Gas Chromatography

1
Gas Chromatography

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

Gas Chromatography A Practical Course by
Gerhard Schomburg (QD79.C45 S3913 1990), 29-30,
31-36, 38-60, 66-73
2
General Design of a GC
3
Some of the designdetails

• 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

5
(No Transcript)
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
9
(No Transcript)
10
(No Transcript)
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
• GCs usually setup for either packed or capillary
• Lets say you choose a capillary column, theres
  more to think about!

12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
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.

17
(No Transcript)
18
(No Transcript)
19
Capillary vs. Packed Columns

• Packed Columns
• Greater sample capacity
• Lower cost (can make your own)
• More rugged
• Most common in process labs or separating/determin
  ing major components in a sample (prep GC)
• Limited lengths reduces R and N
• Not compatible with some GC detectors

• 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

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!

21
(No Transcript)
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

23
(No Transcript)
24
GC Carrier Gases (the mobile phase)

• Usually inert gases (dont 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)

25
(No Transcript)
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

27
(No Transcript)
28
(No Transcript)
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
Dont 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
• Ive 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.

34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
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
40
(No Transcript)
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!

42
(No Transcript)
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 hexachlorocyclohexa
  ne by contaminating it with excess analyte
• Widely used for the determination of pesticides,
  herbicides and PCBs in environmental samples.
• Non-destructive