1. Experiments in Analytical Chemistry
-Gas Chromatographic analysis of a gas mixture

2. Types of chromatography
Adsorption chromatography: solute is adsorbed on the surface of solid particles
Partition chromatography: solute equilibrates between the stationary liquid and mobile phase
Ion-exchange chromatography: solute ions of the opposite charge are attracted to the stationary phase
Molecular exclusion chromatography: the larger solutes pass through most quickly
Affinity chromatography: one kind of molecule in complex mixture becomes attached to molecule that is covalently bound to stationary phase

3. Basic equations of chromatography
Relative retention
Retention factor
Resolution
Plate height
s = standard deviation of band
Number of plates on column
Another equation for resolution
g = unadjusted relative retention

4. Plate height equation Van Deemter equation Longitudinal diffusion
A: multiple paths
B: longitudinal diffusion
C: equilibration time
Longitudinal diffusion
Equilibration time
Hmass transfer = Cux = (Cs + Cm)ux
Cs: mass transfer through stationary phase
Cm: mass transfer through mobile phase

5. Gas chromatography
Schematic diagram of a gas chromatography

6. Open tubular columns
(a) Typical dimensions of open tubular gas chromatography column. (b) Fused-silica column with a cage diameter of 0.2 m and column length of m (c) Cross-sectional view of wall-coated, support-coated, and porous-layer columns

7. Gas chromatography detectors
1. Thermal conductivity detector
105 linearity
H2 and He give lowest detection limit
Sensitivity increases with
Increasing filament current
Decreasing flow rate
Lower detector block temperature

8. 2. Flame ionization detector
N2 gives best detection limit
Signal proportional to number of susceptible carbon atoms
100-fold better detection than thermal conductivity
107 linear response range
3. Electron capture detector
Particularly sensitive to halogen-containing molecules, conjugated carbonyls, nitriles, nitro compounds, and organometallics
Carrier: either N2 or 5% methane
Gas entering the detector is ionized by high energy electrons

9. Experiments
Purpose
Students are expected to learn the principles of gas chromatography (GC) by practicing analysis of a mixture in which several hydrocarbons are present. They will learn how to interpret gas chromatogram to obtain important parameters such as retention time, capacity factor, plate height, and number of plates to evaluate a GC column.
GC involves injection of a small amount of sample into a moving stream of gas, which is termed the mobile phase or the carrier gas. The sample is carried by the gas stream through a column that consists of a tube packed with solid particles called the stationary phase. Separation of a sample mixture into its individual components is achieved if the components are retained in the column to different extents.
In the elution method of chromatography, a carrier gas more lightly adsorbed than any component in the sample is passed through the column into which a sample has been introduced. Each component of the sample will then partition between the stationary phase and the gas phase, and that in the gaseous phase will be move by the flow of carrier gas. As a result, the sample will be carried through the column in a definite time (the retention time), which depends on the affinity of the column packing material for that component, the temperature, and the flow rate for the carrier gas. Figure 1 is a graph that shows the detector response as a function of time

11. Column: 3% OV-1 Chromosorb W-HP, 6ft Injector: maintained at 150 ℃
Procedure
You will be given a liquid mixture of C5-C8 (n-pentane, n-hexane, n-heptane, n-octane). Before injecting the sample into the column, it is better to evaporate it.
Measure Use a syringe to inject ca. 0.1 mL liquid sample to the Tedlar bag and put it in the oven that is already at ca. 120 ℃. At this temperature all the components will be evaporated.
Use a gas-tight syringe to take ca. 500 L of a gas sample. Inject it to the injection port of a GC.
Follow the RA’s instruction to operate the GC. The following conditions are proven adequate.
Column: 3% OV-1 Chromosorb W-HP, 6ft
Injector: maintained at 150 ℃
Carrier gas: Ar
Flow rate: 20 mL/min
Detector: Thermal conductivity detector maintained at 150 ℃, current: 100 mA
Reference gas: Ar, 20 mL/min
Column oven temperature programming: 40 ℃ (3min)  10V/min  60 ℃
Approximate analysis time: 5 min

12. Report
Submit the chromatogram and identify each peak.
Measure adjusted retention time, capacity factor, plate height, and plate number for each peak. Use the air peak to measure tm.
Compare the peak areas for each component. What conclusions can be drawn?