Dionex ASE 150, Operator'S Manual

Page: 160 / 176

ASE 150 Operator’s Manual
152
Doc. 065207-02 9/08
E.5 Selectivity in ASE
Selectivity in extraction is defined here as being able to extract compounds of
interest with little or no interfering coextracted compounds. ASE is generally
considered to be an exhaustive extraction technique, and often the extracts
obtained from complex samples contain compounds that can interfere with the
determination of the desired analytes. Selectivity in ASE could come from the
manipulation of the extraction conditions to minimize coextractables while
maximizing analyte recovery. There are three basic procedures to obtain selective
extraction in ASE or, in other words, to generate extracts that contain the
compounds of interest and few, if any, interfering compounds. The three
techniques are choice of temperature, choice of solvent, and use of adsorbents in
the sample cell. Of course, the most powerful method is to use variations of all
three to fine-tune the selectivity during the ASE process.
The choice of temperature alone can affect selectivity. The higher the temperature
for the extraction, the less selective the results. Lowering the temperature will
make ASE more selective, but the recovery of analytes can diminish unless the
time is increased. Similarly, selectivity is decreased when using more polar
solvents. However, one can use a series of solvents of increasing polarity to obtain
selective extractions or what could be called fractionation.
Dark-colored fruits such as blueberries are being studied for their antioxidant
content. These compounds are polyphenolic in nature and require polar solvents
for extraction. However, if the fruit sample is extracted with a polar solvent like
methanol, acetonitrile, ethanol, or water, the resulting extract contains many
compounds that make the analysis for antioxidant compounds more challenging.
We have found that the samples can first be extracted with nonpolar solvents like
hexane or DCM to remove unwanted wax compounds. Then, by extracting the
same sample with solvents of increasing polarity and collecting the fractions in
separate vessels, one can obtain extracts that are easier to analyze.