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Intro to Chromatographic Separations Chap 26

Intro to Chromatographic Separations Chap 26. CHROMATOGRAPHY. Originally based on separation and identification by color All have in common: Stationary phase Mobile phase (eluent) Separation based on differences in migration rates of components of a mixture

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Intro to Chromatographic Separations Chap 26

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  1. Intro to Chromatographic Separations Chap 26

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

  3. Quantitative Description of Column Efficiency • Two related terms used: • Plate height, H • Number of theoretical plates, N • Related by: (Eqn. 26-16) • Efficiency increases as N↑and as H↓ (Eqn. 26-17) (Eqn. 26-21)

  4. Effect of Particle Size on Plate Height Fig. 26-11 • Smaller particles reduce H and increase resolution

  5. Optimization of Column Performance • Objectives: • reduce zone broadening • alter relative migration rates of solutes • Column Resolution (RS) ≡ • (Eqn. 26-24) • Recall thatefficiency increases as N and as H • lengthening column increases N • lower H by altering flow rate of mobile phase or • decreasing particle size of packing

  6. (Eqn. 26-17) (Eqn. 26-21) tR 2σ W

  7. The General Elution Problem in Chromatography Fig. 26-15 Desire RS > 1.50

  8. Effect of Solvent Variation on Chromatograms Fig. 26-14

  9. The General Elution Problem in Chromatography Fig. 26-12

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

  11. Result of Temperature Programming Detector Signal

  12. Gas Chromatography Chap 27

  13. Schematic of a Gas Chromatograph Fig. 27-1

  14. A Soap-Bubble Flowmeter Fig. 27-2

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

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

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

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