GC

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Chapter 22

• GC LC

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22.1 Gas Chromatography

1. Schematic diagram

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22.1 Gas Chromatography

1. Columns open tubular columns

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22.1 Gas Chromatography

• m.p.(gas) - s.p.
• s.p. solid(using adsorption)ex SiO2
• column ages Si-O-H cause tailing peak.
• 2) s.p. liquid(GLC, using partition)
• a range of polarities (Table 22-1),
  like dissolves like
• Decrease thickness of stationary phase leads to
• Resolution? (H?)
• tr?
• Sample capacity?

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22.1 Gas Chromatography

• B) The effects of column polarity on separation

Like dissolves like (a) S.P nonpolar, b.p.
dependent (b) S.P polar
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How changing the S.P. can affect separation
Figure 22-4 Resolution of trans fatty acids in
hydrogenated food oil improves when the
stationary phase is changed from DB-23 to HP-88
(aryl group)
P.484
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22.1 Gas Chromatography

• C) Common solid s.p.
• a) Porous carbon larger molecules bind more
  tightly than small ones, flexible molecules bind
  more than rigid ones
• b) Molecular sieves inorganic materials with
  nanometer-size cavities that retain separate
  small molecules H2, O2, N2, CO2, CH4. (Fig.
  22-5)
• c) Guard column
• Collect nonvolatile components that would
  otherwise be injected into a column and never be
  eluted.

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22.1 Gas Chromatography

• packed column vs. open tubular column

higher resolution lower sample capacity
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22.1 Gas Chromatography

• Temperature programming
• ? temp of column ? ? v.p. solute,
• ? ? tr
• ? sharpens peaks
• isothermal constant temp.
• temp. programming (gradient)
• raise the column temp. during the separation.

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22.1 Gas Chromatography -9
Figure 22-6 (a) Isothermal and (b) programmed
temperature chromatography of linear alkanes
through a packed column with a nonpolar
stationary phase.
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22.1 Gas Chromatography

• 4. Carrier Gas

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22.1 Gas Chromatography
5. Sample Injection 1) gasses, liquids, or solids
? vaporized, not decomposition 2) injection
time ? ? bands broader 3) injected by syringe
(manual or automatic injection)
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22.1 Gas Chromatography
Figure 22-7 Injection port operation for (a)
split, (b) splitless, and (c) on-column injection
into an open tubular column.
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22.1 Gas Chromatography

• split injection (350?) (only 0.1-10 sample)
• Routine method
• concentrated sample
• high resolution
• dirty samples
• could cause thermal decomposition
• splitless injection (220?) (80)
• For quantitative analysis and for analysis of
  trace components of mixture
• high resolution
• solvent trapping (Tsolvent lt 40?) for dilute
  sample
• cold trapping (Tsolute lt 150?) for high-boiling
  solutes
• on-column injection (50?) (100)
• best for thermally unstable solutes.

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22.1 Gas Chromatography

• 5. Detectors
• Qualitative analysis mass spectrometer, IR
• Quantitative analysis area of a chromatographic
  peak.

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22.1 Gas Chromatography

• a) Thermal conductivity detector
• -most general way
• -responds to everything
• -not sensitive enough for high resolution.
• b) Flame ionization detector
• -most popular
• -mainly responds hydrocarbons (C-H)
• c) Electron capture detector
• -for compounds containing atoms with high
  electron affinities.
• -sensitive for halogen, CO, NOx, orgaometallic
  compounds.

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22.1 Gas Chromatography

• d) Mass Spectrometric Detection and Selected
  Reaction Monitoring
• - A mass spectrometer is the single most
  versatile detector.
• - Total Ion Chromatogram (TIC)
• - selected ion monitoring (SIM) at on value of
  m/z
• - selected reaction monitoring (SRM) tandem
  mass
• MS/MS
• - Multiple reaction monitoring (MRM)

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QQQ Mass Spectrometer
Precursor ion (parent ion) vs. Product ions
(daughter ion) Solid phase extraction (SPE)
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Caffeine as example
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Caffeine (13C) as an internal standard
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22.2 Liquid Chromatography

• 1. open, gravity-feed column
• 2. closed column (under high pressure) packed
  with micron-size particles. (HPLC)
• 3. stationary phase
• a. adsorption silica (SiO2?xH2O), alumina
  (Al2O3?xH2O),
• b. molecular exclusion,
• c. ion-exchange, ?affinity

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22.2 Liquid Chromatography
?compete with ? for binding on s.p.
the more strongly bind to s.p.??eluent
strength ?
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22.2 Liquid Chromatography

• 4. Eluent strength Table 22.2
• The more polar solvent
• ? ? eluent strength
• ? ? tr
• 5. Gradient elution increased the eluent
  strength during the separation in liquid
  chromatography.

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22.3 High-Performance Liquid Chromatography (HPLC)
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22.3 High-Performance Liquid Chromatography (HPLC)

• 1. Through a closed column, and needs high
  pressure.
• 2. s.p. particles size ?
• microporous particles of silica
• with diameters of 1.5-10 um
• s.p. ? m.p. faster,
• i.e. C? in van Deemter eqn.
• ? resolution ?

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22.3 HPLC
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22.3 HPLC

• 3. Stationary phase
• a) Normal-phase chromatography polar s.p. and
  less polar solvent. Eluent strength is increased
  by adding a more polar solvent.
• b) Reversed-phase chromatography low-polarity
  s.p. and polar solvent. Eluent strength is
  increased by adding a less polar solvent.

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22.3 HPLC

• c) Bonded stationary phase.
• polar vs. nonpolar
• d) Optical isomers
• D- L-amino acids
• for drug industry

see p.494 for R polar or nonpolar
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22.3 HPLC

• d) Optical isomers separation
• ex for ant-inflammatory drug Naproxen

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22.3 HPLC

• 4. Column
• Guard column
• Injection valve

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22.3 HPLC

• 5. Solvents
• a) Isocratic elution
• elution with single solvent or a constant
  solvent mixture
• b) Gradient elution
• solvent is changed continuously from a weak
  eluent strength to a strong eluent strength by
  mixing more and more of a strong solvent to a
  weak solvent during the chromatography.

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22.3 HPLC

• Figure 22-20 Isocratic HPLC
• separation of a mixture of aromatic
• compounds at 1.0 mL/min on a
• 0.4625 cm Hypersil ODS column
• (C18 on 5-µm silica) at ambient
• temperature (22?)
• benzyl alcohol
• phenol
• 3, 4-dimethoxyacetopheneone
• benzoin
• ethyl benzoate
• toluene
• 2,6-dimethoxytoluene
• o-methoxybiphenyl.

A KH2PO4(aq) B CH3CN(l)
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22.3 HPLC

• The gradient can be used to resolve all peaks
  by reducing the time from 2 h to 38 min.

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Detectors - Ultraviolet detector -
Electrochemical detector redox reaction -
Fluorescence detector LC-MS - ESI
(Electrospray ionization) - APCI (atmospheric
pressure chemical ionization)
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Figure 22-23 Atmospheric pressure chemical
ionization interface between liquid
chromatography column and mass spectrometer.
P.500