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What Can SLE Do For Me?

An interesting and useful variation of liquid-liquid extraction (LLE) is supported liquid extraction (SLE).  Compounds are separated on the same basis of differential solubility, as with conventional liquid-liquid extraction.  While the chemistry of this technique is very similar to that of LLE the physical nature of the technique offers distinct benefits. 

  1. Automation - the use of column format tubes or 96-well plates for SLE allows this process to be easily automated using liquid handling instrumentation.

  2. Cost effective - less solvent is used to process samples, therefore, there are  reduced solvent purchase and disposal costs.

  3. Alleviation of emulsions - LLE is prone to emulsion formation when extracting samples containing surfactant-like compounds (e.g. phospholipids).  However, SLE is an emulsion free technique as there is no vigorous shaking.  As a result recoveries are often higher and demonstrate better reproducibility from sample to sample when handling samples which may form emulsions.

  4. Improved recoveries and reproducibility - the intimate contact between the aqueous and organic phases allows very efficient partitioning and analyte recoveries can sometimes be higher than in conventional LLE. Furthermore, the SLE process is more technique independent than conventional LLE; this often means that the experiments are more reproducible.1 

In SLE, an aqueous sample is applied to a high-surface area matrix such that the sample is dispersed over the surface of the matrix, creating an interface for extraction
(Figure 1).  This is most often executed in a syringe barrel-type column or 96-well plate format, similar to those used in solid phase extraction (SPE).  The most common matrix employed for sample dispersion is purified diatomaceous earth. 

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Figure 1: Supported liquid extraction (SLE).

Often the aqueous sample is pretreated (i.e. the pH is adjusted or an ion-pairing agent or buffer is added) such that the analyte or analytes of interest are in a suitable form to be extracted into an organic solvent, just as in conventional LLE.  In order to promote partitioning of analytes into the organic phase in supported liquid extraction, the charge on any acidic or basic groups should be suppressed wherever possible.

This is particularly important for more polar analytes.  Suppression of ionizable functional groups can be carried put using the 2 pH rule (Figure 2).  For neutral analytes, that do not have any ionizable functional groups, the choice of the correct extraction solvent is the most important factor.  A water-immiscible organic solvent in which the analyte is freely soluble is a good choice.  

For very non-polar analytes, non-polar solvents such as heptane, hexane, or dichloromethane may be most appropriate.  

For more polar compounds, more polar extraction solvents such as MTBE, dichloromethane or ethyl acetate are useful. For polar molecules that do not elute well in these solvents, 5% (v/v) of a polar modifier such as isopropanol can be added to the extraction solvent to enhance extraction efficiency and raise analyte recovery.

Figure 2: 2 pH rule.

A small volume of water-immiscible organic solvent is subsequently passed over the matrix holding the aqueous layer and the analytes partition into the organic phase.  The extraction solvent is allowed to percolate by gravity or sometimes a gentle vacuum or pressure is applied.  The organic solvents most commonly used are the same as those in LLE; ethyl acetate, methyl tert-butyl ether (MTBE), dichloromethane, hexane, and mixtures thereof. 

In SLE the entire sample volume is absorbed onto the extraction bed, therefore, it is vital to use a format with sufficient capacity to absorb the whole sample volume. For example, to extract 200 µL of plasma, diluted 1:1 with buffer, a 400 µL capacity product should be used.  When using internal standards, they should be added to the raw sample, mixed, and allowed to equilibrate prior to any other sample pre-treatment.

Some commercial SLE products are available pre-buffered.  This allows for the pH of the sample to be changed simply by applying the sample to the column (Figure 3).  This approach is useful for analytes where pH can improve extraction into the organic phase.  For amines, basic-buffered supports would be used, and for acids and acidic-buffered support.

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Figure 3: Pre-buffered SLE matrix (1) and analyte elution (2).

As in conventional LLE, this technique may be used in two ways:

  1. The target analytes are transferred into the extracting solvent leaving the contaminants behind in the sample solvent on the support.
  2. Contaminants are transferred into the extracting solvent leaving the target analytes behind on the solid support.  In this case it is necessary to recover the analytes by displacing the original sample solvent from the support using an appropriate solvent.

The benefits of using this technique (cost efficiency, automation, alleviation of emulsions etc.) can be realized for any sample preparation method currently being carried
out via LLE.

 

References

  1. Majors, R. E. LCGC Europe 2012, 25, 430-435.
 

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Dr. Dawn Watson
 

This article was written by Dr. Dawn Watson.

Dawn received her PhD in synthetic inorganic chemistry from the University of Strathclyde, Glasgow. The focus of her PhD thesis was the synthesis and application of soft scorpionate ligands. As well as synthetic skills, this work relied on the use of a wide variety of analytical techniques, such as, NMR, mass spectrometry (MS), Raman spectroscopy, infrared spectroscopy (IR), UV-visible spectroscopy, electrochemistry, and thermogravimetric analysis.

Following her PhD she spent two years as a postdoctoral research fellow at Princeton University studying the reaction kinetics of small molecule oxidation by catalysts based on Cytochrome P450. In order to monitor these reactions stopped-flow kinetics, NMR, HPLC, GC-MS, and LC-MS techniques were utilized.

Prior to joining the Crawford Scientific and CHROMacademy technical team she worked for Gilson providing sales and support for the entire product range including, HPLC (both analytical and preparative), solid phase extraction, automated liquid handling, mass spec, pipettes, and laboratory consumables.

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