Dionex ASE 150, Operator'S Manual, Page: 156 / 176

ASE 150 Operator’s Manual
148
Doc. 065207-02 9/08
soluble in the extraction process and then be deposited in the exit lines.
Oven drying and freeze drying are other viable alternatives for sample
drying prior to extraction; however, the recovery of volatile compounds
may be compromised by these procedures.
The sample drying or dispersing agent should be mixed thoroughly with
the sample in a small vial, beaker, or mortar, and then added to the sample
cell. For quantitative transfer, the mixing vessel can be rinsed with 1 to
2 mL of the extraction solvent using a Pasteur pipette, and this volume
added directly to the sample cell. The main purpose of mixing with a
drying agent like diatomaceous earth is not to remove all water from the
sample but to make it easy to transfer the sample into the sample cell. If
elevated temperatures are used, then some water will be removed from
the sample during the extraction process. In this case, sodium sulfate can
be added to the extract after collection to remove water.
E.3.2 Extraction Parameters
Solvent
For an efficient extraction, the solvent must be able to solubilize the target
analytes while leaving the sample matrix intact. The polarity of the
extraction solvent should closely match that of the target compounds. The
common adage of “like dissolves like” is very applicable in ASE. Mixing
solvents of differing polarities can be used to extract a broad range of
compound classes. Generally, if a particular solvent has been shown to
work well in a conventional procedure, it will also work well in ASE.
Compatibility with the postextraction analytical technique, the need for
extract concentration (solvent volatility), and the cost of the solvent
should all be considered. While many ASE methods recommend solvents
or solvent mixtures for specific analyte classes, there may be alternatives
that better fit the needs of a particular laboratory. For example, Schantz
(11) showed that dichloromethane/acetone or acetonitrile can be used to
get complete extraction of polycyclic aromatic compounds. If GC or GC-
MS is the analytical method, then dichloromethane/acetone would be the
solvent of choice. If HPLC is the determinative step, then acetonitrile
would be the solvent of choice. Solvent choice will also determine the
level of coextractables along with the analytes. Generally, the more polar
the solvent or solvent mixture, the less selective it will be.