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Chapter 27 Gas Chromatography 1. Principles

Chapter 27 Gas Chromatography 1. Principles. Two types - Gas-solid chromatography (limited because of semipermanent retention of polar molecules) Gas-liquid chromatography Retention volume F: average volumetric flow rate mL/min, can be determined

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Chapter 27 Gas Chromatography 1. Principles

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  1. Chapter 27 Gas Chromatography1. Principles Two types - Gas-solid chromatography (limited because of semipermanent retention of polar molecules) • Gas-liquid chromatography Retention volume F: average volumetric flow rate mL/min, can be determined by measuring flow rate exiting the column using a soap bubble meter Fig. 27-2 (p.790) A soap-bubble flow meter

  2. But VR and VM depend on pressure inside column, and column has resistance to flow - At inlet, P high , F low - At outlet, P low, F high  PF = constant Pressure drop correction factor j - to calculate average pressure from inlet pressure Pi and outlet pressure P

  3. Specific retention volume Vg - semi-useful parameter for identifying species eluting Relationship between Vg and k

  4. 2. Instrumentation 2.1 Carrier gas (mobile-phase gas) He (common), N2, H2, Pi 10-50 psi above atom, F = 25-150 mL/min for packed column, 1-25 mL/min for open tubular capillary Fig. 27-1 (p.790) Block diagram of a typical GS

  5. 2.2 Sample injection - Direct injection into heated port to promote fast vaporization - Sample volume 1-20 L for packed column 10-3L for capillary column, a sample splitter is needed (1/50-1/500 to column, rest to waste)or use purge valve Fig. 27-4 (p.791) Cross-section view of a microflash vaporizer direct injector

  6. 2.3 Column - Packed column: 1- 5m - Open tubular (capillary): 1m to 100m Both constructed of coiled stainless steel/glass/Teflon, with coil diameter of 10-30 cm 2.4Oven accurate to < 1C equal or slight high than average boiling point of sample For broad boiling range--temperature programming

  7. 2.3 Column - Packed column: 1- 5m - Open tubular (capillary): 1m to 100m Both constructed of coiled stainless steel/glass/Teflon, with diameter of 10-30 cm 2.4Oven: accurate to < 1C equal or slight high than average boiling point of sample For broad boiling range--temperature programming 2. 5 Detectors Requirement and desire: 1. Sensitive (10-8 -10-15 g solutes/s) 5. fast response wide dynamic range 2. Stability and reproducibility 6. simple (reliable) 3. Linear response 7. uniform response to all analytes 4. Operate at high T (RT-400 C) 8. nondestructive

  8. Flame Ionization Detector (FID) • Sample pyrolyzed and produce current in electrical field. • Signal depends on #C atoms in organic analyte –mass sensitive not concentration sensitive • Weakly sensitive to carbonyl, amine, alcohol, amine groups • Insensitive to non-combustibles – H2O, CO2, SO2, NOX Advantages • Rugged • Sensitive (10-13 g/s) • Wide dynamic range (107) Disadvantage • Destructive Fig. 27-8 (p.794) FID

  9. Thermal Conductivity Detector (TCD) - Thermal conductivities of He and H2 much larger than organics • Organics cause T rise in detector Advantages • Simplicity • Wide linear dynamic range (105) • Responds to both organic and inorganic • Nondestructive Disadvantage • Low sensitive (10-8 g/mL carrier gas) Fig. 27-9 (p.794) TCD

  10. Electron Capture Detector (ECD) - electron from -source ionize carrier gas • organic molecules capture electron and decrease current Advantages • Simple and reliable • Sensitive to electronegative groups (halogens, peroxides) • Largely non-destructive Disadvantages • Insensitive to amines, alcohols, and hydrocarbons • Limited dynamic range (102) Other detectors: AES, AAS, chemiluminescence reaction, MS, FTIR Fig. 27-10 (p.795) ECD

  11. 3. Column and Stationary Phases 3.1 Open tubular columns (Capillary) - Wall coated (WCOT) <1 m thick liquid coating on inside of silica tubing • support-coated (SCOT) 30 m thick coating of liquid-coated support on inside of silica tubing • Best for speed and efficiency but only small samples 3.2 Packed columns • Liquid coated silica particles (<100-300 m diameter) in glass tubing • Best for large scale but slow and inefficient

  12. 3.3 Immobilized liquid stationary phases • Low volatility • High decomposition temperature • Chemically inert (reversible interaction with solvent) • Chemically attached to support (prevent “bleeding”) • Appropriate k and  for good resolution • Many based on polysiloxanes or polyethylene glycol (PEG) • [Check Table 27-2 for common stationary phases for GLC]

  13. Stationary phases usually bonded and/or cross-linked for longer-lasting - Bonding – covalent linking of stationary phase to support - Cross-linking – polymerization reactions after bonding to join individual stationary phase molecules Film thickness (0.5-1 M) affects retention and resolution - thicker films for volatile analytes Non-polar stationary phases best for non-polar analytes - non-polar analytes retained preferentially Polar stationary phases best for polar analytes - polar analytes retained preferentially Chiral phases being developed for enantiomer separation (pharmaceutical)

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