Supported liquid extraction (SLE) has been around for a long time, but surprisingly it is seldom used to replace classical
liquid–liquid extraction (LLE). In SLE, the same aqueous phases used in LLE are coated onto an inert diatomaceous earth support,
but instead of shaking the two immiscible phases together, the organic phase is passed through the column (or cartridge) and
a very efficient extraction takes place. SLE offers many advantages over LLE, including equivalent or more efficient extraction,
no emulsion formation, easy automation, less organic solvent use, less labour and less glassware.
Liquid–liquid extraction (LLE) is probably the most widely used sample preparation process. It has been around for many decades,
the theory is fairly easily understood and thousands of publications have demonstrated its usefulness. Most chromatographers
are first exposed to the technique in their high school or undergraduate chemistry class. I covered the basics of LLE in a
previous "Sample Prep Perspectives" instalment (1), and they will not be repeated here.
There are many advantages to the use and application of LLE. First, the operation is fairly simple and the glassware used
is inexpensive. Common solvents are often used for extraction, and most applications involve an aqueous phase and an immiscible
organic phase. Initially, the sample can be in either phase, and, depending on the partition coefficients of sample and matrix
components in the two phases, compounds of interest often can be quantitatively transferred without the presence of interferences.
Adjustments to extraction conditions such as changing pH and ionic strength of the aqueous layer or addition of a second organic
solvent to the organic layer are easily made. Recoveries can be quite high and reproducible.
On the other hand, there are also some disadvantages to traditional LLE. First, it involves a great deal of time with laborious
shaking that can be user dependent. Vigorous shaking (or even non-vigorous shaking) can result in emulsion formation. Emulsions
can be difficult to break, and additional time is usually involved in performing this activity. In its traditional application,
the technique consumes large amounts of organic solvents and generates considerable waste. Glassware must be cleaned after
each use. Automation is very difficult; the technique is performed serially rather than in parallel so throughput is reduced.