Gang Wang Instrumental_Week10-12_CH26_28-2010

Gang Wang Instrumental_Week10-12_CH26_28-2010 - Chapter 26...

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Chapter 26: An Introduction to Chromatographic Separations Column Chromatography Migration Rates Distribution Contstants Retention Times Selectivity Factor Zone Broadening & Column Efficiency Optimizing Performance Resolution Intro to Chromatography Chromatography is a separation technique Many determinations involve separation followed by analysis Chromatography electrophoresis HPLC & GC are our primary focus Also discuss low pressure column chromatography & TLC (thin layer) All chromatographic techniques have Stationary phase – solid or viscous liquid phase typically in a column Mobile phase–moves sample in contact with stationary phase
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Chromatography: sample transported by mobile phase electrostatic or van der Waals' some components in sample interact more strongly with stationary phase and are more strongly retained sample separated into zones or bands Elution Chromatography: flushing of sample through column by continual mobile phase (eluent) addition migration rate fraction time spent in mobile phase Planar chromatography - flat stationary phase, mobile phase moves through capillary action or gravity Column chromatography - tube of stationary phase, mobile phase moves by pressure or gravity
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Overview of chromatographic process – packed column Inject at t o Separate t 1 to t 3 Detect at t 4 Å Resulting chromatogram A & B retained by column differently B has higher K B takes longer to elute from column Detector sees A first then B Peak heights & peak areas are proportional to conc. Band broadening
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Important: chromatogram (concentration versus elution time) • more strongly retained species elutes last (elution order) • analyte is " diluted " during elution (dispersion) zone broadening proportional to elution time By changing experimental conditions, non-separated bands can be separated (A) adjust migration rates for A and B (increase band separation) (B) adjust zone broadening (decrease band spread)
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Partitioning = type of equilibrium where the analyte divides itself between two phases For liquid liquid extraction–two liquids For chromatography – mobile vs. stationary phases Analyte A in equilibrium with two phases A mobile A stationary Define a partition ratio K (or distribution constant ) C s where C s & C M are K = ‐‐‐‐‐‐‐ C M concentrations of analyte in stationary & mobile phases Prefer if K is constant over conc. range If not constant we can work in a narrow range where it is constant This is linear chromatography From now on everything is linear chromatography In linear chromatography a constant flow rate of mobile phase moves through column K is typically constant or nearly constant Elution = process by which analyte is flushed through the column by mobile phase (which could be a liquid or a gas)
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tM = time for unretained molecule to reach detector or dead time
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Gang Wang Instrumental_Week10-12_CH26_28-2010 - Chapter 26...

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