Solid-Phase Extraction Columns for Lipid Analysis

Solid-phase extraction methods are being used for increasing numbers of applications in lipid analysis, but especially as a rapid means of isolating particular components for further analysis. Many different types of packing material are available enabling most different modes of chromatography to be used, including adsorption, reversed-phase, silver-ion and ion-exchange. However, there are occasions when too much is expected of the methodology. Solid-phase extraction columns makes use of small columns made of an impermeable plastic material and packed with a variety of adsorbents held in place by frits, under trade names such as Bond Elut, Sep-Pak, etc. The first reaction of most analysts to their introduction was probably one of pleasant surprise and wonderment that no one had had such a simple idea before. This was certainly my first thought, then my Scottish blood (we have a reputation for thriftiness) asserted itself and I began to wonder whether the costs could be justified in comparison to purchasing adsorbents separately and using disposable pasteur pipettes as mini-columns. In time, I have come to the conclusion that both approaches have their merits. In simple clean-up procedures with silica gel or Florisil as the adsorbent, the pasteur pipette technique is often suitable. The commercial pre-packed columns are available in such a wide range of packing materials, with uniform reproducible properties, that their value for specific purposes cannot be denied. I will discuss some of their applications here. I reviewed the methodology in relation to lipids more comprehensively earlier [1], and this review is available online here. Others have also reviewed the topic [2]. In addition, you will find more than 300 references to the use of the technique in the literature survey section of the website. The term "solid-phase extraction column" now appears to be widely accepted for the commercial columns, largely because one of the main uses is in the extraction of materials of interest from biological fluids such as plasma or urine. It is a rather clumsy expression, which can perhaps be made more tolerable by abbreviating it to "SPE column", although "sorbent column" might be neater. Apart from silica gel and Florisil, which have been much used by lipid analysts as adsorbents over the years, SPE columns are available packed with materials to which various functional groups are bonded by chemical means to the silanol groups on the surface of the silica gel particles. Those most widely used are probably those with chemically bonded octadecylsilyl (ODS or C18) moieties, which are of particular value in the extraction of lipids from aqueous media. In addition, there is a variety of packing materials with cationic or anionic functional groups. Only a few of these appear to have been explored in analytical applications with lipids, but I will describe some examples below. frits