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

Figure 1

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