Quantitative Gas Chromatography Chem 2223 Lab Prep

Goals and Objectives Goals Specific Objectives 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

Agilent 6850 Gas Chromatograph Top view showing injection port

Setup

The Chromatogram tR

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 point Fused silica tubing dc  0.3 mm df  1 m L = 15 - 60 m

GC Analysis Conditions Inlet Split mode, 250 C. Split ratio: 5:1 Sample injection volume 0.5 L Carrier gas Helium Column ZB-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 Signal Date rate: 20 Hz

Sample Injection Rubber septum

Solutes and Internal Standard Compound Structure or Formula Boiling Point, oC Relative Polarity Methanol (solvent) CH3OH 64.6 Polar Toluene 110.6 Nonpolar Ethylbenzene 135.2 p-Xylene 138.4 Bromobenzene (internal standard) 156.0

Sample Chromatogram and Integration Report

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.

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 Ax, Ais are the peak areas for the analyte and internal standard mx, mis 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 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.

Sample Chromatogram and Integration Report IS X AX = 27.01 AIS = 17.80

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 *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.