Practical Aspects of Solvent Extraction - Columnist Ron Majors discusses some of the practical considerations in the successful application of the popular yet age-old technique of solvent extraction (

Practical Aspects of Solvent Extraction

Columnist Ron Majors discusses some of the practical considerations in the successful application of the popular yet age-old technique of solvent extraction (also known as liquid–liquid extraction, or LLE). After a brief review of the basics, guidelines on the selection of the appropriate extraction solvents and how to use acid–base equilibria to ensure efficient extractions of ionic and ionizable compounds are provided. Problems in LLE and the solutions to these problems are highlighted. A..

Mar 1, 2009
By: Ronald E. Majors
LCGC Europe
Volume 22, Issue 3

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Recently, I had the opportunity to participate in Colacro XII, a chromatography symposium held in a Latin America country every two years. This year's meeting was held in Florianopolous, Brazil, 27–30 October. During the symposium, I perused the large number of applications posters for the sample preparation techniques being used. I was surprised to find many of the investigations of such diverse matrices as natural products, fruits and vegetables, petroleum products and body fluids used solvent extraction (liquid–liquid extraction, or LLE) as the initial (or the only) sample preparation technique. So, I thought that I would devote an instalment of "Sample Prep Perspectives" to this well used technique. In our last comprehensive sample preparation survey, nearly 40% of the respondents reported on the use of LLE as at least one of their routine sample preparation procedures.¹

Earlier, I wrote on the basics of LLE providing some of the theory (which will not be repeated here), examples of classical LLE including continuous and countercurrent chromatography, and some newer techniques of the time.² More recently, I covered approaches for miniaturization of classical LLE.³ In this instalment, I will provide some practical hints to those who are considering the use or already are using LLE in hopes of improving your methodology.

Quick Review of LLE

LLE is performed using two immiscible liquids and soluble samples. LLE is useful for separating analytes from interferences by partitioning the sample between these two immiscible liquids or phases. Usually, one phase in LLE will be aqueous (often the denser or heavier phase) and the second phase is an organic solvent (usually the lighter phase). The more hydrophilic compounds prefer the polar aqueous phase, while more hydrophobic compounds will be found mainly in the organic solvent. Analytes extracted into the organic phase are recovered easily by evaporation of the solvent, while analytes extracted into the aqueous phase can often be injected directly onto a reversed-phase high performance liquid chromatography (HPLC) column. Obviously, when the analyte of interest is found in the aqueous phase, evaporation is a much slower process. However, the following discussion assumes that an analyte of interest is concentrated preferentially into the organic phase, but similar approaches are used when the analyte is extracted into an aqueous phase.

Figure 1

The generic flow diagram of Figure 1 summarizes the steps involved in a typical LLE separation. Because extraction is an equilibrium process with limited efficiency, significant amounts of the analyte can remain in both phases. Chemical equilibria involving changes in pH, ion-pairing and complexation can be used to enhance analyte recovery and eliminate interferences.

In its classical and simplest form, LLE is performed conveniently in a separatory funnel where the two immiscible phases are added from the top and, after the extraction process, the heavier phase is drained out the bottom stopcock. However, LLE can be performed in other devices such as beakers, vials, centrifuge tubes and graduated cylinders. In these instances, the liquids must be removed by some type of pipette and great care must be exercised not to disturb the interface layer. Otherwise, the extraction process may be incomplete.

Selection of the Organic Solvent

The LLE organic solvent is chosen for the following characteristics:

A low solubility in water (<10%).
Volatility for easy removal and concentration after extraction.
Compatibility with the HPLC or gas chromatography (GC) detection technique to be used for analysis (for example, avoid solvents that are strongly UV-absorbing for LC and chlorinated solvents for GC using electron-capture detection).
High purity because extracted analytes are often recovered by the evaporation of large volumes of organic solvent; impurities in the solvent can be concentrated as well.
Polarity and hydrogen-bonding properties that enhance recovery of the analytes in the organic phase — that is, increase the value of K_D, the distribution constant.¹

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