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Gas Chromatography Gas Chromatography

From Harris, Quantitative Chemical Analysis, 6e, Chapter 24From Harris, Quantitative Chemical Analysis, 6e, Chapter 24

Created with MindGenius Business 2005Created with MindGenius Business 2005®®

Gas Chromatography Gas Chromatography

Principles Principles Stationary phase types Stationary phase types Detection Systems Detection Systems

Created with MindGenius Business 2005Created with MindGenius Business 2005®®

GC Principles GC Principles

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

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GC Stationary phase types GC Stationary phase types (1) (1)

Formats include WCOT (wall coated Open tube), SCOT (support coated) and PLOT (porous-layer)

From Harris, Quantitative Chemical Analysis, 6e, Chapter 24From Harris, Quantitative Chemical Analysis, 6e, Chapter 24

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GC Stationary phase types GC Stationary phase types (2) (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 24From Harris, Quantitative Chemical Analysis, 6e, Chapter 24

Created with MindGenius Business 2005Created with MindGenius Business 2005®®

GC Detection Systems (1) 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

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GC Detection Systems (2) GC Detection Systems (2)

c) Electron Capture detector Senses reduction in standing current Normally operated at 300ºCAdvantages: Extremely sensitive, Selective for halogens, nitro

groups, peroxides, quinones, non-destructiveDisadvantages: 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 possibleAdvantages: Extremely sensitive, and selective by massDisadvantages: most easily interfaced with a low flow rate system