Basic Gas Chromatography
Prepared by: Mina S. Buenafe
Gas Chromatography
•Chromatography – A Very Brief History
•Definitions / Terminologies in GC
•Instrumentation Overview
•System Modules
•Mobile Phase (Carrier Gas)
•Inlets
•Stationary Phase(s)
•Columns (Packed and Capillary)
•Detector(s)
•Troubleshooting
Chromatography – A (Very) Brief History
IN THE EARLY 1900’S
M. Tswett published his work on separation of plant pigments. He
coined the term chromatography (literally translated as color writing)
and scientifically described the process – earning him the title “Father
of Chromatography”
W. Ramsey published his work on separation of mixture of gases and
vapors on adsorbents like charcoal.
IN THE EARLY 1940’s
A. Martin and R. Synge first suggested the possibilities of gas
chromatography in a paper published in Biochem. J., v.35, 1358, (1941).
Martin won a Nobel Prize for his work in Partition chromatography.
IN THE EARLY 1950’s
A. Martin and A. James published the epic paper describing the first
gas chromatograph
Definition of Terms
Chromatography:
A physical method of separation in which
the components to be separated are
distributed between two phases, one of
which is stationary while the other moves
in a definite direction
“Official” IUPAC definition
Definition of Terms
Chromatogram
It is the output signal from
the detector of the instrument.
Definition of Terms
Distribution Constant (KC)
It is the tendency of a given component to be
attracted to the stationary phase. This can be
expressed in chemical terms as an equilibrium
constant. Also called the partition coefficient
(KP) or the distribution coefficient (KD)
KC = [A]S/[A]M
Mathematically, it is defined as the concentration of solute
A in the stationary phase divided by its concentration in the
mobile phase.
Definition of Terms
The attraction to the stationary phase can also be
classified according to the type of sorption by the solute.
Adsorption: sorption on the surface of the stationary phase
Absorption: sorption into the bulk of the stationary phase
(usually called ‘partition’ by chromatographers)
Definition of Terms
Retention Volume (VR)
It is usually defined as the distance
between the point of injection to the
peak maximum. It is the volume of the
carrier gas necessary to elute the solute
of interest.
Mathematically: VR = FC x tR
Where FC is the constant flow rate
tR is the retention time
Definition of Terms
Phase Ratio (b)
For packed columns:
b = Mobile Phase Volume
Stationary Phase Volume
For capillary columns:
b = rc/2df
Where rc is the radius of the column
df is the thickness of the film
Definition of Terms
Retention Factor (k)
It is the ratio of the amount of the solute (NOT
concentration) in the stationary phase to the
amount in the mobile phase. It is also called
capacity factor (k’), capacity ratio, or partition
ratio
Mathematically:
k = (WA)S/(WA)M = KC/b
Also k = (tR - t0) = time in stationary phase
t0
time in carrier gas
k is temperature and flow dependent. Best separations occur when
k is between 5 and 7
Definition of Terms
Theoretical Plates (N)
This is the most common measure of
column efficiency in chromatography
N = 16(tR/Wb)2
= 5.54(tR/ Wh)2
Where Wb is the peak width at the base
Wh is the peak width at half-height
Definition of Terms
Height Equivalent to a Theoretical Plate (H)
This is a related parameter that also defines column
efficiency. Also identified as HETP
Mathematically:
H = L/N
Where L is the Column Length
(An efficient column will have a large N and a small H)
Definition of Terms
Separation Factor (a)
It is a measure of relative distribution
constants. Also known as selectivity and/or
solvent efficiency.
Mathematically: a = k2/k1 = (KC)2/(KC)1
It is dependent on:
 Chemical composition of the phase
 Partitioning between the two phases
Definition of Terms
Resolution (Rs)
It is the degree to which adjacent
peaks are separated.
Mathematically:
Rs =
(tR)B – (tR)A
[(Wb)B + (Wb)A]/2
—
Also Rs = L/H x k/(k+1) x a-1/a
Instrumentation Overview
Schematic of a Typical Gas Chromatograph
System Modules
Carrier Gas
Main purpose: carries the sample through the column
Secondary purpose: provides a suitable matrix for the
detector to measure the sample component.
Detector
Carrier Gas
Thermal Conductivity
Helium
Flame Ionization
Helium or Nitrogen
Electron Capture
Very dry Nitrogen or Argon, 5% Methane
Carrier gases should be of high purity (minimum of 99.995%).
• Oxygen & water impurities can chemically attack the liquid phase of the column and
destroy it.
• Trace water content can desorb other column contaminants and produce high
detector background or ‘ghost peaks’.
• Trace hydrocarbon contents can cause high detector background with FID’s and
limit detectability.
System Modules
Carrier Gas
Flow Measurements and Control:
• Essential for column efficiency and qualitative analysis (e.g.
reproducibility of retention times)
Average linear flow velocity (ū) in OT columns:
ū = L/tm
where L is column length in cm
tm is the retention time of an unretained peak (e.g.
methane) in sec
To convert linear flow velocity to flow rate (Fc) in mL/min:
Fc = ū x Pr2 x 60sec/min
System Modules
Carrier Gas
Effect of mobile phase (carrier gas) density on column efficiency. Van Deemter plots for
the 3 common carrier gases for a column of capacity factor k’ = 7.90. The low density
gases (H2 & He) have optimum efficiency at slightly higher flow rates than N2. The much
lower slopes of H2 and He curves allow them to be used at higher flow rates (compared
to N2) with very little loss of separation efficiency.
System Modules
Inlets
Inlets are the points of sample introduction
Ideal Sample Inlets for Column Type:
Packed Columns
Flash Vaporizer
Capillary Columns
Split
On-Column
Splitless
On-Column
System Modules
Inlets
Split Injector
The oldest, simplest, and easiest injection
technique.
Cross section of a typical split injector
Advantages to Split Injection:
Disadvantages:
•High resolution separations
•Trace analysis is limited
•Neat samples can be introduced.
•Process sometimes discriminates
between high molecular weight
solutes so that the sample entering
the column is not representative of
the sample injected.
•Dirty samples can be introduced by
putting a deactivated glass wool plug in
the liner to trap non-volatile components
System Modules
Inlets
Splitless Injector
Samples have to be diluted in a volatile
solvent and 1-5mL is injected in the
heated injection port. Septum purge is
essential in splitless injections.
Cross section of a typical splitless injector
Advantages to Splitless Injection:
Disadvantages:
•Improved sensitivity over a split
injector
•Time consuming
•Initial temperature and time of
opening the split valve needs to be
optimized.
•Not well suited for volatile
compounds (boiling points of peaks of
interest have to be about 30oC
higher than solvent.
System Modules
Inlets
Other Types of Inlets:
• Direct Injection: involves injecting a small sample into a
glass liner where vapors are carried directly into the
column.
•On-Column Injection: inserting the precisely aligned
needle into the capillary column and making injections
inside the column.
•Flash Vaporization: involves heating the injection port to
a temperature well above the boiling point to ensure rapid
volatilization
•Static Headspace: concentrates the vapors over a solid
or liquid sample (best for residual solvent analysis)
System Modules
Stationary Phase
Sub-classification of GC Techniques
• GSC: gas solid chromatography stationary phase is solid
• GLC: gas liquid chromatography stationary phase is liquid
System Modules
Stationary Phase
Gas Solid Chromatography (GSC)
• Solids used are traditionally run in packed columns
• These solids should have small and uniform particle sizes (e.g.
80/100 mesh range)
Some Common GC Adsorbents
Commercial/Trade Names
Silica Gel
Chromasil®, Porasil®
Activated Alumina
Alumina F-1, Unibeads-A®
Zeolite Molecular Sieves
MS 5A, MS 13X
Carbon Molecular Sieves
Carbopack®, Carbotrap®, Carbograph®, Graphpac®
Porous Polymers
Porapak®, HayeSep®, Chromosorb®
Tenax Polymers
Texan TA®, Tenax GR®
•Some of these solids have been coated on the inside walls of capillary
columns and are called “Support Coated Open Tubular” or SCOT
columns.
System Modules
Stationary Phase
One major application of Packed Column GSC is in Gas Analysis.
Reasons:
• Adsorbents provide high surface areas for maximum interaction with gases that
may be difficult to retain on liquid stationary phase.
• Large samples can be accommodated, providing lower absolute detection limits.
• Some packed column GC’s can be configured to run below ambient temperature
which will also increase the retention of the gaseous solutes.
• Unique combinations of multiple columns and/or valving make it possible to
optimize analysis of a particular sample.
Packed Columns also provide the flexibility of allowing mixed packings for
special applications (e.g. 5% Fluorcol on Carbopack B® for analysis of Freons)
System Modules
Stationary Phase
Gas Liquid Chromatography (GLC)
To use liquid as stationary phase,techniques were applied to hold the liquid in a
column.
•
For packed columns: liquid is coated onto a solid support, chosen for its high
surface area and inertness. The coated support is then dry-packed into a column
as tightly as possible.
•
For capillary or open tubular (OT) columns: liquid is coated on the inside of the
capillary. To make it adhere better, the liquid phase is often extensively crosslinked and sometimes chemically bonded to the fused silica surface.
Schematic representation of (a) packed column and (b) capillary column
System Modules
Stationary Phase
Gas Liquid Chromatography (GLC)
Requirements for the stationary liquid phase:
•Low vapor pressure
•Thermal stability
•(if possible) Low viscosity (for fast mass
transfer)
•Should interact with the components of the
sample to be analyzed (“Like dissolves like”)
System Modules
Stationary Phase
Gas Liquid Chromatography (GLC)
Types of Capillary Columns (OT)
WCOT: Wall-coated open tubular column
(provides the highest resolution of all OT’s –
i.d.’s range from 0.1mm to 0.53mm and film
thickness from 0.1 – 5.0m)
PLOT: Porous layer open tubular column (less
than 5% of all GC use these days)
SCOT: Surface-coated open tubular column (no
longer available in fused silica)
System Modules
Detectors
The part of the system that ‘senses’
the effluents from the column and
provides a record of the analysis in
the form of a chromatogram. The
signals are proportional to the
quantity of each analyte.
System Modules
Detectors
FID: Flame Ionization Detector
The most common GC detector used.
The column effluent is burned in a small oxy-hydrogen flame
producing some ions in the process. These ions are collected
and form a small current that becomes the signals. When no
sample is being burned, there should be little ionization, the
small current is produced from impurities from the from the
hydrogen and air supplies.
Hydrogen flow rate is commonly set to 40 – 45mL/min, Air,
350- 450mL/min, and for OT columns (with flows of about 1
mL/min), Make-Up gases is added to carrier gas (to make up
the flow to 30mL/min)
System Modules
Detectors
TCD: Thermal Conductivity Detector
This is a differential detector that measures the thermal
conductivity of the analyte in the carrier gas compared to
the thermal conductivity of the pure gas. At least two
cavities are required. These cavities are drilled into a
metal block and each contain a hot wire or filament. The
filaments are incorporated into a Wheatstone Bridge
Circuit (for resistance measurements).
The choice of carrier gas will depend on the thermal
conductivity of the analyte (H2 and He have highest TC’s,
N2 gives rise to unusual peak shapes)
Detectors
System Modules
NPD: Nitrogen Phosphorus Detector
A bead of Rb or Cs is electrically
heated when flame ionization
occurs. The detector shows
enhanced detectability for
nitrogen-, phosphorus-, or halogencontaining samples.
System Modules
Detectors
MSD: Mass Spectrometric Detector
Analyte molecules are first ionized in
order to be attracted or repelled by
the proper magnetic or electrical
fields.
Troubleshooting
Common GC Problems:
• Retention Time Problems
• Resolution Problems
• Baseline Problems
• Peak Problems
Retention Time Problems
Retention Time Shift
Possible Cause
Solution
Comments
Change in carrier
velocity
Check the carrier gas
velocity
All peaks will shift in the
same direction by
approximately the same
amount
Change in column
temperature
Check the column
temperature
Not all peaks will shift by
the same amount
Change in column
dimensions
Verify the column identity
Large change in
Try a different sample
compound concentration concentration
May also affect adjacent
peaks. Sample overloading is
corrected with an increased
split ratio, sample dilution,
or decreased injection
volume.
Leak in the injector or
column connection
Usually accompanied by peak
size change.
Leak-check the injector and
column installation
Retention Time Problems
Retention Time Shift
Possible Cause
Solution
Comments
Blockage in a gas line
Clean or replace the plugged
line
More common for the split
line; also check flowcontrollers and solenoids.
Septum leak
Replace the septum
Check for needle barb.
Sample solvent
incompatibility
Change solvent.
Use a retention gap.
For splitless injector.
Contamination
Trim the column.
Remove ½ - 1 meter from
the front of the column.
Only for bonded and crosslinked phase.
Solvent-rinse the column.
Resolution Problems
Loss of Resolution
Decrease in separation
Possible Cause
Solution
Comments
Different column
temperature
Check column temperature
Differences in other peaks
will be visible
Different column
dimensions or phase
Verify column identity
Differences in other peaks
will be visible
Co-elution with another
peak
Change the column
temperature
Decrease column
temperature and check the
appearance of a peak
shoulder or tail.
Column contamination –
resulting in a change in
column selectivity
Trim the column
Remove ½ - 1 meter from
the front of the column.
Only for bonded and
cross-linked phase.
Solvent-rise the column
Resolution Problems
Loss of Resolution
Increase in peak width
Possible Cause
Solution
Comments
Change in carrier gas
velocity
Check carrier gas velocity
A change in retention time also
occurs
Column contamination
Trim the column
Remove ½ - 1 meter from the
front of the column.
Only for bonded and crosslinked phase.
Solvent-rise the column
Inlet liner
contamination
Clean or replace liner
Change in the injector
Check the injector settings
Typical areas: split ratio, liner,
temperature, injection volume
Change in the sample
concentration
Try a different sample
concentration
Peak width increases at higher
concentration
Improper solvent
effect lack of focusing
Lower oven temperature.
Choose different solvent for
better solvent/sample/phase
polarity match.
Use a retention gap
For splitless injection.
Baseline Problems
• Excessive Column Bleed
Possible Cause
Thermal damage to the
column
Oxygen damage to
column
Chemical phase damage
to column
Solution
Comments
Remove column from detector
and bake-out overnight,
reinstall and condition as usual
Use GC maximum temperature
function
Columns damaged by oxygen will
usually need to be replaced
although an overnight bake-out
may be attempted
Perform periodic leak checks.
Change septa regularly. Use high
quality carrier gases. Install and
maintain oxygen traps
Remove ½ to 5 meters from the
front of the column
Perform sample prep to remove
inorganic acids and bases from the
sample. Install guard column and
trim frequently. If acids or bases
must be used, choose HCl or
NH4OH, or an organic alternative.
Baseline Problems
• Erratic Baseline (drift, wander)
Possible Cause
Inlet
contamination
Solution
Clean the injector
Comments
Try a condensation test; gas lines may
also need cleaning. Take steps to
prevent sample backflash (reduce
injection volume, lower inlet
temperature, use larger volume liner
Column
contamination
Bake-out column.
Solvent-rinse the
column
Limit bake-out to 1 – 2 hours
Only for bonded and cross-linked
phases.
Check for inlet contamination
Incompletelyconditioned column
Fully condition the
column
More critical for trace analysis
Un-equilibrated
detector
Allow the detector
to stabilize
Some detectors may require up to 24
hours to fully stabilize
Change in carrier
gas flow-rate
during the
temperature
program
Normal in many
cases
MS, TCD, and ECD respond to carrier
gas flow rate changes
Baseline Problems
• Erratic Baseline (drift, wander)
Possible Cause
Solution
Comments
Contaminated gases
Use appropriate purifier to
remove contaminants
More of a problem for
detector gases.
Column and inlet liner
misaligned
Check installation of column
end and inlet liner, adjust if
necessary
Causes a baseline change
after a large peak
Large leak at the septum Replace septum
during injection and for
Use smaller diameter needle
a short time thereafter
Sample decomposing
Remove inlet liner and check
cleanliness.
Use new, deactivated liner or
replace glass wool and
packing.
Causes a baseline change
after a large peak.
Common with large diameter
needles.
Causes a baseline rise
before and after a peak.
Baseline Problems
·Noisy
Baseline
Possible Cause
Inlet contamination
Solution
Comments
Clean the injector,
replace liner, gold seal
Try a condensation test; gas
lines may also need cleaning.
Bake-out the column.
Solvent-rinse the
column.
Limit bake-out to 1 – 2 hours
Only for bonded and crosslinked phases.
Check for inlet contamination
Column contamination
Detector contamination
Clean the detector.
Contaminated or low
quality gases
Replace spent gas
purifier.
Use purifiers to remove
contaminants.
Use better grade gases.
Column inserted too far
into the detector
Reinstall the column.
Usually the noise increases over
time and not suddenly.
More of a problem for detector
gases.
Consult the GC manual for the
proper installation distance.
·Noisy
Baseline Problems
Baseline
Possible cause
Solution
Comments
Incorrect detector gas
flow rates
Adjust the flow rates to
the recommended values.
Consult the GC manual
for appropriate flow
rates
Leak when using an MS,
ECD, or TCD
Find and eliminate the
leak.
Usually at the column
fittings or injector.
Old detector filament,
lamp, or electron
multiplier, NPD head
Replace appropriate part.
Septum degradation
Replace septum.
For high temperature
applications, use
appropriate septum
Baseline Problems
·Ghost Peaks
Possible Cause
Contaminants introduced
with sample
Solution
Sample or solvent clean-up
Comments
Contaminants in sample process or
solvent.
Clean the injector, replace liner,
gold seal, and septum
Try a condensation test; gas lines
may also need cleaning. Take steps
to prevent sample backflash
(reduce injection volume, lower
inlet temperature, use larger
volume liner)
Septum bleed
Replace septum.
Use a high quality septum
appropriate for the inlet
temperature.
Contamination of sample
prior to introduction to
the GC
Check sample handling steps for
potential contamination sources:
sample clean-up, handling,
transfer, and storage
Semi-volatile
contamination (peak
widths will be broader
than sample peaks with
similar retention
Bake out column,
Solvent-rinse the column.
Check for contamination in the
inlet, carrier gas, or carrier gas
lines.
Inlet contamination
Limit bake-out to 1 – 2 hours.
Only for bonded and cross-linked
phases.
Peak Problems
·Fronting Peaks
Possible cause
Solution
Comments
Reduce mass amount of
the analyze to the column.
Decrease injection volume,
dilute sample, increase
split ratio
Most common cause for
fronting peaks.
Reinstall the column in the
injector.
Consult the GC manual for the
proper installation distance
Injection technique.
Change technique.
Usually related to erratic
plunger depression or having
sample in the syringe needle.
Use an autosampler.
Compound very soluble in
injection solvent
Change solvent. Using a
retention gap may help.
Column overload
Improper column
installation.
Mixed sample solvent
Change sample solvent.
Worse for solvents with large
differences in polarity or
boiling points.
Peak Problems
·Tailing Peaks
Possible cause
Solution
Comments
Severe column
contamination
Trim the column.
Solvent rinse the column
Remove ½ - 1 meter from the front
end of the column.
Only for bonded or cross-linked
phase.
Check for inlet contamination.
Tailing will sometimes increase with
compound retention.
Active column
Cut off 1-meter from the front
end of the column.
Replace column.
Only affects active compounds.
Usually produces tailing that
increases with retention.
Improper column
installation, leak, or
column end poorly cut
Re-cut and reinstall the column
into the inlet.
Replace ferrule.
Confirm installation is leak-free
Make a clean square cut with a
reliable cutting tool.
Consult GC manual for the proper
installation distance.
More tailing for early eluting peaks.
Contaminated or active
liner or gold seal
Use new, deactivated liner.
Clean or replace gold seal.
Only affects active compounds.
Peak Problems
·Tailing Peaks
Possible cause
Solution
Solid particles in liner
Clean or replace liner.
Needle hitting and breaking
packing in inlet liner
Partially remove packing from
liner or use without packing.
Solvent/column not compatible
Use a different solvent.
Use a retention gap.
Comments
More tailing for the early
eluting peaks or those closest
to the solvent front.
3 – 5 meter retention gap is
sufficient.
Split ratio too low
Increase split ratio.
Flow from split vent should be
 20mL/min
Solvent effect violations for
splitless or on-column
injections
Decrease the initial column
temperature to 10 - 25°C
below solvent boiling point
Peak tailing decreases with
retention.
Poor injection technique
Change technique
Usually related to erratic
plunger depression or having
sample in the syringe needle.
Use an autosampler.
Peak Problems
·Tailing Peaks
Possible cause
Solution
Comments
Inlet temperature too high
Decrease inlet temperature by
50°C
Tailing generally worse for
early eluters.
Inlet temperature too low
Increase inlet temperature by
50°C
Tailing usually increases with
retention
Dead volume in system
Reduce dead volume. Transfer
line connections, fused silica
unions, etc.
Peak tailing decreases with
retention.
Cold spots (condensation)
Eliminate cold spots.
Commonly at transfer lines.
Tailing usually increases with
retention
Overloading of PLOT columns
Reduce the amount injected
onto column.
Peak Problems
·Split Peaks
Possible cause
Solution
Comments
Reinstall the column in the
injector
Consult the GC manual for the
proper installation distance
Change technique.
Usually related to erratic
plunger depression or having
sample in the syringe needle.
Use an autosampler.
Mixed sample solvent
Change sample solvent.
Worse for solvents with large
differences in polarity or boiling
points
Poor sample focusing
Use a retention gap
Solvent/column not
compatible
Use a different solvent.
Use a retention gap.
Column installation
Injection technique
For splitless, on-column, and
PTV injectors
Peak Problems
·Split Peaks
Possible cause
Sample degradation in
injector (only some peaks
show splitting)
Severe detector overload
Solution
Reduce inlet temperature.
Derivatize sample to make
compounds thermally
stable.
Change to an on-column
injector.
Reduce the amount of
sample on-column.
Comments
Peak broadening or tailing
may occur if the
temperature is too low.
Requires an on-column
injector
May only affect some
peaks.
Peak Problems
·Changes in Peak Size
Possible cause
Solution
Comments
Change in detector
response
Check gas flow,
temperature and settings.
Check background level or
noise
All peaks may not be
equally affected
May be caused by the
system contamination, not
the detector.
Change in the split
ratio
Check the split ratio
All peaks may not be
equally affected.
Change in the purge Check the purge activation
activation time
time
For splitless injectors
Change in injection
volume
Check injection technique
Injection volumes are not
linear.
Change in injector
discrimination
Maintain the same injector Most severe for spit
parameters: flows,
injections. All peaks may
temperatures, liners, etc. not be equally affected
Peak Problems
·Changes in Peak Size
Possible cause
Solution
Comments
Change in sample
concentration
Check and verify sample
concentration
May be caused by
degradation, evaporation,
or variances in sample
temperature or pH
Leak in the syringe
Use a different syringe
Sample leas past the
plunger or around the
needle; leaks are often not
readily visible
Column
contamination
Trim the column
Remove ½ - 1 meter from
the front of the column.
Only for bonded and
cross-linked phases
Solvent-rinse the column
Column activity
Trim or replace the column Only affects active
compounds
•Peak Problems
·Changes in Peak Size
Possible Cause
Solution
Comments
Co-elution
Change column
temperature or stationary
phase
Decrease column
temperature, and check
for the appearance of peak
shoulder or tail
Sample backflash
Inject less, use larger
liner, or reduce the inlet
temperature
Less solvent and higher
flow rates are most helpful
Decomposition from
inlet contamination
Clean the inlet, replace
the liner, replace the gold
seal
Only use deactivated liners
and glass wool in the inlet.
Loss of sample prior Check sample handling,
to introduction into sample preparation,
the GC
transfer and storage
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