1. Overview Determining Identity Quantitative Analysis
Gas Chromatography
Overview
Determining Identity
Quantitative Analysis

2. Gas Chromatography Gas Chromatography (GC)
A type of partition chromatography used to analyze very small quantities of volatile materials
Qualitative technique:
Can be used to identify compounds if standards are used
Quantitative technique:
Can be used to determine relative amounts of compounds present in a mixture

3. The Instrument heated Injection port computer detector column printer
Gas cylinder
Injection port
heated

4. Overview During gas chromatography:
The injected sample vaporizes and is carried onto the column by an inert carrier gas (mobile phase)
Nitrogen
Helium
The sample adsorbs on the stationary phase, a nonvolatile liquid.
Usually liquid polymers with a high BP:
Polydimethylsiloxane
Carbowax (polyethylene glycol)

5. Overview
As the mobile phase passes across the stationary phase, the compounds in the mixture partition between the stationary phase and the mobile phase.
Partitioning depends on:
Column temperature
Gas flow rate
Affinity of the components for the stationary phase

6. Overview
Partitioning between the stationary and mobile phases depends on both a compound’s:
affinity for the stationary phase and
its vapor pressure or boiling point.
In general, compounds with low boiling points (and high vapor pressures) spend more time in the mobile phase and elute from the column in a shorter amount of time than compounds with high boiling points.

7. Overview
When each component reaches the detector (OCCC’s instrument uses a TCD), the filament in the detector heats up, ultimately resulting in an electrical signal that is digitized and sent to the attached computer.
A chromatogram is generated.
A graph of the intensity of
the detector’s response as
a function of time

8. Determining Identity
The number of components present in a mixture is indicated by the number of peaks observed.
Under a very specific set of conditions, the retention time for a compound can be used to help identify a compound.
The amount of time from injection for a sample to reach the detector

9. Determining Identity
Retention time varies with changes in the identity of a compound and with experimental conditions including:
type and amount of stationary phase
length of column,
gas flow rate,
temperature profile
If the same experimental conditions are used, the retention time of standards can be used to identify the components of a mixture.
You must also have reason to expect that particular compound is present.

10. Determining Identity
Example: If an unknown must contain one or more of the following compounds and all standards and the unknown are analyzed under the same conditions, determine the identity of the compounds present in the unknown.
Standard A: retention time = 1.22 min
Standard B: retention time = 2.47 min
Standard C: retention time = 3.76 min
Standard D: retention time = 4.93 min
Unknown 17:
Peak 1: retention time = 2.45 min
Peak 2: retention time = 3.73 min

11. Determining Identity
Remember, more than one compound in the world can have the same retention time under the same conditions.
Therefore, simply matching the retention times of two peaks does not necessarily mean they are the same compound.
You must have reason to expect a particular compound is present or you must use additional data to verify the identity.

12. Quantitative Analysis
The relative amount of each component in a mixture can be determined by comparing the relative area under each peak in the chromatogram.
% A = area for peak A x 100
total area all peaks

13. Quantitative Analysis
Example: Calculate the % composition of each component present in the mixture.
Retention
Time (min)
Area Count %
Peak 1
2.15
125,000
Peak 2
3.64
150,000
Peak 3
4.88
250,000

14. Quantitative Analysis
Example: Calculate the % composition of each component present in the mixture.
Retention
Time (min)
Area Count %
Peak 1
2.15
125,000
23.8
Peak 2
3.64
150,000
28.6
Peak 3
4.88
250,000
47.6