Presentation on theme: "Quantitative Gas Chromatography"— Presentation transcript:
1 Quantitative Gas Chromatography Chem 2223 Lab Prep
2 Goals and Objectives Goals Specific Objectives To become familiar with basic methods of quantitative analysis by gas chromatographySpecific ObjectivesUse the standard additions technique to determine the identities and concentrations of the components in a mixture of volatile organic compounds
3 Agilent 6850 Gas Chromatograph Top view showing injection port
6 A Capillary Column for GLC A thin layer of nonvolatile stationary phase is coating on the inner wall of the tubing (WCOT)Polydimethylsiloxane (silicone)Good for retaining and separating nonpolar solutes by boiling pointFused silica tubingdc 0.3 mmdf 1 mL = m
7 GC Analysis Conditions InletSplit mode, 250 C. Split ratio: 5:1Sample injection volume0.5 LCarrier gasHeliumColumnZB-5. Poly(dimethylsiloxane) with 5% phenyl substitution. 15 m long x 0.32 mm i.d., 1.0 m film thickness.Oven Temperatures (C) and timesInitial: (4.50 min)Ramp: 45.0/minFinal: (1.00 min)DetectorThermal conductivity, 300 CSignalDate rate: 20 Hz
9 Solutes and Internal Standard CompoundStructure or FormulaBoiling Point, oCRelative PolarityMethanol(solvent)CH3OH64.6PolarToluene110.6NonpolarEthylbenzene135.2p-Xylene138.4Bromobenzene(internal standard)156.0
11 Internal Standard Method DescriptionIn 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.
12 Internal Standard Method DescriptionIn 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. It is measured by injecting a mixture containing known amounts of analyte and standard. For some analyte X, the response factor isAx, Ais are the peak areas for the analyte and internal standardmx, mis are the masses of analyte and internal standard injectedApplicationIn the analysis of an unknown sample, a known amount of internal standard is added to the sample prior to injection.In practice, one should prepare a series of standards at various mx/mis. A plot of Ax/Ais vs mx/mis should be linear with slope = RF. You measure Ax/Ais from the chromatogram and calculate mx/mis for the unknown from the regression fit. Since mis is known, mx can be caluclated.
14 Calibration Curve with Internal Standard StandardsEach contains fixed mass of internal standard, various masses of std analyteCalibration curve shows linear response. Slope = response factor*UnknownAdd known amount of internal standardInject and measure Ax/AisDetermine cx/cis for your unknown from calibration curve. Since cis is known, cx for your unknown is simplycx = (cx/cis)cis*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.