chapter 24 gc

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