ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 22

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 22
DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 22 GAS AND LIQUID CHROMATOGRAPHY
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GAS CHROMATOGRAPHY
- Mobile phase is a gas - Column is open
tubular - Sample is injected through a thin
rubber disc (septum) - Sample is vaporized upon
entering a heated glass port - A carrier gas
(He, N2, H2) carries vaporized sample to a
detector - Detector temperature is higher than
column temperature to keep solutes in the gaseous
state
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GAS CHROMATOGRAPHY
Liquid Sample Size 0.1 to 2 µL for analytical
chromatography 20 to 1000 µL for perspective
chromatography Gas Sample Size 0.5 to 10 mL
(gas tight syringe should be used)
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GAS CHROMATOGRAPHY
Open Tubular Column - Usually made of fused
silica (SiO2) - Liquid or solid stationary
phase is coated on the inner wall - Stationary
phase may be porous carbon
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GAS CHROMATOGRAPHY
Open Tubular Column - Polar column is usually
used for polar solutes - Nonpolar column is
usually used for nonpolar solutes - Tailing is
seen when stationary phase bakes and SiOH
groups (silanol) forms on the silica surface
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GAS CHROMATOGRAPHY
Nonpolar Stationary Phase - Solutes are eluted in
order of increasing boiling point - Solutes with
higher vapor pressure are eluted faster Polar
Stationary Phase - Solutes are separated based on
polarity - Less polar solutes are eluted faster
than strongly polar solutes
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GAS CHROMATOGRAPHY
Molecular Sieves - Included in the stationary
phase - Has cavities and made of inorganic
materials - Used to dry gaseous solutes -
Strongly retains H2O - Separates other small
molecules (CH4, H2, O2, N2, CO2)
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GAS CHROMATOGRAPHY
Guard Column - Collects nonvolatile solutes that
are not eluted - Attached to the front of a
chromatography column 5 to 10 meters long - Has
no stationary phase and is silanized - Ends are
cut off with time to discard nonvolatile solute
buildup
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GAS CHROMATOGRAPHY
Comparing Open Tubular Column to Packed Column -
Gives better separation - Narrower peaks -
Handles smaller samples (analytical
chromatography)
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GAS CHROMATOGRAPHY
Effect of Column Temperature Increase in column
temperature - Increases solute vapor pressure -
Decreases retention time - Results in sharp
peaks Temperature Programming - Used to separate
compounds with a wide range of boiling points
and polarities
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GAS CHROMATOGRAPHY
Sample Injection - Sample is injected through a
thin rubber disc (septum) heated glass port where
it is vaporized
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GAS CHROMATOGRAPHY
Split Injection - Good for open tubular
columns - Complete injection may be too much for
an open tubular column - 0.1 to 10 of the
injected sample reaches the column - Not good
for quantitative analysis (higher boiling point
components may not be vaporized)
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GAS CHROMATOGRAPHY
Splitless Injection - Suitable for quantitative
analysis - Suitable for analysis of low
concentrations of solutes (trace componenets) -
Dilute solution with low-boiling solvent is
used - About 80 of the injected sample reaches
the column
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GAS CHROMATOGRAPHY
Solvent Trapping - Sample is initially injected
at about 40o below boiling point of sample - A
thin band of solute is trapped - Column
temperature is later raised
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GAS CHROMATOGRAPHY
Cold Trapping - Used for high-boiling solutes -
Sample is initially injected at about 150o below
boiling point of solutes of interest - Solvent
and low-boiling solutes are eluted -
High-boiling solutes are trapped in a narrow
band - Column temperature is later raised
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GAS CHROMATOGRAPHY
On-column Injection - Sample is injected
directly into the column - Used for compounds
that decompose at temperatures above their
boiling points - Solvent trapping or cold
trapping is employed to trap a narrow band of
analyte - Column temperature is increased
afterwards to initiate chromatography
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GAS CHROMATOGRAPHY
Detectors Mass Spectrometer - Very sensitive and
the most versatile Flame Ionization - Low
detection limits Thermal Conductivity - Not
sensitive to narrow columns (diameter lt 0.53 mm)
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GAS CHROMATOGRAPHY
Detectors Electron Capture - Very sensitive to
halogen-containing compounds - Insensitive to
ketones, alcohols, and HCs Sulfur
Chemiluminescence Alkali Flame - Selectively
sensitive to nitrogen and phosphorus - Used for
analysis of drugs Flame Photometric
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LIQUID CHROMATOGRAPHY
Stationary phase - Usually silica (SiO2
xH2O) - Alumina (Al2O3 xH2O) - Adsorption of
water slowly deactivates adsorption sites of
silica - May be reactivated by heating to about
200oC in an oven
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LIQUID CHROMATOGRAPHY
Eluent Strength (eo) - A measure of adsorption
energy of solvent - eo of pentane is 0 - More
polar solvents have greater eluent strengths -
Solutes elute more rapidly when eluent strength
is greater
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LIQUID CHROMATOGRAPHY
Eluent Strength (eo) - Weakly retained solutes
are first eluted with a low eluent strength
solvent - Eluent strength is increased by adding
a polar solvent to elute strongly retained
solutes - Eluent strength is increased by making
mobile phase more like the stationary phase
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HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
- Most common LC - Uses closed columns under
high pressure - Resolution increases with
decreasing particle size of stationary phase -
Decreasing particle size decreases plate
height - Small particle size reduces the terms A
and C in the van Deemter equation
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HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Stationary Phase - Microporous particles of
silica - Octadecyl (C18)
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HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Norman-Phase Chromatrography - Stationary phase
is polar - Solvent is less polar - Eluent
strength is increased by adding a more polar
solvent
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HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Reversed-Phase Chromatrography - More common -
Stationary phase is weakly polar or nonpolar -
Solvent is more polar - Eluent strength is
increased by adding a less polar solvent -
Insensitive to polar impurities and eliminates
tailing
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HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Column - Irreversible adsorption to the columns
is very common - Disposable guard column
containing the same stationary phase is attached
to the column entrance
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HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Isocratic Elution - Elution with a single
solvent - Elution with a constant solvent
mixture Gradient Elution - Solvent is changed
continuously from weak to strong eluent
strength - Used when the solvent does not
efficiently elute all components
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HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
Detectors - Ultraviolet (most common) -
Electrochemical - Refractive index -
Evaporative light-scattering - Charged aerosol
(most sensitive)