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1
Quantitative Gas Chromatography
Chem 2223 Lab Prep
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Goals and Objectives
• Goals– To become familiar with basic methods of
quantitative analysis by gas chromatography
• Specific Objectives– Use the standard additions technique to determine
the identities and concentrations of the components in a mixture of volatile organic compounds
4
Setup
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Compound Structure or Formula Boiling Point, oC Relative Polarity
Methanol(solvent)
CH3OH 64.6 Polar
Toluene 110.6 Nonpolar
Ethylbenzene 135.2 Nonpolar
p-Xylene 138.4 Nonpolar
Bromobenzene(internal standard)
156.0 Polar
Solutes and Internal Standard
Br
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Internal Standard Method• Description
– In this approach, an internal standard is added to the sample, and the response from the analyte peak is compared to the internal standard. This approach corrects for minor variations in the injection volume.
• Response Factor (RF)– The response factor accounts for differences in the
detector response between the analyte and standard.
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Sample Chromatogram and Integration Report
ISX
AIS = 17.80
AX = 27.01
isx
isxisx cc
AAR /
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Calibration Curve with Internal Standard
Standards• Each contains fixed mass of internal
standard, various masses of std analyte
• Calibration curve shows linear response. Slope = response factor*
Unknown• Add known amount of internal standard• Inject and measure Ax/Ais
• Determine cx/cis for your unknown from calibration curve. Since cis is known, cx for your unknown is simply
cx = (cx/cis)cis
isx
isxisx cc
AAR /
*This expression for the response factor is not used directly in your calculations. The following expression which accounts for the intercept is more rigorous (in practice the intercept is very near zero). Calculations based on the calibration data do take the intercept into account.
/( intercept)x is
x isx is
A A yR
c c