gas chromatography

1. Gas Chromatography
•General Design of a Gas Chromatograph
•Separation Processes in Gas Chromatography
•GC Columns
•GC Injectors
•GC Detectors

2. General Design of a GC

3. Some of the design“details”
• Gas supplies usually have either in-line or instrument
mounted traps to remove any water, oxygen,
hydrocarbons or other “contaminants” from compressed
gases
• Gas flows can be controlled using either needle valves or
mass-flow controllers (electronic sensors)
• Instruments can have multiple injectors, detectors or
columns
• Injectors and detectors usually have their own
temperature controlled zones (small heaters)
• The GC oven has a large fan and a vent door to help with
rapid cooling of the oven
• Data collection (and integration) can be done using a chart
recorder, integrator or a computerized data system

4. Separation Processes in GC
• Gas Chromatography as it is usually performed is correctly called gas-
liquid chromatography
• the analyte is in the gas phase in the GC and partitions between the mobile
phase (carrier gas) and the liquid stationary phase that is coated on the
inside of an open-tubular capillary column or on particles inside a packed
column
• Some packed-column GC uses non-coated solid stationary phases, in
which case one is performing gas-solid adsorption chromatography
• Capillary, open-tubular (WCOT specifically) column GC is the primary
type of GC used in quantitative analysis:
• higher resolution = greater ability to discriminate between components
• smaller capacity of the column is not important as long as sufficient analyte is
available for detection
• pg/mL (ppt) to g/mL (ppm) concentration range for liquid analytes

6. The Objective in
Chromatography (all types)
• Separate your analytes (resolution of 1.5 or better) in
the shortest amount of time possible and detect
them….
• How can we do this in GC?
• Use different columns for different analyte types
• stationary phase
• diameter of column, stationary phase thickness
• column length
• Use different injection types/temperatures to optimize the
process of loading the sample on the column
• Use different temperature (or pressure) programs for the
column
• Select and use a detector that is suitable for the analyte(s) of
interest

7. GC Columns (concentrating on open-tubular capillary
columns)
• Column “frame” constructed of fused silica tubing
• Polyamide coating on the outside gives it strength
• Liquid stationary phases coated or bonded to the inside of the
tubing
• 0.1 - 0.53 mm + ID, 5-100 meters in length, stationary phases
usually 0.10 to 1.5 m in thickness
• Mounted on a wire cage to make them easier to handle
• 5-150 meters long.

8. Capillary
Column
Stationary
Phases

11. Choosing a GC Column…
• Is the column compatible with your analytes
• polar analytes require polar stationary phases so they will
spend some of their “time” in the stationary phase
• non-polar analytes require non-polar stationary phases
• You usually have to compromise on the stationary phase to
get a good column for your analytes (which are probably a
mix of polar and non-polar)
• DB-5, HP-5, EC-5, RTX-5 (5% dimethyl, 95% diphenyl
polysiloxane) most common general use column.
• Temperature range, solvent and carrier gas
compatibility
• Sample capacity versus resolution
• usually determines packed vs.. capillary
• GC’s usually setup for either packed or capillary
• Let’s say you choose a capillary column, there’s more
to think about!

16. For capillary GC columns….
 Increased length = greater N, therefore a greater R
 expense is possible band broadening if analytes are on the
column too long!
 Increased length leads to longer separations. Do you have
the time?
 Increased stationary phase thickness and column
diameter provides increased sample capacity and
can provide increased resolution
 tradeoffs are a longer analysis time and more column bleed
with thicker stationary phases
 Is the column compatible with the detector?
 Thick stationary phases bleed more and will contaminate a
mass spectrometer.
 For most analytical work, a best “compromise”
column is chosen and other variables (temp, etc.) are
altered to optimize the separation.

19. Capillary vs. Packed Columns
 Capillary Columns:
 Higher resolution (R)
 Greater HETP and N
 Shorter analysis time
 Greater sensitivity
 Most common in analytical
laboratory GC instruments
 Smaller sample capacity
 Higher cost/column
 Columns more susceptible
to damage
 Packed Columns
 Greater sample capacity
 Lower cost (can make your own)
 More rugged
 Most common in process labs or
separating/determining major
components in a sample (prep
GC)
 Limited lengths reduces R and N
 Not compatible with some GC
detectors

20. Temperature Programming in GC
 The “simplest” way to alter the separation in GC is to
alter the temperature program in the oven. You can
also alter the pressure of the carrier gas, but this is
less common (much).
 Isothermal = constant temperature
 Gradient = varied temperature
 By altering the temperature, you vary the rate of the
reaction for any analyte:
 they spend more or less time in the stationary phase
 the greater the difference in the times between analytes, the
better the separation!
Analyte
Analyte phase
stationary
phase
mobile 

22. The traps of temperature…
If your temperature at a given time is too high, you
can cause the peaks to co-elute
poor resolution vs but a faster separation
If your temperature at a given time is too low, you
can get still get a good separation
adequate resolution, but a separation that takes very
long
You have to choose a compromise temperature
program

24. GC Carrier Gases (the mobile phase)
 Usually “inert” gases (don’t react with analytes except
sometimes in the detector)
 Purpose:
 sweep sample through the column
 protect column from oxygen exposure at temperature
 assist with function of the detector
 Most common:
 Helium (available relatively pure without extensive purification after
it leaves a compressed gas cylinder)
 Nitrogen (usually requires an oxygen and water trap)
 Hydrogen
normally used only with flame ionization detectors (FID) since the FID
needs it as fuel for the flame
still rarely used due to safety concerns (and chromatographic ones)

26. GC Injection….
 Samples are injected through a septum:
 keeps oxygen out of the column
 provides a seal to keep the carrier gas pressure up at the
head of the column
carrier gas flow rate is determined by the pressure or the gas at
the opening of the column
 Many different (mostly proprietary) materials
red rubber (bleeds at about 250 C)
Thermogreen (good up to about 300 C)
High-temperature blue (good a little over 300 C)
 The injector is usually lined with a de-activated glass
liner
 prevents metal injector-sample reactions that would alter
analytes or damage the metal of the injector
 Can be cleaned/replaced regularly

29. Injection types

30.  On-Column Injection:
 used widely in packed-column GC, less in capillary GC
 sample is deposited directly on the column
Good for thermally unstable compounds
Good for quantitative analysis at low concentrations
 all sample is available to travel to the detector
BUT, you can inject only a relatively small amount of sample in
capillary GC anyhow.
 Splitless Injection:
 Sample is vaporized in the injector itself and ALL of the
sample is swept onto the column by the carrier gas
 Again, relatively small samples are injected (10  L or less in
capillary GC)
 Sample spends a large amount of time in the injector
 Best for trace (1 -100 ppm range) concentrations of high
boiling point analytes in low boiling point solvents
extra time in the injector helps volatilize the analytes.

31. Split Injection:
the injection is split, with only a portion of the
sample (usually 1% - 20%) actually making it to
the column
the most common method of injecting samples
onto small diameter, open-tubular columns.
Even if you inject 20 L, only a fraction (adjustable)
makes it on to the column
Not good for analytes with a wide range of
boiling points
some may be swept out the split vent before they are
volatilized
Modern capillary GCs come with a
Split/Splitless injectors standard
you switch between modes by changing the split
vent gas flow and using a different injection liner.

32. Don’t Forget SPME (Solid Phase Microextraction)

33. GC Detectors
 A dozen or more varieties (some obscure)
 Must be:
 sensitive to the analytes of interest
 compatible with the column, carrier gas, solvent, etc.
 rugged enough to withstand general unattended used
I’ve run our new GC for 36 hours straight without touching it!
 Should have a known linear range
 if the detector response is very linear, you can use a response
factor instead of a calibration curve for quantitation!
 Usually require separate gas supplies (other than the
carrier gas), have their own temperature control.
 Measure nothing more than a voltage or a current.

38. FID
FPD

39. Thermal Conductivity
(TCD)
 The carrier gas has a known
thermal conductivity.
 As the thermal conductivity of
the column eluent (gas flow in)
changes, the resistance of the
filament changes.
 The presence of analyte
molecules in the carrier gas
alter the thermal conductivity of
the gas (usually He)
 There is normally a second
filament to act as a reference
(the carrier gas is split)
 Increased sensitivity with
decreasing temperature
(detector), flow rate and applied
current.
 Filaments will burn out
(oxidized) in the presence of
oxygen if hot!
Non-destructive

41. FID
 Destructive, sample lost.
 Analytes containing C burn
in a hydrogen-oxygen flame
and produce ions
 CHO+ ions are collected on a
cathode and the current they
produce results in the signal
 WILL NOT detect non-C
containing compounds!
 Requires H2 supply (tank or
generator) and O2 supply
(compressed air)
 H2 carrier gas can be used,
eliminating the need for a
supply for the detector
 A makeup gas can also be
required!
-
Flame
O
,
H
e
CHO
O
CH
2
2


 

43. ECD
 Particularly sensitive to halogens nitriles,
carbonyls, nitro compounds
 Analytes pass through a cell, in which
electrons are traveling between a 63Ni
electrode and a collector electrode
 As analytes with “electron capturing
ability” pass through the cell, the flow of
electrons is interrupted.
 The change in current, due to reduced
flow of electrons, is recorded.
 EXTREMELY SENSITIVE TO
HALOGENS
 could ruin detector with 1 ppm
hexachlorocyclohexane by contaminating it
with excess analyte
 Widely used for the determination of
pesticides, herbicides and PCBs in
environmental samples.
 Non-destructive