Gas chromatography is a technique used to separate and analyze mixtures that rely on differences in volatility and affinity of compounds for a mobile and stationary phase. The key components of a gas chromatography system are a carrier gas, sample injection system, column, and detector. Factors like carrier gas type, column temperature, length, diameter, and stationary phase influence separation of compounds on the column. Common detectors include thermal conductivity, flame ionization, and electron capture detectors which have different properties in terms of sensitivity, selectivity, and response characteristics.

1. Gas Chromatography
Mobile phase: Gas (H2, He, N2)
Stationary phase: Nonvolatile liquid or solid
Solute: gas or volatile liquid
GC considerations: analyte must be volatile and thermally stable
Factors that influence separation
Carrier gas type and velocity
Column temperature
Column length
Column diameter
Film thickness
Stationary phase type

2. Instrument
Apparatus – major components
1. Carrier Gas
2. Sample Injection System
3. Column
4. Detector

3. Gas Chromatography
1. Carrier Gas
• Inert
• Hydrogen, helium, nitrogen
• Often dictated by detector
2. Sample Injection System
•Temp. very important
•Avoid band broadening
•Suitable size (µL)
•Liquid/Gas: syringes

5. Gas Chromatography
3. Columns
A. Packed
B. Open Tubular
A. Packed
Stainless steel or glass
3-8 mm dia
1-5 m in length
Fine solid support coated with nonvolatile liq
Strong solid support

6. Gas Chromatography
B. Open Tubular
Fused silica with
nonvolatile liquid
coated on it
Advantages
Higher Rs
Shorter time
Less sample

7. 1. wall coated
2. support coated
3. porous layer

9. Gas Chromatography
A. Solid Support
Small uniform particles
Good mechanical strength
High surface area
Inert at high temperature
Usually 150 – 250 µm dia

10. B. Stationary Phase: Volatile liquid
Low volatility
Good thermal stability
Chemical inertness
“appreciable” solvent power
“like dissolves like”

12. Temperature
Programming
Broad range of boiling
points
Increase temperature
during sepn

14. Column Diameter

15. Column Length

16. Stationary Phase Thickness

18. Gas Chromatography
4. Detector
– Sensitive
– Stable
– Linear
– Versatile
– Response time
– Selective
Most Common
A. Thermal conductivity
B. Flame Ionization
C. Electron capture

19. Gas Chromatography
A. Thermal conductivity
– Simple
– Universal
– Change in thermal
conductivity of a gas
when an analyte is
present
– He, H2 have high TC
Properties
4-5 order of magn linear response
Simple
Responds to all analytes
Low sensitivity relative to others

20. Temp  f(TC)
R  f(Temp)
Analyte dec TC,
inc Temp, inc R
TC detector

21. Gas Chromatography
B. Flame Ionization
– Burn elutate in a mixture of H2 and air – create
ions and electrons
– Response prop. to solute mass
– Insensitive to H2, He, N2, CO2, NH3, carbonyl…
Properties
High sensitivity
Large linear response  107
Destructive
Good detection limits

22. CH + O  CHO+
+ 1e-
FI detector

23. Gas Chromatography
C. Electron Capture
– Ionize gas entering detector with high energy
electrons
– Measure current
– Analyte that has affinity for e-, captures them
Properties
Sensitive to halogens, conj. Carbonyls, nitriles, nitro compds
Selective
Small linear range, 102
Non-destructive