Chapter 23:

Gas Chromatography

Mobile phase (carrier gas): gas (He, N2, H2)

- do not interact with analytes - only transport the analyte through the column

Analyte: volatile liquid or gas

Stationary phase:

- solid (GSC) or non-volatile liquid (GLC)

GSC (gas-solid adsorption chromatography)- semi-permanent retention of active or polar molecules- severe tailing of elution peaks

GLC (gas-liquid partition chromatography)

- non-volatile liquid is coated on the inside of the column or on a fine solid support- In 1955, the first commercial apparatus for GLC appeared on the market

Gas Chromatography

23-1. The separation process in gas chromatography

Temp of a sample injector port:50 oC above the b.p. of least volatile component of the sample rapidly evaporates

(thermostated)

(2-50 m)

The column should be hot enough to provide sufficient vapor pressure for analyte to be eluted in a reasonable time.

Open Tubular ColumnsThin coating: small C-term (decreased H) :

Compared with packed columns,OTC offers higher resolution, shorter analysis time, greater sensitivity, lower sample capacity

Length: 15-100 m

Open Tubular Columns

Because of increased surface area, support-coated columns can handle larger samplethan can wall-coated column

- Solid particles are active stationary phase

Porous-Layer Design in Stat. Phase

Vapors from a beer can

Effect of Column Length

The number of theoretical plates, N, on a column is proportional to the lengthResolution is proportional to (N)1/2 and therefore, the square root of column length

Effect of Stationary Phase Thickness

Liquid Sta. Phase

Choice of liquid phase for a given problem:“like dissolves like”

- Nonpolar columns: best for nonpolar solutes- Polar columns for polar solutes

- As a column ages,

stationary phases bakes off surface silanol groups (Si-OH) are exposed peak tailing (polar analyte)

Therefore, stationary phase is covalentlyattached to silica surface

Polar Stat. Phase:-CN, -CO, -OH, -ester Non-polar Stat. Phase: hydrocarbon

Chiral Separation

Chiral Separation

Separation of amino acid enatiomers

Chiral Separation: Cyclodextrin Stat. Phase

Packed Columns

Packed columns contain - a fine solid support itself- a fine solid support coated with nonvolatile liquid stationary- solid support: silanized diatomite to reduce hydrogen bonding to a polar solute

Packed columns- are made of stainless steel or glass- 3-6 mm diameter, 1-5 m in length

Packed columns are good for preparative separations

Uniform particle size decrease A term reduce H better resolution

Small particle size decreased time for solute equilibration improved column efficiency

(problem: higher pressure for carrier gas)

Chromatogram of alcohol mixture using packed column

In comparison to the chromatogram using OTC, broader peaks, longer retention times, and less resolution

Non-polar column Polar column (retention time: hydrocarbon<ketone<alcohol)

H-bonding

Dipole interaction

The Retention Index

Column oven temp = 70oC

The Retention Index

Retention index relates the retention time of a solute to the retention times of linear alkanes

Kovat retention index (I) for a linear alkane ≈ 100 times the number of carbon atoms

(e.g.) octane, I = 800, nonane, I = 900

Temperature ProgrammingTemp of column (oven) increases Solute vapor pressure increase decrease retention time

Isothermal at 150 oC

Temp programming:

50 – 250 oC at 8 oC/min

Precaution: at too high temp.thermal decomposition of analyte

Temperature Programming

Carrier Gas in GC

Carrier gas: chemically inert

Good resolution but slow analysis

React with alkeneat high temp& dangerous

H = A + B/U + CU

In GC, B term is important (Do of gas: 104 greater than liquid)

Carrier Gas in GC

Sample Injection in GCLiquid samples are injected into GC by syringe through a rubber septum into a heated portGaseous samples use gas-tight syringe

<Sample size>Packed column: sub L – 20 L, Capillary column: 10-3 L (split injection)

Spilt injection delivers only 0.2-2% of the sample to the column

Sample Injection in Capillary GC

Split injection: routine meansSplitless injection: best for trace analysisOn-column injection: best for thermally unstable solutes

Quantitative and Qualitative Analysis by GCQualitative analysis:

- retention time (GC-FID, TCD, ECD…): comparison with authentic sample - mass (GC-MS)

Quantitative analysis:

- peak area or peak height

Thermal Conductivity Detector (TCD)

- TCD responds to the changes in thermal conductivity of carrier gas stream (high conductivity) by the presence of analyte molecules (low conductivity)

- The carrier gas of choice: H2 or He (highest thermal conductivity)

Pt, W, or thermister

Temp of the filament depends on the thermal conductivity of surroundings

Thermal Conductivity Detector (TCD)

<Advantages of TCD>

- simple system- wide linear dynamic range (~ 104)- general response to organic and inorganic species- non-destructive

<Limitation of TCD>

- relatively low sensitivity

Flame Ionization Detector (FID)

- Most widely used and generally applicable detector- Column eluate is mixed with H2/air and then burned in flame.

CH + O CHO+ + e-

- Most organic compounds, when pyrolyzed at the temp of H2/air flame, produce ions (~1/105) and electrons that can conduct electricity through the flame

- FID responds to the # carbons entering the detectorper unit time

: mass sensitive rather than concentration-sensitive

- Functional groups (carbonyl, carboxyl, halogen) yield fewer ions

- FID is insensitive to noncombustible gases (H2O, CO2,O2, N2, SO2, and NOx)

Sulfur Chemiluminescence Detector

Sulfur compounds SO + productsSO + O3 SO2* + O2SO2* SO2 + light (blue)

H2-O2 flame

* very small amount of sulfur compounds<FI Detector>

<SC Detector>

GC-Mass Spectrometer

Mass spectrometer is a powerful detector for both qualitative and quantitative analysisof analyte in gas or liquid chromatography

Analyte: ionized Ionized analyte:

separated according to mass

Ions: detected

GC-Mass Spectrometer

GC-Mass SpectrometerIonization: electron impactMass analyzer: quadrupole mass analyzer compact, rugged, less expensive than magnetic sector, high scan rate ( <10 ms)

70 eV = 6.7 x 103 kJ/molTypical chemical bond: 200-600 kJ/mol

GC-Mass Spectrometer<Electron impact ionization>hard source much fragmentation

Electron Impact: good for structural informationChemical ionization: good for M.W. information

CH4 + e- CH4+ + 2e-

CH4+ + CH4 CH5

+ + CH3CH5

+ + M CH4 + MH+

<Chemical ionization>soft source, less fragmentation>

MW=226

GC-Mass Spectrometer

GC-Mass Spectrometer