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Transcript
Chapter 27 Gas Chromatography 1. Principles Two types - Gas-solid chromatography (limited because of semipermanent retention of polar molecules) Gas-liquid chromatography Retention volume VR t R F retained VM t M F non retained F: average volumetric flow rate mL/min, can be determined by measuring flow rate exiting the column using a soap bubble meter Tc ( P Pwater ) F Fm T P Fig. 27-2 (p.790) A soap-bubble flow meter 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 PF = constant Pressure drop correction factor j - to calculate average pressure from inlet pressure Pi and outlet pressure P 3[( Pi / P) 2 1] j 2[( Pi / P) 3 1] corrected retention volume VR0 j t R F VM0 j t M F Specific retention volume Vg - semi-useful parameter for identifying species eluting Vg VR0 VM0 ms mass of stationaryphase 273 Tc jF (t R t M ) 273 ms Tc column temperat ure Relationship between Vg and k (k tR tM ) tM VM0 k 273 Vg mS Tc (k KVS ) VM Vg KVS 273 mS Tc K S densityof liquid on stationaryphase 273 Tc 2. Instrumentation Fig. 27-1 (p.790) Block diagram of a typical GS 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 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 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 2.4 Oven accurate to < 1C equal or slight high than average boiling point of sample For broad boiling range--temperature programming 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 2.4 Oven: accurate to < 1C 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) 2. Stability and reproducibility 3. Linear response 4. Operate at high T (RT-400 C) 5. fast response wide dynamic range 6. simple (reliable) 7. uniform response to all analytes 8. nondestructive 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 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 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 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 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] 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)
