Organic Chemistry.

1. CHAPTER 5CHAPTER 5
GAS CHROMATOGRAPHYGAS CHROMATOGRAPHY

2. 5.1 APPARATUS
• Gas Chromatography as it is usually performed is correctly called
gas-liquid chromatography :
» The analyte is in the gas phase in the GC and partitions between
the mobile phase (carrier gas) and the liquid stationary phase that
is coated on the inside of an open-tubular capillary column or on
particles inside a packed column.

3. • General Design of GC

5. 5.1.1 Carrier Gas Supply
• Mobile phase in GC is called ‘carrier gas’.
• Flow rates are controlled by a two-stage pressure regulator at the
gas cylinder and pressure regulator/flow regulator.
• Usually “inert” gases (don’t react with analytes except sometimes
in the detector).
• Purpose :
sweep sample through the column
protect column from oxygen exposure at temperature
assist with function of the detector
• Most common :
Helium (available relatively pure without extensive
purification after it leaves a compressed gas cylinder)
Nitrogen (usually requires an oxygen and water trap)

6. Hydrogen : normally used only with flame ionization detectors
(FID) since the FID needs it as fuel for the flame still rarely used
due to safety concerns (and chromatographic ones).
• Flow rates are controlled by a two-stage pressure regulator at the
gas cylinder and pressure regulator/flow regulator.
• Inlet pressure : 10 – 50 psig
• Flowrate :
a. packed column » 25 – 150 mL/min
b. open tubular column » 1 – 25 mL/min

7. 5.1.2 Sample Injection System
• The injector is a hollow, heated, glass-lined cylinder where the
sample is introduced into the GC.
• The temperature of the injector is controlled so that all components
in the sample will be vaporized.
• Sample injection » direct injection into heated port (>Toven) using
microsyringe :
a. 1 – 20 μL packed column
b. 10-3 μL capillary column

10. • Split Injection :
Routine method.
0.1 – 1.0% sample to column.
Reminder to waste.
Not good for analytes with a wide range of boiling points
some may be swept out the split vent before they are volatilized.
• Splitless Injection :
All sample to column.
Good for quantitative analysis.
Sample is vaporized in the injector itself and ALL of the sample
is swept onto the column by the carrier gas.
Again, relatively small samples are injected (10 μ L or less in
capillary GC).

11. Sample spends a large amount of time in the injector
Best for trace (1 – 100 ppm range) concentrations of high
boiling point analytes in low boiling point solvents » extra time in
the injector helps volatilize the analytes.
• On-Column Injection :
Sample that decomposes above boiling point » no heated
injection port.
Column at low temp to condense sample in narrow band.
Heating of column starts chromatography.
Used widely in packed-column GC, less in capillary GC
sample is deposited directly on the column.
• Good for thermally unstable compounds.
• Good for quantitative analysis at low concentrations » all
sample is available to travel to the detector.

12. • In general, BUT, it can inject only a relatively small amount of
sample in capillary GC anyhow.

14. 5.1.3 Column
• In general, two types of column in GC :
a. Open-tubular capillary column
b. Packed column
• GC Columns are :
i. varied in length from less than 2 m to 50 m or more.
ii. constructed of stainless steel, glass, fused silica or teflon.
iii. fit into oven for thermostating, they are
usually formed into coils having
diameters of 10 to 30 cm.

15. • Packed Columns
Greater sample capacity
Lower cost
More rugged
Most common in process labs or separating/determining major
components in a sample (prep GC)
Limited lengths reduces R and N
Not compatible with some GC detectors

17. • Open-tubular (capillary) columns
Higher resolution (R)
Greater HETP and N
Shorter analysis time
Greater sensitivity
Most common in analytical laboratory GC instruments
Smaller sample capacity
Higher cost/column
Columns more susceptible to damage

20. 5.1.4 Column Thermostating
• The “simplest” way to alter the separation in GC is to alter the
temperature program in the oven.
• The pressure of the carrier gas can be altered, but this is less
common (much).
• Isothermal = constant temp, Gradient = varied temperature.
• In general, the temp programming : as column temp raised » vapor
pressure analyte increases, eluted faster.
• Thus, raising the column temp during separation » can separate
species with wide range of polarities or vapor pressures.
• By altering the temperature, the rate of the reaction for any
analyte can be varied :
• they spend more or less time in the stationary phase
• the greater the difference in the times between analytes, the
better the separation!

23. 5.1.5 Detector
• Need :
Sensitive (10-8 – 10-15 g solute/s).
Operate at high Temperature (0 – 400°C).
Stable and reproducible.
Linear response.
• Desired :
Wide dynamic range.
Fast response.
Simple (reliable)
Nondestructive.
Uniform response to all analytes.

25. 5.1.5(a) Thermal Conductivity Detectors
• The carrier gas has a known thermal conductivity.
• As the thermal conductivity of the column eluent (gas flow in)
changes, the resistance of the filament changes.
• The presence of analyte molecules in the carrier gas alter the
thermal conductivity of the gas (usually He)
• There is normally a second filament to act as a reference (the carrier
gas is split)
• Increased sensitivity with decreasing temperature (detector), flow
rate and applied current.
• Filaments will burn out (oxidized) in the presence of oxygen if hot!

26. • This kind of reactors are :
Rugged.
Wide dynamic range (105).
Nondestructive.
Insensitive (10-8 g/s) » non-uniform.

28. 5.1.5 (b) Flame Ionization Detector
• Destructive, sample lost.
• Analytes containing C burn in a hydrogen-oxygen flame and produce
ions.
• CHO+ ions are collected on a cathode and the current they produce
results in the signal.
• WILL NOT detect non-C containing compounds!
• Requires H2 supply (tank or generator) and O2 supply (compressed
air).
• H2 carrier gas can be used, eliminating the need for a supply for the
detector.
• A make up gas can also be required!

29. • This kind of reactors are :
Rugged.
Wide dynamic range (107).
Sensitive (10-13 g/s).
Signal depends on no of C atom in organic analyte » mass
sensitive not concentration sensitive.
Weakly sensitive to carbonyl, amine, alcohol, amine groups.
Not sensitive to non-combustible » H2O, CO2, SO2, NOx.
Destructive.

33. 5.2 STATIONARY PHASE FOR GLC
• Column Stationary Phase :
• Packed
Liquid coated with silica particle (<100 – 300 μm) in glass tube
Best for large scale but slow and inefficient.
• Capillary/ Open Tubular
Wall-coated (WCOT)< 1 μm thick liquid on inside of silica tube
Support-coated (SCOT) 30 μm m thick coating of liquid-coated
support on inside of
silica tube.
Best for speed and efficiency
but only small samples.

39. • Immobilized Liquid Stationary Phase :
Low volatility.
High decomposition temperature.
Chemically inert (reversible interactions with solvent).
Chemically attached to support (prevent “bleeding”).
Appropriate k and α for good resolution
• Many based on polysiloxanes or polyethylene glycol (PEG) :

40. • Some Common Stationary Phases for Gas-Liquid Chromatography
(GLC):

42. • Stationary phase usually bonded and/or cross-linked
Bonding – attach a monomolecular layer of the stationary
phase to the silica surface of the column by a chemical
reactions.
Cross-linking – polymerization reactions after bonding to join
individual stationary phase molecules.
• Non-polar stationary phases best for non-polar analytes » non-polar
analytes retained prefentially.
• Polar stationary phases best for polar analytes » polar analytes
retained prefentially.

46. 5.3 APPLICATIONS OF GAS-LIQUID
CHROMATOGRAPHY
• Gas-liquid chromatography is applicable to species that are
appreciably volatile and thermally stable at temperature up to a few
hundred degrees °C.
• Consequently, GC has been widely applied to the separation and
determination of the components in a variety of sample types.

47. 5.3.1 Qualitative Analysis

49. 5.3.2 Quantitative Analysis
• Quantitative GC is based on comparison of either the height or the
area of an analyte peak with that of one or more standards.
• Both of these parameters vary linearly with concentrations.
a. Analysis based on peak height
• Peak heights are more easily measured, however, for narrow peaks,
more accurately determination.
b. Analysis based on peak area
• Peak area is independent of the broadening effects. Peak area is a
more satisfactory analytical paramater than peak height

50. 5.3.3 Calibration With Standards
• The most straightforward method of quantitative gas
chromatography.
• Chromatograms for standards are obtained, peak heights or areas
are plotted as a function of concentration to obtain working curve.
• A plot of data should yield a straight line passing through the origin.
• Quantitative analysis are based on this plot.
5.3.4 Internal Standard Methods
• The highest precision for quantitative GC is obtained using this
method because the uncertainties introduced by sample injection,
flow rates and variations in column conditions are minimized.

51. • The ratio of analyte peak to internal-standard peak area (or height) is
used as analytical parameter.
• For this method to be successful, it is necessary that the internal
standard peak be well separated from the peaks of all other
components in the sample With suitable internal standard precisions
of 0.5% to 1% relative are reported.