistry of the solvent extraction process and the second part the techniques for its use in industry with a large number of applications. In this introductory chapter we try to put solvent extraction in its chemical context, historical as well as modem. The last two chapters describe the most recent applications and theoretical developments. 

The studies described above have the purpose of identifying the reacting species in a solvent extraction process and developing a quantitative model for then-interactions. The fundamental parameter measured is the distribution ratio, from which extraction curves are derived. Solvent extraction work can still be carried out with simple batchwise (or point-by-point) technique, but continuous on-line measurements give faster and more accurate results. 

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