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Speakers
John V Hinshaw
GC Dept. Dean
CHROMacademy
Tony Taylor
Technical Director
CHROMacademy
Moderator
Dave Walsh
Editor In Chief
LCGC Magazine
An Introduction to Column Selection for Capillary GC
Aims & Objectives
1. Important analyte / stationary phase
interactions
2. GC stationary phase polymer types
3. Selecting an appropriate stationary
phase
4. Stationary Phase Selectivity Effects –
tuning the chemistry
5. Choosing column dimensions
6. Effects of column dimensions on
selectivity, retention, efficiency and
resolution
7. Bringing it all together from a practical
standpoint
Capillary GC column selection - what's important?
Stationary Phase
Length (m) x Internal Diameter (mm) x Film Thickness (mm)
L
rc
5% Diphenyl dimethylpolysiloxane
30m x 0.25mm x 0.25mm
Efficiency (N)  carrier gas / L / rc
Retention (k)  oC / rc / df
Selectivity (a)  oC / phase
df
Stationary phase selection:
Major analyte / stationary phase interactions
Major Analyte / Stationary Phase Interactions
Electronegativity / Dipole
Moments / Polarity
Non-polar
Polar - Dipole
Stationary phase selection:
Dispersive interactions
1. All substances contain small dipoles (small
electronegativity differences)
2. Instantaneous dipoles fluctuate throughout
the molecule (electron / nuclei vibration)
3. As two molecules approach – transient
dipoles can induce the opposite dipole in the
other molecule and a small attractive effect
is seen
4. Often called a ‘dispersive interaction’ and
occurs between compounds which are
predominantly non-polar
5. Dispersive interactions occur with all
substances, regardless if there is another
overriding interaction
Van der Waals
Forces
p8.flv
Stationary phase selection:
Dipole interactions
1. Dipole interactions come in two sorts:
Dipole-Dipole / Dipole-Induced Dipole
1. Dipole interactions occur between substances
whose permanent dipoles come into close
contact with each other
2. Dipole-Induced dipole interactions occur when
a polar substance meets a polarisable
compound (typically containing pi-electrons)
3. The stronger dipole induces a more
permanent dipole in the other substance and
an intermolecular attraction occurs
Dipole – Dipole
Interaction
p10.flv
p11.flv
Stationary phase selection:
Hydrogen Bonding Interactions
1. A special case of a dipole-dipole
interaction
2. Dipoles associated with the functional
groups of two molecules come into close
proximity
3. Hydrogen bonding interactions are very
strong compared to dispersive interactions
4. In the extreme (e.g. the association of
water with methanol) the dipole-dipole
interaction energy can approach that of a
chemical bond
5. Still an underlying weak dispersive
interaction occurring simultaneously
Hydrogen Bonding
Interactions
Modern Stationary Phase Chemistry
1. Immobilized Polymeric Liquids bonded to the inner surface of a
silica capillary via silyl-ether linkages
2. Deactivation treatments applied before and/or after bonding
‘Polysiloxane’ Phases
Typical Ratio of
monomers (X:Y):
5:95
35:65
50:50
Higher percentage of functional monomer
indicate higher degree of that interaction
50:50 phase shows stronger Induced Dipole
Interactions with Aromatics
Modern Stationary Phase Chemistry (III)
‘Polysiloxane’ Phases
Typical Ratio of monomers (X:Y):
6:94
14:86
50:50
Typical Ratio of monomers (X:Y):
35:65
50:50
Modern Stationary Phase Chemistry (III)
‘Glycol / Wax’ Phases
Stationary Phase Interaction Summary
Lower polarity phases bleed less!
Stationary Phase Selection
1. Critical – Phase and Temperature directly
effect selectivity
2. Principle of ‘like dissolves like’ holds well
3. Separate polar analytes using a more polar
phase and vice versa
4. The skill is knowing the degree of polarity
required to avoid overly long retention
times whilst still obtaining a satisfactory
separation
5. Separating compounds of intermediate
polarity or mixed polarity & functionality
requires knowledge of the retentivity and
selectivity of each phase
6. May require ‘fine tuning’ of the phase
chemistry using the monomeric ratios
Like Dissolves Like
Stationary Phase Selection:
Test Probe Chemistry
Stationary Phase Selection:
Dispersive Interactions
100% Methyl Polysiloxane
Boiling Point Column?
2
1
3
6
4
5
1.
2.
3.
4.
5.
6.
Toluene
Hexanol
Phenol
Decane (C10)
Naphthalene
Dodecane (C12)
Strong Dispersion
No Dipole
No H Bonding
110oC
156oC
182oC
174oC
218oC
216oC
Stationary Phase Selection:
Dipole (Induced Dipole) Phases
5% Phenyl Phase
5%
Phenyl
100%
Methyl
5,6
1
2
?
1 2
3
4
Strong Dispersion
Weak (Induced) Dipole
No H Bonding
3
4
5
6
Strong Dispersion
No Dipole
No H Bonding
1.
2.
3.
4.
5.
6.
Toluene
Hexanol
Phenol
Decane (C10)
Naphthalene
Dodecane (C12)
Stationary Phase Selection:
Dipole (Induced Dipole) Phases
50% Phenyl Phase
50%
Phenyl
1 2
4
36
5
Strong Dispersion
Weak (Induced) Dipole
No H Bonding
100%
Methyl
?
1 2
3
4
6
5
Strong Dispersion
No Dipole
No H Bonding
1.
2.
3.
4.
5.
6.
Toluene
Hexanol
Phenol
Decane (C10)
Naphthalene
Dodecane (C12)
Stationary Phase Selection:
Dipole & Hydrogen Bonding Phases
14% Cyanopropylphenyl
Phase
14% Cyano-propylphenyl
1
100%
Methyl
?
1 2
24
6
3 5
3
4
Strong Dispersion
None/Strong Dipole (Ph/CNPr)
Weak/Moderate H Bonding (Ph/CNPr)
6
5
Strong Dispersion
No Dipole
No H Bonding
1.
2.
3.
4.
5.
6.
Toluene
Hexanol
Phenol
Decane (C10)
Naphthalene
Dodecane (C12)
Stationary Phase Selection:
Dipole & Hydrogen Bonding Phases
50% Cyanopropylphenyl
Phase
50% Cyano-propylphenyl
41
100%
Methyl
6
?
1 2
2
5
3
4
3
Strong Dispersion
Strong Dipole
Moderate H Bonding
6
5
Strong Dispersion
No Dipole
No H Bonding
1.
2.
3.
4.
5.
6.
Toluene
Hexanol
Phenol
Decane (C10)
Naphthalene
Dodecane (C12)
Stationary Phase Selection:
Dipole & Hydrogen Bonding Phases
35% Trifluoropropyl
35% Trifluoropropyl Phase
100% PDMS
1,2
1
DB-1
DB-200
2
3
3
Strong Dispersion
Moderate Dipole
Weak H Bonding
1. p-Xylene, 2. m-Xylene, 3. o-Xylene
Column:
DB1 / DB200 (30m x 0.25mm, 0.25µm)
Carrier:
Helium @ 32 cm/sec.
Oven:
45 – 115oC @ 5oC/min.
Stationary Phase Selection:
Dipole (& H Bonding) Phases
100% Polyethylene Glycol
100% PEG
4 1
6
2
5
100%
Methyl
?
1 2
3
4
3
Strong Dispersion
Strong Dipole
Moderate H Bonding
6
5
Strong Dispersion
No Dipole
No H Bonding
1.
2.
3.
4.
5.
6.
Toluene
Hexanol
Phenol
Decane (C10)
Naphthalene
Dodecane (C12)
Stationary Phase Selection:
PLOT columns
1. Porous Layer Open Tubular (PLOT)
for Gas Solid Chromatographic (GSC)
applications
2. PLOT columns are coated with small,
solid porous particles using a binder.
Particles are Alumina or Molecular
sieve
3. Solutes are separated on differences
in their adsorption properties, size and
shape
4. PLOT columns used to separate highly
volatile liquids and permanent gases
without the need for cryogenic or
subambient cooling of the GC oven
Stationary Phase Selection:
PLOT columns
5. Alumina columns are well
suited to the analysis of C1 – C10
hydrocarbons and small aromatics
6. KCl derivatised columns
produce altered selectivity
7. The Q designated columns
show better selectivity for C1-C3
hydrocarbons (not good for >C6)
8. Q columns are also able to
separate sulphur gases and
most light hydrocarbons.
9. Molecular sieve columns used for noble
and permanent gas samples - also good for
the separation of solvents.
50m, 0.53mm ID
Rt-Alumina™ PLOT
Practically Speaking!
1. If no information or ideas about which stationary phase to use is
available, start with a DB-1 or DB-5
2. Low bleed ("ms") columns are usually more inert and have higher
temperature limits
3. Use the least polar stationary phase that provides satisfactory
resolution and analysis times. Non-polar stationary phases have
superior lifetimes to polar phases
4. Use a stationary phase with a polarity similar to that of the
solutes. This approach works more times than not; however, the
best stationary phase is not always found using this technique
Practically Speaking!
5. If poorly separated solutes possess different dipoles or hydrogen
bonding strengths, change to a stationary phase with a different
amount (not necessarily more) of the dipole or hydrogen
bonding interaction
6. If possible, avoid using a stationary phase that contains a
functionality that generates a large response with a selective
detector
7. 100% Methyl or 5% Phenyl, 50% Phenyl, 14% Cyanopropylphenyl
and WAX (PEG) cover the widest range of selectivities with the
smallest number of columns
8. Use PLOT columns for the analysis of gaseous samples at above
ambient column temperatures
Column Dimensions - Column Length
Efficiency (N)  carrier gas / L / rc
Retention (k)  oC / rc / df
Selectivity (a)  oC / Phase
1.
2.
3.
4.
Doubling column length doubles efficiency
Doubles analysis time (or by 1.5 – 1.75x for temperature gradient)
Increases column costs
Improves resolution by a factor of 1.4 x
p30.flv
Column Internal Diameter (or rc)
Efficiency (N)  carrier gas / L / rc
Retention (k)  oC / rc / df
Selectivity (a)  oC / Phase
1. Affects efficiency, retention, carrier flow rate, capacity and pressure
drop across the column
2. Inversely proportional to column efficiency – halve diameter,
double efficiency, increase resolution by factor of 1.4
Column Internal Diameter (or rc) (II)
Efficiency (N)  carrier gas / L / rc
Retention (k)  oC / rc / df
Selectivity (a)  oC / Phase
3. Inversely proportional to analyte retention for isothermal
but NOT GRADIENT
4. Consider gradient temperature programming in conjunction
with pressure programming for constant flow
Column Internal Diameter (or rc) (III)
Efficiency (N)  carrier gas / L / rc
Retention (k)  oC / rc / df
Selectivity (a)  oC / Phase
5. Column head pressure an inverse square function of column radius
6. Column capacity increases with column internal diameter
7. Capacity also depends on the stationary phase type, film thickness
and the nature of the analytes
p34.flv
Phase Film Thickness (df)
Efficiency (N)  carrier gas / L / rc
Retention (k)  oC / rc / df
Selectivity (a)  oC / Phase
1. Affects retention, inertness,
capacity, resolution and bleed
2. Film thickness is directly
proportional to retention
time (1.5:1 for gradient)
3. Thick stationary phase films
give retention for highly volatile analytes
4. Increasing film thickness allows retention of volatile analytes
at temperatures at or above ambient
5. Doubling df gives an increase of around 20oC in elution
temperature
Phase Film Thickness (df) (II)
Efficiency (N)  carrier gas / L / rc
Retention (k)  oC / rc / df
Selectivity (a)  oC / Phase
6. Retention of late eluting (high boiling point) analytes is reduced
using thinner film columns
7. Early eluting analytes (k<2) are better resolved using thicker film
columns
8. Resolution may DECREASE for analytes with k values between 5
with INCREASING film thickness
9. Thicker films bleed more - upper temperature limits of thick film
columns will be lower
10. Thicker film columns are more inert as the film shields the analyte
from active sites on the silica tubing
11. Thicker film columns have higher analyte capacity, and so may
reduce peak fronting
p37.flv
Phase Ratio (b)
1. Stationary phase to
mobile phase ratio
2. Increasing the phase ratio
will result in decreased
analyte retention
(increasing the column
radius or decreasing the
film thickness)
(reduction in capacity?)
Phase Ratio (b)
3. Use to keep retention
time approximately
constant whilst
increasing efficiency
(reducing the column
internal diameter) and
reducing film thickness
to keep b constant
4. Net result is a more
efficient separation
within the same
timescale as the original
separation!
Phase Ratio (b) (II)
Practically Speaking!
1. Capillary GC columns are typically 10, 15, 20, 25, 30, 50, 60,120 (m)
2. Extending the column length is the least favoured option for
increasing resolution and should be avoided if possible
3. Cost and analysis time are proportional to column length
4. Use the shortest column that will give you the required resolution
(begin with 25-30 m columns if the number/ nature of samples is
unknown).
5. To increase resolution try changing the stationary phase or column
internal diameter first
6. Narrow internal diameter columns are capable of separating
multiple analytes in a single analysis
Practically Speaking!
7. Increase film thickness when volatile analytes are involved or
reduce film thickness to decrease retention of highly adsorbed
analytes
8. Use phase ratio to increase separation efficiency in the same
timeframe as the original separation
9. Column head pressure and bleed increase with column length.
10. 10-15 m columns are well suited to samples containing well
separated analytes or where the number of analytes is low
11. 50-60m columns should be used only where very large numbers of
components need to be separated and as a last resort when
reducing the column internal diameter and changing the stationary
phase and temperature program have failed!
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