16
Quantitation
Because splitless injections suffer from irreproducibility, quantitation for semivolatile
compounds is by internal standard, using a single ion for each analyte. Internal standards at
known concentrations areused to correct for variances in the amount ofmaterial transferred
to the columnwithdifferent injections, and also to trackMS sensitivity.
All sample analyte concentrations are calculatedusing response factors obtained from the
calibration curve. Thismethoduses single ions (i.e., extracted ions) for each compound so
that chromatographic resolutionof each compound is not necessary. It is acceptable for
compounds to co-elute, as long as theydonot have any common ions used for quantitation.
Typically, only isomers have similar spectra, so chromatographic resolution is requiredonly
for these compounds.
Figure 9 shows the possibilities for calibration curves for this analysis. CurveA is the desired
result, indicating aproportional responsewith increasing concentration. This implies that
there is nodetector saturationor reactivity for this compound. CurveB is indicative of a
compound that undergoes a reaction, usually in the injectionport or on the headof the
column. If a calibration curve likeCurveB is observed, injectionport or columnmaintenance
is required. Finally, CurveC shows high-end roll-off, indicating saturation. If a calibration
Figure9.
Calibration curves showing linear response, adsorption, andhigh-end roll-off.
responce (areacounts)
concentration
C
A
B
20000
40000
60000
80000
100000
120000
140000
160000
180000
200000
20
40
90
80
100 120 140 160
curve likeCurveC is observed, select a columnwithhigher capacityor reduce detector
sensitivityby lowering themultiplier voltage.
Summary
Although theanalysisof semivolatileorganiccompounds isoneof themoredifficult tests
performedbyenvironmental laboratories, usingRestek’sRtx
®
-5SilMSorRtx
®
-5MScolumns
and following the tipspresented in thisguidecanmake it easier toperform.Correct sample
preparation, extract cleanup, injection technique, analytical columns, standards, andquantitation
canhelpminimizeproblemsnormallyassociatedwith semivolatileorganicanalyses.When
problemsoccur, use themost appropriate troubleshootingandmaintenanceprocedures toquickly
optimizeyour analytical system.When facedwithdifficulties inyour semivolatileanalysis,
remember that themajorityof problemsoccur in the samplepreparationandcleanup step, or in
theGC injectionport. If you still arehavingdifficulties after reading through thisguide, contact
Restek’sTechnical ServiceTeamviaemail at
viaphoneat
800-356-1688or814-353-1300, ext. 4.
Website :
E-mail :
TelNo : 03 9762 2034 . . . inAUSTRALIA
