6
Obtaining consistentGPC results beginswith the extraction and concentrationprocedures
because slight changes inmobilephase and sample solvent composition can result in some
target compounds beinguncollected. Because the typical sample solvent forGPC is pure
dichloromethane, it is critical that all extracts be reduced to as small a volume as possible
before reconstitution indichloromethane to avoid large amounts of acetonebeing applied to
the column. Soil andbiota samples typically are extractedwith a solventmixture of acetone
anddichloromethane. It is critical that all extracts be reduced to as small a volume as possible
before reconstitution indichloromethane to avoid large amounts of acetonebeing applied to
the column.Dichloromethane has a lower boilingpoint than acetone, so itwill evaporate first
during sample concentration,whichwill leavenearly100% acetone in the concentration
vessel. If dichloromethane is then added to adjust the extract tovolume, therewill be
significant amounts of acetone introduced to theGPC column. Thiswill lead to “solvent
shock” and the formationof a voidwill be observed at the front of the column. This, in turn,
will affect the retention times of the compounds eluting from theGPC column andultimately
will result in some target compounds beinguncollected.Table II lists the common
semivolatile compound elutionvolumes usingGPC.
Analysis
CalibrationStandards
Calibration standards arepurchased asmixtures andusually aredivided among three to seven
separate ampuls due to the cross-reactivityof several compounds. It is importantwhen
making the actualworking standard that the solutionbe storedunder refrigerated conditions in
aMininert
™
vial (Restek cat.#21050 and21051) due to the volatilityof some of the com-
pounds. Failure toproperly store the calibration standardswill result in evaporative loss of the
early-eluting compounds and the solvent. Thiswill, in effect, concentrate the late-eluting
compounds and cause continuing calibration failure andquantitation errors. Evenwhen stored
under the correct conditions, there stillwill be degradationof some compounds due to cross-
reactivity. This is observed as a loss of the target compound and commonlyoccurswith
benzidine, 3,3'-dichlorobenzidine, 4-chloroanaline,N-nitrosodiphenylamine, and toa lesser
extentwith the phenols andother anilines. These standards are stable in the separate ampuls
supplied from themanufacturer, but problems arisewhen all of the compounds aremixed
together tomake theworking calibration standard. Therefore, it is important tomonitor the
responseof themore active compounds andmake freshmixtureswhen the calibration
standardsdegrade.
InjectionPortConfiguration
Several of the compounds listed inTable I are prone tobreakdownor adsorptionon active
surfaces. Typically thiswill occur in the injectionport; therefore, careful attentionmust be
given to the configuration andmaintenanceof the injection system.
On-column injection
techniques can eliminatebreakdownor adsorption in the injection
system and improve chromatographic analysis for drinkingwater extracts or extractswith
littleor nonon-volatile residues.However,wedonot recommendon-column injections for
soil andbiota extracts or extracts that contain large amounts of non-volatile residue, because
the analytical column canbe contaminatedquickly.
Thepreferred injection technique for analyzinghighlycontaminatedextracts is
direct injection,
but direct injectioncancause solvent peak tailingand result in someof the target compounds
elutingclose to the solvent peak.
To reduce solvent peak tailing,
splitless injection
ismost commonlyused forGC/MSanalysisof
semivolatilecompounds.Thereare somedrawbacks to splitless injection includingmolecular
weight discrimination, incomplete sample transfer, and reactivity.Theseproblemscanbe
minimized if the technique isproperlyoptimized. Splitless injection requires an injection system
that is equippedwitha solenoidvalvecontrolling the flow toa split vent.The solenoidvalve is
closedduring the injectionprocess, so themajorityof thevaporized samplemoves to the front of
Restek
Tip
MixingCalibrationStandards
Whenblending several ampuls to
produce a calibration standard, it is
important that all the compounds are
completelydissolved in the solvent.
This is particularly importantwith
someof thehighmolecularweight
polycyclicaromatichydrocarbons
(PAHs) andpesticides that can
separate from solutionduring
refrigerated storage.Beforeopening
ampulscontaining semivolatile
compounds, allow them towarm to
room temperature. Somemixtures
may require sonication to ensure
complete solubility. Follow the
manufacturer’s recommendations for
proper handlingof the standard
mixture.Because some semivolatile
compounds are light sensitive, it is
recommended that calibration
standards be stored in amber vials.
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