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 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. Agilent 6850 Gas Chromatograph
Top view showing injection port

4. Setup

5. The Chromatogram tR

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

7. 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: (4.50 min)
Ramp: 45.0/min
Final: (1.00 min)
Detector
Thermal conductivity, 300 C
Signal
Date rate: 20 Hz

8. Sample Injection
Rubber septum

9. 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

10. Sample Chromatogram and Integration Report

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. 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.

13. Sample Chromatogram and Integration ReportIS X
AX = 27.01
AIS = 17.80

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 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.