5
Restek
Tip
StabilizingDichloromethane
Dichloromethane requires a stabilizer to
prevent the formationof hydrochloric
acid (HCl).Without a stabilizer, HCl
will form and injectionof acidic
dichloromethanewill cause inlet liners
and columns tobecome reactive.There
are two types of stabilizers: stabilizers
that keepHCl from forming, and
stabilizers that eliminateHCl upon
formation.Methanol is a stabilizer that
preventsHCl from forming;whereas
cyclohehane, cyclohexene, 2-
methylbutene, andamylene scavenge
theHCl after its formation.
Dichloromethaneused in liquid
extractors should containboth types of
stabilizers.Methanol is a better
stabilizer, acting as a free radical
inhibitor, butmethanol partitions into
the thewater phase.This could leave an
unstabilized extract unless a scavenger
stabilizer also is used.
as diatomaceous earth is added to the sample prior tobeing loaded into the extractionvessels.
The type of samples being extracted aswell as the requiredmethoddetection limits shouldbe
considered as part of the evaluationof pressurized fluid extraction.
Microwaveextraction
(USEPAMethod3546) canbeuseful for automatedextractionaswell.
Thismethod typicallyperforms theextractionof 12 samples simultaneously, but requires slightly
moreoperatorhandling than thepressurized fluidextraction instruments.Microwaveextraction
instrumentation is less expensive, but can suffer from the same sample size limitations.
Supercritical fluid extraction (SFE)
has beenpromoted for a number of years as ameans of
“solventless” extraction for environmental samples. SFEhas been added toSW-846 as
Methods 3560, 3561, and3562but its application is limited. SFE suffers from severematrix-
relatedvariation, requiringmodificationof its conditions dependingon soil type,water
content, sample size, and typeof analytes.Doing soultimately requires additional sample
preparationprior to the actual extraction. These requirements, added to the high cost of the
instrument, havevirtuallyprecluded theuseof SFE for environmental samplepreparation.
Cleanup
Sample extract cleanupmaybe themost important step inmaintaining long-term instrument
performance.Many times,when instrument problems arise, they are causedby exposureof
the injectionport and the column tomaterial in the sample extracts other than the target
compounds.While all contaminants cannot be eliminated, reducing themwillminimize
injectionport and columnmaintenance.Most semivolatile extracts, especially those extracts
from soil andbiota samples, containhigh-boilinghydrocarbons and lipids. Thedifficulty in
attempting to remove these compounds usingoneof the common solid-liquid cleanup
techniques (e.g., Florisil
®
and silica gel) is that the cleanup technique also removes some of
the target compounds. In addition, because the analyticalmethodusually calls for the
reportingof several tentatively identified compounds (TICs), it is not desirable to clean the
extracts of compounds thatwouldnormally elute in the range of the target compounds. For
these reasons, gel permeation chromatography (GPC) is theonlyuniversal cleanup technique
for semivolatile extracts.
Gel PermeationChromatography
Gel permeation chromatography (GPC) is apreparative scale chromatographicmethodof
separationbasedonmolecular size. Because the target compounds are similar inmolecular
size, they elute as a bandofmaterial and are easily separated from lighter andheavier
contaminants.However,GPC systems are expensive and the processing time per sample is
between30 to70minutes. For these reasonsmany laboratories choose not touseGPC.
However, it is very efficient for removing sulfur, highmolecularweight hydrocarbons, and
lipids from semivolatile extracts; andmaybe prudent for soil andbiota samples.
Although sulfur canbe removedusingother techniques suchasmercuryor activatedcopper
powder, theseprocedures, especiallycopperpowder,maydegrade someof the target compounds
andwill not remove thehigh-boilinghydrocarbonsor lipids.The lipidcontent of biotaextracts
canbe significant andmayoverloadmost SPEclean-up techniques. If a sampleextractwitha
high lipidcontent is injected into theGC, the injectionport and front of thecolumnwill become
contaminatedquickly.Thiswill result in failureof check standards and the lossof active
compounds suchasnitroanilines, nitrophenols, carbazole, andpentachlorophenol (PCP). In spite
of theaddedexpenseand time required forGPC, it is thebest alternative for extract cleanup.
USEPAMethod3640details the requirements forGPC cleanupof extracts for semivolatile
analysis.One of the important steps ofGPC cleanup is to ensure eachday that the instrument
iswithin its retention time calibration.Althoughnot requiredby themethod, it is good
practice to run adaily calibration check standardbeforeprocessing thenext batchof samples.
If anumber of samples havebeenprocessed that contain large amounts of contamination, the
front of theGPC column canbecome reactive.This typically is observed in the loss of 2,4,6-
tribromophenol for semivolatile extracts. If the columnbecomes reactive, injectingblanks
may return the system to control and save the time required to change the column.
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