CHAPTER 5CHAPTER 5GAS CHROMATOGRAPHYGAS CHROMATOGRAPHY

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.

• General Design of GC

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 columnprotect column from oxygen exposure at temperatureassist 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)

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

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

• 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).

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 GCsample 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.

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

5.1.3 Column• In general, two types of column in GC :

a. Open-tubular capillary columnb. 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.

•

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

•

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

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!

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.

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!

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

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!

• 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.

5.2 STATIONARY PHASE FOR GLC

• Column Stationary Phase :• Packed

Liquid coated with silica particle (<100 – 300 μm) in glass tubeBest for large scale but slow and inefficient.

• Capillary/ Open TubularWall-coated (WCOT)< 1 μm thick liquid on inside of silica tubeSupport-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.

• 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) :

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

• 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 analytesretained prefentially.

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 anddetermination of the components in a variety of sample types.

5.3.1 Qualitative Analysis

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

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.

• 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.