Gas Chromatography
Schematic Diagram of a Gas
Chromatograph
Open Tubular Columns
• The vast majority of analyses use long
narrow open tubular columns made of fused
silica (SiO2) and coated with polyimide
Porous-layer open tubular column (porous carbon)
Chromatogram of vapors from the headspace
of a beer can on a porous carbon column
Capillary Columns
• Inner diameters are typically 0.10 to 0.53
mm and lengths are 15 to 100 m
• Narrow columns provide higher resolution
but require higher operating pressure and
have less sample capacity
Effect of OT column inner diameter on Rs
N
Resolution Increases in Proportion to N
Effect of Thickness of Stationary
Phase on Resolution
• Increasing the thickness of the stationary
phase increases retention time, sample
capacity and resolution of early peaks
(k’ ≤ 5)
• Thick films of stationary phase can shield
analytes from the silica surface and reduce
tailing but can also increase bleed of the
stationary phase at elevated temperature
Effect of Thickness of Stationary
Phase on Resolution (cont.)
• A thickness of 0.25 μm is standard, but
thicker films are used for volatile analytes
Effect of Stationary Phase Thickness on OT
Column Performance
The Choice of Liquid Stationary
Phase
• Based on the rule “like dissolves like”:
nonpolar columns are best for nonpolar
solutes and strongly polar columns are best
for strongly polar solutes
• To reduce the column bleed, it is usually
bonded (covalently) to the silica surface or
covalently cross-linked to itself
Chiral Phases for Separating Optical
Isomers
Enantiomers of an amino acid
Volatile derivative for gas chromatography
Structure of β-cyclodextrin, a cyclic sugar
made of seven glucose molecules
Primary –OH groups lie on one face and the secondary –OH groups
lie on the other face. The hydroxyls are capped with alkyl groups
to decrease the polarity of the faces
Chlorinated pesticide impurity separated on the chiral stationary phase
Chiral separation on a 25-m x 0.25 mm OT column; 0.25-μm phase:
10% methylated β-cyclodextrin chemically bonded to PDMS
Adsorbents Used for PLOT Columns
•
•
•
•
•
Alumina
Silica gel
Porous polymers
Graphitized carbon blacks
Molecular sieves
– Inorganic (zeolites)
– Organic (carbon)
Structure of the Zeolite Molecular
Sieve
Packed Columns
• Provide greater sample capacity, but give
– Broader peaks
– Longer retention times
– Less resolution
• Typically 3-6 mm in diameter and 1-5 m in
length
Alcohols separated on a 2 mm x 76 cm column with 20%
Carbowax 20M on Gas-Chrom R
Retention Index
• Relative retention times of polar and
nonpolar solutes change with the polarity of
the stationary phase
• On nonpolar stationary phases, solutes are
eluted in order of increasing boiling points
(retention is determined by the volatility of
the solutes)
Retention Index (cont.)
• On strongly polar stationary phases, the
order of elution is determined by the
intermolecular forces between the solutes
and the stationary phase (hydrogen bonding,
dipole-dipole)
PDMS – nonpolar
stationary phase
Poly(ethylene glycol) – strongly polar
stationary phase
Retention Index (cont.)
• The Kovats retention index, I, for a linear
alkane equals 100 times the number of
carbon atoms (for octane, I = 800; for
nonane I = 900)
• A compound eluted between octane and
nonane has a retention index between 800
and 900 computed by the formula
⎡
log t 'r (unknown) - log t 'r (n) ⎤
I = 100 ⎢n + ( N − n )
⎥
log
t
'
(
N
)
−
log
t
'
(
n
)
⎣
⎦
r
r
n – the number of carbon atoms in the smaller
alkane (8 in octane)
N – the number of carbon atoms in the larger
alkane
t’r(n) and t’r(N) – the adjusted retention times of
the smaller and larger alkane, respectively
The above formula is valid for isothermal
conditions only.
Example
tr(CH4) = 0.5 min; tr(octane) = 14.3 min
tr(unknown) = 15.7 min; tr(nonane) = 18.5 min
The retention index for the unknown is
⎡
log 15.2 - log 13.8 ⎤
= 836
I = 100 ⎢8 + (9 − 8)
⎥
log 18.0 − log 13.8 ⎦
⎣
The Retention Index in TemperatureProgrammed Chromatography
• Uses total (unadjusted) retention times and not
their logarithms
⎡
t r (unknown) - t r (n) ⎤
I = 100 ⎢n + ( N − n )
⎥
t r ( N ) − t r ( n) ⎦
⎣
T
• The value of IT will usually differ from the value
of I measured under the same conditions
Useful Practical Properties of I
• By definition a methylene group adds 100
to the retention index
• A functional group (phenyl, hydroxyl) adds
an increment (Xn) to the retention index
• Generally the Xn are additive if the groups
are separated by a few C atoms in a chain
• Retention indices are independent of flow
rate and film thickness
Useful Practical Properties of I
(cont.)
• RI are independent of column dimensions
and can be extrapolated even from packed
column data to capillary columns
• RI is only slightly dependent on column
temperature (generally within 5 units over a
50 °C temperature range)
• RI is a characteristic of the liquid phase
type and the solute
McReynolds Classification of
Stationary Phases
• McReynolds selected 10 probe solutes of
different functionality, each designated to
measure a specific interaction with a liquid
phase
• For each probe, a ΔI value is calculated:
ΔI = Iliquid phase – Isqualane
• As ΔI increases, the degree of specific
interaction associated with that probe
increases
McReynolds Classification of
Stationary Phases (cont.)
• The cumulative effect, when summed for
each of the ten probes, is a measure of
overall “polarity” of the stationary phase
• In tables of McReynolds constants, the first
five probes usually appear
• Each probe is assigned a value of zero with
squalane as reference liquid phase
The Structure of Squalane (C30H62)
2,6,10,15,19,23-Hexamethyltetracosane
Hydrogenated squalene from shark liver oil. Completely
nonpolar. The only interactions with the solute are through
dispersion (Van der Waals) forces
Practical Applications for
McReynolds Values
• Comparison of phases for similarity
• Ranking phases by degree of polarity:
– ΔI values between 0 and 100 – nonpolar phase
– ΔI between 100+ and 400 – moderately polar
– ΔI over 400 – a highly polar phase
• Prediction of analyte elution order: ΔI for
the probe indicates the degree of shift from
a boiling point order
If we have an aromatic/alcohol coelution, switch from a
PDEAS column to a LAC-2R-446 column (the shift of 61)
Temperature Programming
• Used to solve general elution problem
(GEP) for mixtures with a wide range of
boiling points
• Temperature of a column is raised during
the separation to increase solute vapor
pressure and decrease retention times of
late-eluting components
Pressure Programming
• Increasing the inlet pressure increases the
flow of mobile phase and decreases
retention time
• At the end of a run, the pressure can be
rapidly reduced to its initial value for the
next run (no need for column cooling)
• Programmed pressure is useful for
thermally labile analytes
Carrier Gas
• Helium – most common, compatible with
most detectors
• For FID – N2 gives a lower detection limit
than He
• H2, He, and N2 give essentially the same
Hmin at significantly different flow rates: uopt
increases in the order N2 < He < H2
Carrier Gas (cont.)
• H2 cannot be used with an MS detector – it
breaks down vacuum pump oil
• H2 and He give better resolution because the
solutes diffuse more rapidly through them
than through N2 (larger diffusion
coefficients) – the Cm term is smaller
Method Development in Gas
Chromatography
1.
2.
3.
4.
5.
Order of decisions:
Goal of analysis
Sample preparation
Detector
Column
Injection
Selecting the Column
•
•
•
•
Stationary phase – nonpolar most useful
Diameter and length
The thickness of stationary phase
Thick-film, narrow-bore columns provide a
good compromise between resolution and
sample capacity
Choosing the Injection Method
In a Nutshell
• Split – routine for introducing small sample
volume into open tubular column
• Splitless – best for trace levels of highboiling solutes in low-boiling solvents
• On-column – best for thermally unstable
solutes and high-boiling solvents; best for
quantitative analysis
Split Injection
Choosing the Injection Method
(cont.)
• Split injection:
–
–
–
–
–
–
Concentrated sample (or gas analysis)
High resolution
Dirty samples (use packed liner)
Could cause thermal decomposition
Poor quantitative analysis
Less volatile components can be lost during
injection
Choosing the Injection Method
(cont.)
• Splitless injection:
– Dilute sample
– High resolution
– Poor quantitative analysis (less volatile
components can be lost during injection)
– Requires solvent trapping or cold trapping
– Cannot be use with isothermal chromatography
Choosing the Injection Method
(cont.)
• On-column injection:
– Best for quantitative analysis
– Thermally sensitive compounds
– Low-resolution technique
CH2Cl2
MIBK
p-xylene