Principles of Extraction

Dr. Safa Majeed Hameed

    This article focuses on three widely used techniques for extraction of semivolatile organics from liquids: liquid–liquid extraction (LLE), solid-phase extraction (SPE), and solid-phase microextraction (SPME). Other techniques may be useful in selected circumstances, but these three techniques have become the extraction methods of choice for research and commercial analytical laboratories. A fourth, recently introduced technique, stir bar sorptive extraction (SBSE), is also discussed. 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 e¤ect 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. Extraction or separation of dissolved chemical component X from liquid phase A is accomplished by bringing the liquid solution of X into contact with a second phase, B, given that phases A and B are immiscible. Phase B may be a solid, liquid, gas, or supercritical fluid. A distribution of the component between the immiscible phases occurs. After the analyte is distributed between the two phases, the extracted analyte is released and/or recovered from phase B for subsequent extraction procedures or for instrumental analysis. The theory of chemical equilibrium leads us to describe the reversible distribution reaction as

And the equilibrium constant expression, referred to as the Nernst distribution law [1], is

Where the brackets denote the concentration of X in each phase at constant temperature (or the activity of X for nonideal solutions). By convention, the concentration extracted into phase B appears in the numerator of equation. The equilibrium constant is independent of the rate at which it is achieved. The analyst’s function is to optimize extracting conditions so that the distribution of solute between phases lies far to the right in equation and the resulting value of KD is large, indicating a high degree of extraction from phase A into phase B. Conversely, if KD is small, less chemical X is transferred from phase A into phase B. If KD is equal to 1, equivalent concentrations exist in each phase.