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6
ingmethanol as the stabilizer, themethanol will partition into thewater, leav-
ing an unstabilized extract. Hydrochloric acid forms quickly in unstabilized
dichloromethane, and injection of an acidic solvent will result in reactivity of
liners and columns. The second type of stabilizers are alkene compounds,
which are used to reduce hydrochloric acidupon formation. It is desirable to
use an alkene stabilizer that is low-boiling to prevent interference with early
eluting target compounds.
Sample CleanupMethods
Sample extract cleanup is probably the most important step in maintaining
long-term instrumentperformance.Generally,when instrumentproblemsarise,
they are caused by exposure of the injection port and the column to contami-
nants in the sample extracts.While all of these contaminants cannot be elimi-
nated,most canbe reduced to levelswhere theybecomemuch less of an issue.
Contained inmany pesticide and PCB extracts are hydrocarbons, sulfur, ph-
thalate esters, and lipids in the case of biota samples. Many of these com-
pounds can be removed from the extract by one ormore of the following tech-
niques,with littleadditional cost or time,whichusuallycanbe recoveredbyan
increase in instrumental stability, adecrease in instrumentmaintanence, and
possible improvements in detection limits.
Sulfur andLipidContaminants: Gel PermeationChromatography
Gel permeationchromatography (GPC) isapreparative-scalechromatographic
method of separation based on molecular size. Since the target compounds
are similar inmolecular size, they elute as a band of material and are easily
separated from lighter and heavier contaminants. For the pesticide and PCB
extracts, GPC is a very efficientmethod for removing sulfur and lipids. GPC is
the only cleanup technique cited here that requires considerable expense,
and the processing time per sample is between 30 to 70minutes. For these
reasons,many laboratories choosenot touseGPC.However, for soil andbiota
samples, GPC is themost prudent cleanupmethod.
Sulfur also can be eliminated usingmercury or activated copper powder, but
lipids arenot as easily removed. Lipid content of biota extracts canbe several
orders ofmagnitude higher than that handledusing SPEmethods, soGPC is
still
a good alternative. If sample extractswithhigh lipid content are injected
into the GC, the injection port and head of the column will quickly become
contaminated, resulting in failure of continuing check standards.
USEPAMethod3640details the requirements forGPC cleanupof extracts for
pesticide andPCB analysis. If the sample is to be analyzed for PCBs only, the
sulfuric acid cleanup (USEPAMethod3665) described onpage8 ismore cost
effective thanMethod 3640, but is not amenable to all the pesticides. When
performing a GPC cleanup, verify the instrument retention time calibration
on a daily basis or before processing the next batch of samples, whichever is
less frequent. If a number of samples have been processed that contain large
amounts of contamination, the front of theGPC column canbecome reactive.
This is typically observed in the loss of 2,4,6-tribromophenol for semivolatile
extracts, but it may not be as easily observed in the pesticide GPC standard.
The use of a 2-3" guard column can prevent repacking of the 70g analytical
column.
GPC columnsalsoare very sensitive to slight changes inmobilephase compo-
sition (solvent variations). Because soil and biota samples typically are ex-
tracted using a solvent mixture, and dichloromethane is the lowest boiling
solvent, it will evaporate first when the extract is concentrated. This leaves
nearly 100% acetone in the concentration vessel. If dichloromethane is then
added to adjust the extract to volume, significant amounts of acetone will be
introduced into the GPC column. This will lead to “solvent shock” and the
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Sample extract cleanup is
probably themost important
step inmaintaining
long-term instrument
performance.
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