1. Chapter 27 Gas Chromatography 1. Principles
Two types
- Gas-solid chromatography (limited because of semipermanent retention of polar molecules)
Gas-liquid chromatography
Retention volume
F: average volumetric flow rate mL/min, can be determined
by measuring flow rate exiting the column using a soap bubble meter
Fig (p.790) A soap-bubble flow meter

2. But VR and VM depend on pressure inside column, and column has resistance to flow
- At inlet, P high , F low
- At outlet, P low, F high  PF = constant
Pressure drop correction factor j
- to calculate average pressure from inlet pressure Pi and outlet pressure P

3. Specific retention volume Vg
- semi-useful parameter for identifying species eluting
Relationship between Vg and k

4. 2. Instrumentation 2.1 Carrier gas (mobile-phase gas)
He (common), N2, H2, Pi psi above atom, F = mL/min for packed column, 1-25 mL/min for open tubular capillary
Fig (p.790) Block diagram of a typical GS

5. - Direct injection into heated port to promote fast vaporization
2.2 Sample injection
- Direct injection into heated port to promote fast vaporization
- Sample volume
1-20 L for packed column
10-3 L for capillary column, a sample splitter is needed (1/50-1/500 to column, rest to waste) or use purge valve
Fig (p.791) Cross-section view of a microflash vaporizer direct injector

6. 2.3 Column 2.4 Oven - Packed column: 1- 5m
- Open tubular (capillary): 1m to 100m
Both constructed of coiled stainless steel/glass/Teflon, with coil diameter of cm
2.4 Oven
accurate to < 1C
equal or slight high than average boiling point of sample
For broad boiling range--temperature programming

7. 2.3 Column 2.4 Oven: 2. 5 Detectors - Packed column: 1- 5m
- Open tubular (capillary): 1m to 100m
Both constructed of coiled stainless steel/glass/Teflon, with diameter of cm
2.4 Oven:
accurate to < 1C
equal or slight high than average boiling point of sample
For broad boiling range--temperature programming
2. 5 Detectors
Requirement and desire:
1. Sensitive ( g solutes/s) 5. fast response wide dynamic range
2. Stability and reproducibility 6. simple (reliable)
3. Linear response uniform response to all analytes
4. Operate at high T (RT-400 C) 8. nondestructive

8. Flame Ionization Detector (FID)
Sample pyrolyzed and produce current in electrical field.
Signal depends on #C atoms in organic analyte –mass sensitive not concentration sensitive
Weakly sensitive to carbonyl, amine, alcohol, amine groups
Insensitive to non-combustibles – H2O, CO2, SO2, NOX
Advantages
Rugged
Sensitive (10-13 g/s)
Wide dynamic range (107)
Disadvantage
Destructive
Fig (p.794) FID

9. Thermal Conductivity Detector (TCD)
- Thermal conductivities of He and H2 much larger than organics
Organics cause T rise in detector
Advantages
Simplicity
Wide linear dynamic range (105)
Responds to both organic and inorganic
Nondestructive
Disadvantage
Low sensitive (10-8 g/mL carrier gas)
Fig (p.794) TCD

10. Electron Capture Detector (ECD)
- electron from -source ionize carrier gas
organic molecules capture electron and decrease current
Advantages
Simple and reliable
Sensitive to electronegative groups (halogens, peroxides)
Largely non-destructive
Disadvantages
Insensitive to amines, alcohols, and hydrocarbons
Limited dynamic range (102)
Other detectors:
AES, AAS, chemiluminescence reaction, MS, FTIR
Fig (p.795) ECD

11. 3. Column and Stationary Phases
3.1 Open tubular columns (Capillary)
- Wall coated (WCOT) <1 m thick liquid coating on inside of silica tubing
support-coated (SCOT) 30 m thick coating of liquid-coated support on inside of silica tubing
Best for speed and efficiency but only small samples
3.2 Packed columns
Liquid coated silica particles (< m diameter) in glass tubing
Best for large scale but slow and inefficient

12. 3.3 Immobilized liquid stationary phases
Low volatility
High decomposition temperature
Chemically inert (reversible interaction with solvent)
Chemically attached to support (prevent “bleeding”)
Appropriate k and  for good resolution
Many based on polysiloxanes or polyethylene glycol (PEG)
[Check Table 27-2 for common stationary phases for GLC]

13. Stationary phases usually bonded and/or cross-linked for longer-lasting
- Bonding – covalent linking of stationary phase to support
- Cross-linking – polymerization reactions after bonding to join individual stationary phase molecules
Film thickness (0.5-1 M) affects retention and resolution
- thicker films for volatile analytes
Non-polar stationary phases best for non-polar analytes
- non-polar analytes retained preferentially
Polar stationary phases best for polar analytes
- polar analytes retained preferentially
Chiral phases being developed for enantiomer separation (pharmaceutical)