Solvent extraction equilibrium steps involved

Chelate formation is the more common means used for solvent extraction, and so we will describe the principles involved. Most chelating agents are weak acids that ionize in water. The usual practice is to add the chelating agent to the organic phase. The extraction process can be thought to consist of four equilibrium steps, each with an equilibrium constant. These are represented by the following ... 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

Liquid-liquid extraction (LLE) has been around for a long time and has been used extensively as an analytical sample pretreatment to remove unwanted matrix components [16,17]. It is based on the principles of differential solubility and partitioning equilibrium of analyte molecules between aqueous (the original sample) and organic phases. Depicted in Fig. 5, LLE initially involves pH adjustment of the sample with an appropriate buffer. This pH adjustment is intended to neutralize the molecule, making it more amenable to extraction. The next step is the addition of an immiscible organic extraction solvent, followed by agitation... 

Solid-phase microextraction (SPME) is a solvent-free sample preparation technique. The volume of the extraction phase is very small compared to the sample volume. The extraction is not exhaustive, but is based on equilibrium between the sample and the extraction phase, which is located on a fiber. SPME involves an adsorption step of the analyte, from a gas headspace or in a liquid sample (direct immersion), and a desorption step, which often is coupled directly with injection in the analytical system. Although SPME is mainly used in combination with GC, it has also been automated for HPLC. Eigure 9.10 shows a schematic representation of an SPME device. 

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