Gas Chromatography Mobile phase: Gas (H 2 , He, N 2 ) 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
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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 separationCarrier gas type and velocityColumn temperatureColumn lengthColumn diameterFilm thicknessStationary phase type
Instrument
Apparatus – major components1. Carrier Gas2. Sample Injection System3. Column4. Detector
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 (L)•Liquid/Gas: syringes
Gas Chromatography
3. Columns
A. Packed
B. Open Tubular
A. PackedStainless steel or glass
3-8 mm dia1-5 m in length
Fine solid support coated with nonvolatile liqStrong solid support
Gas Chromatography
B. Open Tubular
Fused silica with nonvolatile liquid coated on it
Advantages Higher Rs
Shorter time Less sample
1. wall coated2. support coated3. porous layer
Gas Chromatography
A. Solid Support
Small uniform particles
Good mechanical strength
High surface areaInert at high temperatureUsually 150 – 250 m dia
B. Stationary Phase: Volatile liquid
Low volatilityGood thermal stability
Chemical inertness
“appreciable” solvent power“like dissolves like”
Temperature Programming
Broad range of boiling points
Increase temperature during sepn
Column Diameter
Column Length
Stationary Phase Thickness
Gas Chromatography
4. Detector– Sensitive– Stable – Linear– Versatile– Response time– Selective
Most Common
A. Thermal conductivity
B. Flame Ionization
C. Electron capture
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
Temp f(TC)R f(Temp)
Analyte dec TC, inc Temp, inc R
TC detector
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
CH + O CHO+ + 1e-
FI detector
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
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