The Role of Selectivity in Extractions: A Case Study

Feb 01, 2014

Many of the extraction techniques developed over the past generation tout selectivity among their advantages. In reality, solvent selection and the use of stationary (sorbent) phases are the main mechanisms for providing selectivity. Therefore, selectivity is often limited to isolation of classes of compounds rather than individual structures. In this column instalment, the selective removal of a fat substitute in food products is discussed to demonstrate options for obtaining selectivity during extraction.

Over the past generation or so, myriad extraction techniques were developed that have generally improved yields, lessened the amount of organic solvent used, and minimized time. Additionally, many of these techniques claim advantages concerning selectivity.

Selectivity is the ability to determine the analytes of interest in preference to other sample components (potential interferents). A recent instalment of this column (1) advocated that selectivity can stem from any point in the analytical process, but as a general rule selectivity arises from separations, selective detection schemes, and selective chemical reactions. These approaches can balance each other. For example, if an analytical separation is not completely sufficient, the use of a selective detection method like mass spectrometry (MS) or fluorescence spectroscopy can offer the balance of the required selectivity provided that the unseparated components do not suppress the detector signal.


Figure 1: Just-enough sample preparation represents a continuum of methodologies.
Majors described "just enough" sample preparation (2) in which method selectivity is matched to the qualitative or quantitative analytical requirements. For example, the QuECHERS (quick, easy, cheap, effective, rugged, and safe) method for extracting pesticides from fruits and vegetables combines salting out partitioning with dispersive solid-phase extraction (SPE) to remove matrix components, allowing effective chromatography and MS detection. As Majors points out and illustrates in Figure 1 from his original column, increasing complexity in an analytical procedure typically leads to greater selectivity.

Turning our attention back to modern extraction methods, the fundamental driving force of the technique leads to the element of selectivity. A number of sorbent-based methods, such as SPE, solid-phase microextraction, and stir-bar sorbent extraction, use chromatographic stationary phases to isolate solutes of interest from gaseous or liquid samples. Analytes are retained by their attraction to a stationary phase of similar polarity and are selectively eluted via choice of an appropriate solvent. The techniques aimed at solid samples, including supercritical fluid extraction (SFE), pressurized fluid extraction, microwave extraction, and ultrasound extraction, rely on the application of energy (often heat) to drive the analyte into an appropriate solvent. In all of these techniques, both sorbent- and solvent-based, the key to selectivity is the match between analyte polarity and polarity of the extracting phase. In other words, "like dissolves like". Thus, extractions are usually considered crude separation techniques, providing compound class selectivity and less utility for the selective isolation of specific, individual compounds. Of course, volatility is the major contributor to selectivity for gas-phase techniques.

If the primary selectivity mechanism in extractions is solute polarity (that is, matching solute polarity with the solvent or sorbent following the "like dissolves like" principle), is selectivity possible during chemical extraction? Is selectivity beyond compound class selectivity possible? Do extractions need to be selective or is selectivity solely a function of subsequent chromatography and detection?

To look at an example of extraction selectivity within the "like dissolves like" polarity context, let's consider the example of fat analysis in food products and, more specifically, the example of sucrose ester fat substitutes.