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5
Soil Samples
For soil samples, soxhlet orultrasonic extractionhavebeen themost common
extractionmethods; althoughpressurized fluid,microwave, and supercritical
fluid extraction (SFE) are used as well.
Pressurized fluid extraction (USEPAMethod3545) runsunattended, but has
some sample size limitations. Generally, nomore than 10g of sample can be
extractedwithout usingmultiple vessels, so detection limitsmay be compro-
mised for certain analytical methods. It is important to take this into consid-
eration when evaluating the use of either pressurized fluid or microwave ex-
traction.
Although not currently cited by the US EPA, microwave extraction can be
useful for automated extraction as well. Microwave extraction extracts 12
samples simultaneously, but does require slightly more operator handling
than the pressurized fluid extraction instruments. The instrumentation is
less expensive than the pressurized fluid instrumentation, but lack of anEPA
method has limited the use of this technique in the US.
Supercritical fluid extraction has been promoted for a number of years as a
“solventless” extraction technique for environmental samples. SFEwas added
to SW-846 as Method 3560, but its application is limited. SFE suffers from
severematrix-related variation, resulting in theneed tomodify theSFE condi-
tions depending on soil type, water content, sample size, and the type of
analytes. This ultimately requires additional sample preparation prior to the
actual extraction. These requirements, added to thehigh cost of these instru-
ments,hasvirtuallyprecluded theuseofSFE forenvironmental sampleprepa-
ration.
Sonication or Soxhlet Extraction
Sonicationorsoxhletextractionworkswell forchlorinatedpesticidesandPCBs.
Sonication is a faster technique, but requires constant operator attention. In
both techniques, problems such as contamination are attributed to either
contaminated reagents, especially sodium sulfate, or poor laboratory prac-
tices being used when transferring sample extracts. Using sodium sulfate to
removewater (described onpage 4) is important.Mix the samplewith sodium
sulfate to achieve a sandy consistency prior to solvent addition. Using granu-
lar sodium sulfate is recommendedbecause someof thepesticideswill adsorb
to the powderedmaterial.
Solvent Selection
Since soil and biota samples are essentially wet particles, acetone and
dichloromethane usually are used in a 1:1 combination as the extraction sol-
vent. The acetone is needed to adequately penetrate into the soil particle so
that compounds contained in the particle can be extracted. Several other
solvent systems canbeused for unique extractions, but generally this combi-
nation works for most applications. Use pesticide residue grade solvents for
this application and run solvent assays to verify thematerial prior to its use.
To perform a solvent assay, reduce 300 to 400mL of solvent to a final volume
of 1mL, and exchange to hexane for analysis by GC/ECD (electron capture
detection). The extract analysis shouldhave no chromatographic peaks above
50% of the detection limit for any target compound.
Finally, with regards to solvent selection, it is important to note that
dichloromethane will form hydrochloric acid spontaneously without a stabi-
lizer present. There are two classes of stabilizers: stabilizers that keephydro-
chloric acid from forming, and stabilizers that eliminate hydrochloric acid
upon formation.Methanol andcyclohexaneareused tostophydrochloricacid
from forming. If water samples are extracted with dichloromethane contain-
Soxhlet andultrasonic
extraction are themost com-
mon extractionmethods for
soil samples; althoughpressur-
ized fluid,microwave, and
supercritical fluidextraction
can be used aswell.
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