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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

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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