Quantitative Gas
Chromatography
Chem 2223 Lab Prep
1
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
2
Setup
4
Solutes and Internal Standard
Structure or Formula
Boiling Point, oC
Relative Polarity
CH3OH
64.6
Polar
Toluene
110.6
Nonpolar
Ethylbenzene
135.2
Nonpolar
p-Xylene
138.4
Nonpolar
156.0
Polar
Compound
Methanol
(solvent)
Br
Bromobenzene
(internal standard)
9
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.
11
Sample Chromatogram and Integration Report
IS
X
Rx / is 
Ax Ais
c x cis
AX = 27.01
AIS = 17.80
13
Calibration Curve with Internal Standard
Standards
GC Calibration Curve for Cocaine with Internal Standard
• Each contains fixed mass of internal
standard, various masses of std
analyte
• Calibration curve shows linear
response. Slope = response factor*
Standard
1
2
3
4
Ax Ais
c x cis
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
Int. Std.
mg/mL
5.00
5.00
5.00
5.00
cx/cis
0.500
1.000
2.000
5.000
Ax
120
241
480
1198
Aix
600
601
600
600
Ax/Ais
0.200
0.401
0.800
1.997
Cocaine with Interal Standard
1.0 microliter injections
2.500
y = 0.3991x + 0.0013
2.000
Ax/Ais
Rx / is 
Cocaine
mg/mL
2.50
5.00
10.00
25.00
1.500
1.000
0.500
0.000
0.000
1.000
2.000
3.000
4.000
5.000
6.000
cx/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.
Rx / is 
Ax Ais  ( y intercept)
c x cis
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