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1 Quantitative Gas Chromatography Chem 2223 Lab Prep.

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Presentation on theme: "1 Quantitative Gas Chromatography Chem 2223 Lab Prep."— Presentation transcript:

1 1 Quantitative Gas Chromatography Chem 2223 Lab Prep

2 2 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

3 3 Agilent 6850 Gas Chromatograph Top view showing injection port

4 4 Setup

5 5 The Chromatogram tRtR

6 6 A Capillary Column for GLC Fused silica tubing d c 0.3 mm d f 1 m L = 15 - 60 m Polydimethylsiloxane (silicone) Good for retaining and separating nonpolar solutes by boiling point A thin layer of nonvolatile stationary phase is coating on the inner wall of the tubing (WCOT)

7 7 GC Analysis Conditions Inlet Split mode, 250 C. Split ratio: 5:1 Sample injection volume 0.5 L Carrier gasHelium ColumnZB-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 times Initial: 40.0 (4.50 min) Ramp: 45.0/min Final: 120.0 (1.00 min) Detector Thermal conductivity, 300 C SignalDate rate: 20 Hz

8 8 Sample Injection Rubber septum

9 9 CompoundStructure or FormulaBoiling Point, o CRelative Polarity Methanol (solvent) CH 3 OH64.6Polar Toluene110.6Nonpolar Ethylbenzene135.2Nonpolar p-Xylene138.4Nonpolar Bromobenzene (internal standard) 156.0Polar Solutes and Internal Standard

10 10 Sample Chromatogram and Integration Report

11 11 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.

12 12 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. It is measured by injecting a mixture containing known amounts of analyte and standard. For some analyte X, the response factor is –A x, A is are the peak areas for the analyte and internal standard –m x, m is are the masses of analyte and internal standard injected Application –In 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 m x /m is. A plot of A x /A is vs m x /m is should be linear with slope = RF. You measure A x /A is from the chromatogram and calculate m x /m is for the unknown from the regression fit. Since m is is known, m x can be caluclated.

13 13 Sample Chromatogram and Integration Report IS X A IS = 17.80 A X = 27.01

14 14 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 A x /A is Determine c x /c is for your unknown from calibration curve. Since c is is known, c x for your unknown is simply c x = (c x /c is )c is *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.


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