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

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

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

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

Gas chromatography Schematic diagram of a gas chromatography

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 15-100 m (c) Cross-sectional view of wall-coated, support-coated, and porous-layer columns

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

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

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

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

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?