When should I choose SLE or SPE for sample cleanup?

When it comes to sample cleanup the question occasionally comes up asking what technique is best for my application? Solid Phase Extraction (SPE) or Supported Liquid Extraction (SLE). Well, what’s right for you depends on your application and the characteristics of your analytes. Both techniques offer high recoveries, are reproducible, and are easily automated.  In this post I would like to outline some of the advantages, disadvantages and limitations of each technique.

SLE works on the same principle as liquid-liquid extraction (LLE), where you are transferring your analyte from an aqueous layer into an organic layer.  However, the “supported” technique allows for a quick three step extraction (Load, Wait and Elute) versus the five step extraction (Condition, Equilibration, Load, Interference wash, Analyte Elution) required with most SPE methods.

Because  the SLE mechanism is based on partition coefficients, analytes with higher Log P values generally extract better.  However, by treating the sample with small organic acids, analytes with negative Log P values often do well on SLE.  SLE can be the easiest fastest most cost effective way to go. It does need to be pointed out that in most cases, the SLE extract will require evaporation and re-constitution into a more suitable solvent such as mobile phase prior to injection.

Like LLE, SLE works very well with a broad range of analytes, acidic, basic and neutral. Also, it is widely used because it provides extracts virtually free of phospholipids, salts and proteins which can cause ion suppression or enhancement in an LC/MS or GC/MS. However, in some cases, certain molecules are not recovered well using SLE and in these cases, SPE should be considered.

SPE is more selective and therefore, you can often get higher recoveries from problem compounds.  SPE also comes in a variety of modes allowing you to choose from strong or weak cation or anion exchange, mixed mode, reverse phase, etc.  There are many more options of separation techniques to choose from.  Also, with newer chemistries being applied to SPE consumables, you can sometimes eliminate the condition and equilibration steps, making the process faster.  Because SPE is usually a concentration technique, sometimes a method can be developed eliminating evaporation and reconstitution prior to injection.

What have your experiences been? Have you compared any SPE methods to SLE?

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