Chapter 27: Gas Chromatography
Principles
Instrumentation
Detectors
Columns and Stationary Phases
Applications
GC-MS Schematic
Interface less critical for capillary columns
Several types of Mass Specs available
- Rarely magnetic sector or time of flight
- Usually quadrapole or ion trap for GC-MS
- Less expensive
- Less maintenance
- Easy to use
- Normally use electron multiplier as detector
- All MS systems need ion source, either
electron impact or chemical ionization
Ion trap uses radio
frequency to trap
ions, hold or
store them, then
ejects them to
detector
Three modes of operation for GC-MS
1) Spectral mode look at mass spectrum
every second or so during chromatogram
- gives most information for research or
method development
2) Total ion current sum signal for all ions
as one large signal highest sensitivity
3) Selective ion monitoring look at certain
mass/charge ratios for compounds of
interest routine analysis
GC-FTIR
- Powerful technique for identifying compounds
- Use heated light pipe 1 to 3 mm dia
GC-FTIR
- Powerful technique for identifying compounds
- Use heated light pipe 1 to 3 mm dia and 10
to 40 cm long
- Heat to prevent condensation of sample
- Cool detector for sensitivity
- Gives structural information from spectrum
- Not very common
GC Columns & Stationary Phases
- Historically used packed columns
- Stationary phase coated as a thin
film on a high surface area solid support
- Theoretical studies showed that unpacked
columns with narrow diameters were better
- Open tubular columns first developed
- Capillary columns came later because
- Very fragile, difficult to construct, hard to
connect to GCs, small samples hard to detect,
difficult to coat column walls, etc.
Packed Columns
- Tubing of metal, glass, Teflon, etc.
- 2 to 3 m long and 2 to 4 mm in dia
- Packed with diatomaceous earth (SiO2), clay,
carbon particles, glass microbeads, polymer
- Diameter 150-250 m (60-100 mesh) 1 m2/g
- Thin coating of liquid stationary phase
- Can dissolve liquid in solvent, mix with support &
evaporate solvent
- Very tricky to do correctly
- Condition column at 5 oC above operating temp
Stationary Phase Coating
good
bad
Silica
Particle
Silica
Particle
Dont want stationary phase liquid coating to bleed
or puddle in column gives zone broadening &
poor resolution
Open Tubular Columns Capillary Columns
Column evolution
Three types
Wall Coated Open Tubular (WCOT) open
tube with coating on wall duh
Support Coated Open Tubular (SCOT)
open tube with particles of support material
stuck to the walls
Fused Silica Open Tubular (FSOT) WCOT
made of fused silica
Surface chemistry glass & silica
are SiO2 with -OH at surface
OH OH OH OH
-SiOSiOSiOSiOO
O
O
O
-SiOSiOSiOSiO-
Silanol
group
Silica
core
OH is a problem because it can adsorb polar
substances with strong affinity causing
peak tailing must deactivate by reacting
React Si-OH groups with silane
Sometimes still have OH groups
If silica not pure may have metal impurities M-OH
typically use high purity silica acid wash
Same chemistry to making specialty bonded phase
Liquid coatings on stationary
phase should exhibit:
1) Chemical inertness
2) Low volatility (b.p. 100 oC > max temp)
3) Thermal stability
4) Good solvent characteristics (i.e. k &
suitable)
Many different liquid coatings have been
used or attempted for GC, only about 10
have withstood the test of time
Retention time of a solute depends on K
(partition coefficient) which is dependent on
stationary phase must have different Ks for
different analytes
However, if Ks too large long retention time
if Ks too small short retention time
resulting in incomplete separation
In choosing a stationary phase use general
principles such as like dissolves like, polar
groups interact with polar groups, non polar
with non polar, etc.
Polar groups include CN, CO, OH
Polar analytes include alcohols, acids, amines
Non polar hydrocarbons
Where analyte & stationary phase match is
good elution order is determined by
boiling points
Bonded Stationary Phases
Use silylation chemistry to
covalently attach stationary
phase to solid support or column wall
Bonded Stationary Phases
Advantages
- monolayer coverage can be obtained
- reduced bleeding of stationary phase
- longer lasting
- better stability
- can be solvent washed
Chiral Stationary Phases separating stereoisomers is the ultimate in chromatography,
separate molecules that are mirror images
Predicting retention
I) Selectivity Factors
If B is a standard compound & we know , can
then be able to identify compound A even if we
change the the chromatographic conditions
or go to another chromatograph, etc.
This is limited to specific applications where a
database is available, not universally applicable
II) Retention Index (I)
Proposed by Kovats in 1968
Index based on normal alkanes
If have a mixture of 2 known alkanes & 1
unknown compound & the 2 knowns
bracket unknown in tR can then
determine I for unknown & identify it
I = 100 x # of carbon atoms
Regardless of column packing, temp. or
other conditions
Kovats Retention Index
Doesnt work as well for other types
of compounds (Hc), but useful in
some cases e.g. homologous series
Plot log adjusted retention time (tR = tR tM)
vs number of carbon atoms is linear
Useful in particular fields petroleum
industry, cosmetics, pharmaceuticals, etc.
since have their own unique standards
Note number of
carbons that
would be
calculated for
these 3
compounds
based on I
Chapter 28: High-Performance
Liquid Chromatography (HPLC)
Scope
Instrumentation eluants, injectors, columns
Modes of HPLC
Partition chromatography
Adsorption chromatography
Ion chromatography
Size exclusion chromatography
