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Instrumental Lecture 14-2006

Instrumental Lecture 14-2006 - Chapter 26 An Introduction...

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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
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Intro to Chromatography Chromatography is a separation technique 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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Partitioning = type of equilibrium where the analyte divides itself between two phases For liquid-liquid extraction – two liquids For chromatography – mobile vs. stationary phases 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
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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 Example of non-linear chromatography – Pour aqueous solution of A & B on column – Allow water to drain out, both A & B stick – Wash column with 50% MeOH, A removed – Wash column with 100% MeOH, B removed Used extensively for sample cleanup in GC
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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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