1. No category

Chapter 27: Gas Chromatography
Sample is vaporized and injected onto column
• Retention Volume: accounts for pressure, temperature
Temperature Programming
• Retention time of the solute
• Resolution
• Peaks shape
• Useful when?
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MP: Carrier Gas System
• Doesn’t interact with analyte molecules
• Only function is to transport analyte through column
• Chemically Inert:
• Choice:
• System has pressure regulators, gauges, flow meter,
and molecular sieves (to remove water and impurities)
• Pi ~ 10 to 50 psi above room pressure
• Soap bubble flow meter
Sample Injection System
Microsyringe:
Flash vaporizer port at the column head:
Capillary columns:
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Columns
Contained in an thermostatted oven
Wall-Coated Open Tubular Columns (WCOT):
• Fused-Silica Open Tubular Columns (FSOT):
• Thin Walled:
• Flexible:
• Inside diameters:
• Support-Coated Open Tubular Columns (SCOT):
• More SP than WCOT
o
o
Packed Columns: not commonly used
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Stationary Phase
• Volatility:
• Stability:
• Inertness:
• Solvent Characteristics:
o
o
Examples:
• Polydimethylsiloxanes
R – Si – O -[- Si – O -]n- Si – R
• Polyethylene glycol
OH – CH2 – CH2 -[- O – CH2 – CH2 -]n- OH
•
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• Bonded columns:
- Attach monomolecular layer of SP to
column’s silica surface
O
O
• Film Thickness:
• Chiral SP
Detectors
1)
2)
3)
4)
5)
6)
7)
8)
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Thermal Conductivity Detector (TCD)
Measures ability of substance to transport
heat from a hot region to a cold region
• High thermal conductivity • Responds to changes in thermal conductivity
• Tungsten-rhenium filament gets hotter as gas stream
emerges from column
Advantages:
•
•
•
•
Disadvantages
•
•
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Flame Ionization Detector (FID)
• Most widely used and generally
applicable
• Effluent from column is
mixed with hydrogen & air
and ignited electrically
• “Most” organic compounds
pyrolyzed in this flame:
• Apply potential of a few
hundred volts to the burner tip
and a collector electrode above
the flame
o
• Mass-sensitive:
• Insensitive to carbonyls, alcohols, halogens,
amines, water, carbon dioxide, sulfur dioxide and
nitrogen oxides
o
Advantages:
Disadvantages:
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Electron-Capture Detector (ECD)
• Effluent from column
passes over a β–emitter
• Electron from the
emitter causes
ionization of the carrier
gas and production of a
burst of electrons
• In the absence of organic species:
• Current decreases in presence of organics
Advantages:
•
•
Disadvantages:
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Qualitative Analysis - Retention Index
• Use two normal alkanes that bracket unknown
o
o
o
⎛ log t ( x) − log t (n) ⎞
⎟⎟ + 100n
I = 100⎜⎜
⎝ log t ( N ) − log t (n) ⎠
'
'
R
R
'
'
R
R
• I = Kovats Index of unknown, x
• n = # of carbons atoms for the smaller alkane
• tr’(n) = adjusted retention time for smaller alkane
• tr’(N) = adjusted retention time for the larger alkane
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Kovats Index for Linear Alkane
Air
n-heptane
n-octane
Unknown
tr = 1.1 min
tr = 4.0 min tr’ = 2.9 min
tr = 6.5 min tr’ = 5.4 min
tr = 5.3 min tr’ = 4.2 min
Quantitative Analysis
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log tr’= 0.46
log tr’= 0.73
log tr’= 0.62