Intro to Chromatographic Separations Chap 26

Originally based on separation and identification by color Originally based on separation and identification by color All have in common: All have in common: (a) Stationary phase (a) Mobile phase (eluent) (a) Separation based on differences in migration rates of components of a mixture rates of components of a mixture i.e., The extent of which components are partitioned between mobile and stationary phases between mobile and stationary phases A mobile ⇌ A stationary CHROMATOGRAPHY

Quantitative Description of Column Efficiency ► Two related terms used: (1)Plate height, H (2)Number of theoretical plates, N (3)Related by: (Eqn ) ► Efficiency increases as N ↑ and as H ↓ (Eqn ) (Eqn )

Effect of Particle Size on Plate Height Fig Smaller particles reduce H and increase resolution Smaller particles reduce H and increase resolution

Optimization of Column Performance Objectives: reduce zone broadening reduce zone broadening alter relative migration rates of solutes alter relative migration rates of solutes Column Resolution (R S ) ≡ (Eqn ) Recall that efficiency increases as N and as H lengthening column increases N lengthening column increases N lower H by altering flow rate of mobile phase or lower H by altering flow rate of mobile phase or decreasing particle size of packing

tRtRtRtR W 2σ2σ2σ2σ (Eqn ) (Eqn )

Desire R S > 1.50 The General Elution Problem in Chromatography Fig

Effect of Solvent Variation on Chromatograms Fig

Fig The General Elution Problem in Chromatography

Solution to the General Elution Problem For liquid chromatography (LC): For liquid chromatography (LC): vary k A by varying composition of mobile phase during elution (gradient elution or solvent programming) vary k A by varying composition of mobile phase during elution (gradient elution or solvent programming) For gas chromatography (GC): For gas chromatography (GC): use temperature programming use temperature programming

Result of Temperature Programming Detector Signal

Gas Chromatography Chap 27

Schematic of a Gas Chromatograph Fig. 27-1

A Soap-Bubble Flowmeter Fig. 27-2

Sample Port and Injector Must flash volatize and Must flash volatize and introduce sample onto column introduce sample onto column Heated ~ 50°C above BP of Heated ~ 50°C above BP of least volatile compound least volatile compound May be: May be: split (cap columns & GC-MS) split (cap columns & GC-MS) splitless (packed columns) splitless (packed columns) Fig 27-4

Column Configurations Two types: Packed columns (L ~ 1 – 5 m) Packed columns (L ~ 1 – 5 m) Open Tubular Column (L ~ 1 – 100 m) Open Tubular Column (L ~ 1 – 100 m)

Typical Fused Silica Capillary Column ~20 – 100 m Fig. 27-6