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Gas Chromatography

Gas Chromatography. From Harris, Quantitative Chemical Analysis, 6e, Chapter 24. Gas Chromatography. Principles Stationary phase types Detection Systems. GC Principles. Requires volatile analytes Utilises gas/liquid partition

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Gas Chromatography

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  1. Gas Chromatography From Harris, Quantitative Chemical Analysis, 6e, Chapter 24

  2. Gas Chromatography • Principles • Stationary phase types • Detection Systems

  3. GC Principles • Requires volatile analytes • Utilises gas/liquid partition • Most volatile / lowest boiling point normally elutes first • Resolution primarily influenced by a) Temperature (can have gradient) b) Flow rate (affects diffusion and interaction with stationary phase) c) Stationary phase (type and distribution) d) Column dimensions

  4. GC Stationary phase types (1) Formats include WCOT (wall coated Open tube), SCOT (support coated) and PLOT (porous-layer) From Harris, Quantitative Chemical Analysis, 6e, Chapter 24

  5. GC Stationary phase types (2) Chemistry of Stationary Phase • Chemically bonded (as opposed to coated) are the most stable. • Less polar polysiloxanes functionalised with methyl, phenyl, trifluoropropyl are common. • Polar phases include polyethylene glycols (less thermally stable) Mobile phase: Hydrogen (best), helium or nitrogen – very little scope for optimisation by mobile phase change From Harris, Quantitative Chemical Analysis, 6e, Chapter 24

  6. GC Detection Systems (1) More than 10 types available, 4 most common are: a) Flame ionisation detector (FID) • Eluent burnt in a hydrogen fuelled flame • Leads to release of electrons, dependant on [C] • Requires own thermostat at T>column oven Advantages: Robust, sensitive, semi-universal (C only), wide linear range Disadvantages: Non-selective, destructive b) Thermal ionisation detector (or nitrogen detector) • Similar to FID • Additional alkali metal salt (often rubidium chloride) component Advantages: More sensitive than FID, Selective for N or P, wide linear range Disadvantages: needs frequent renewal and careful calibration, destructive

  7. GC Detection Systems (2) c) Electron Capture detector • Senses reduction in standing current • Normally operated at 300ºC Advantages: Extremely sensitive, Selective for halogens, nitro groups, peroxides, quinones, non-destructive Disadvantages: Radioactive, limited range, easily contaminated d) Mass selective detector • Use mass spectrometry (EI or CI) • Focus on monitoring specific molecular ions quantitatively, although simple spectra are also possible Advantages: Extremely sensitive, and selective by mass Disadvantages: most easily interfaced with a low flow rate system

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