GAS CHROMATOGRAPHY

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GAS CHROMATOGRAPHY (GC)
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GAS CHROMATOGRAPHY
Sample is injected (using a syringe) into the
injection port. Sample vaporizes and is forced
into the column by the carrier gas ( mobile
phase which in GC is usually helium) Components
of the sample mixture interact with the
stationary phase so that different substances
take different amounts of time to elute from the
column. The separated components pass through a
detector. Electronic signals, collected over
time, are sent to the GC software, and a
chromatogram is generated.
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GAS CHROMATOGRAPHY
Compounds A and B interact with the stationary
phase through intermolecular forces (van der
Waals or dipole-dipole forces, including hydrogen
bonding). A interacts more strongly with the
stationary liquid phase and is retained relative
to B, which interacts weakly with the stationary
phase. Thus B spends more time in the gas phase
and advances more rapidly through the column and
has a shorter retention time than A. Typically,
components with similar polarity elute in order
of volatility. Thus alkanes elute in order of
increasing boiling points lower boiling alkanes
will have shorter retention times than higher
boiling alkanes.
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GC - Alkane Standards
Pentane
Isooctane
Gas Chromatograph of alkane standard mixture
containing equimolar amounts of n-hexane,
2,3-dimethylbutane, 3-methyl-pentane,
n-heptane, 3-methylhexane, n-octane,
3-methylheptane, 2,2,4-trimethyl-pentane
( isooctane), all dissolved in pentane.
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GC of Alkane Standards vs. distillation fraction
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Standards
Distillation Fraction 1
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GC Peak Areas and Resolution
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GC Isothermal vs Temperature Programming
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GC Example Chromatograms
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GC Packed vs. Capillary Columns
Alltech Chromatography catalog, 550
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GC Packed vs. Capillary Columns
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GC Stationary Phase
Experimental Organic Chemistry D. R. Palleros,
Wiley, NY, 2000
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GC Stationary Phase
Alltech Chromatography catalog, 550
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GC Elution order vs Stationary Phase
Alltech Chromatography catalog, 550
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GC - Derivatization

• Why is chemical derivatization needed?
• GC is best for separation of volatile compounds
  which are thermally stable.
• Not always applicable for compounds of high
  molecular weight or containing polar functional
  groups. These groups are difficult to analyze by
  GC either because they are not sufficiently
  volatile, tail badly, are too strongly attracted
  to the stationary phase, thermally unstable or
  even decomposed.
• Chemical derivatization prior to analysis is
  generally done to
• increase the volatility and decrease the polarity
  of compounds
• reduce thermal degradation of samples by
  increasing their thermal stability
• increase detector response
• improve separation and reduce tailing
• Derivatizing Reagents Common derivatization
  methods can be classified into 4 groups depending
  on the type of reaction applied
• Silylation
• Acylation
• Alkylation
• Esterification

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GC - Derivatization

• Derivatizing Reagents Common derivatization
  methods can be classified into 4 groups depending
  on the type of reaction applied
• Silylation
• Acylation
• Alkylation
• Esterification

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Alltech.com
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GC Resolution and Efficiency
Skoog and Leary Principals of Instrumental
Analysis, 4th ed. Suanders, 1992
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GC Resolution vs Column Efficiency (N, H)
H L / N
van Deemter Equation H A B/u (Cs Cm)u
Skoog and Leary Principals of Instrumental
Analysis, 5th ed. Suanders, 1998
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CHROMATOGRAPHY Preparative vs Resolution vs Speed
vs Expense