Chapter 23: Gas...
Transcript of Chapter 23: Gas...
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