421-821-chapter-28

421-821-chapter-28 - Liquid Chromatography A.)...

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Liquid Chromatography A.) Introduction : Liquid Chromatography (LC) is a chromatographic technique in which the mobile phase is a liquid. LC is a much older technique than GC, but was overshadowed by the rapid development of GC in the 1950’s and 1960’s. LC is currently the dominate type of chromatography and is even replacing GC in its more traditional applications.
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Advantages of LC compared to GC: 1.) LC can be applied to the separation of any compound that is soluble in a liquid phase. LC more useful in the separation of biological compounds, synthetic or natural polymers, and inorganic compounds 2.) Liquid mobile phase allows LC to be used at lower temperatures than required by GC LC better suited than GC for separating compounds that may be thermally labile 3.) Retention of solutes in LC depend on their interaction with both the mobile phase and stationary phase. GC retention based on volatility and interaction with stationary phase LC is more flexible in optimizing separations change either stationary or mobile phase 4.) Most LC detectors are non-destructive most GC detectors are destructive LC is better suited for preparative or process-scale separations Disadvantage of LC compared to GC: 1.) LC is subject to greater peak or band-broadening. much larger diffusion coefficients of solutes in gases vs. liquids
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B.) Low- and High-performance Liquid Chromatography : Many types of liquid chromatography are available, based on different stationary phase and mobile phase combinations. - each type may be further characterized based on its overall efficiency or performance Molecular mass
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Low-performance liquid chromatography – LC methods that use large, non-rigid support material particles > 40 μ m in diameter poor system efficiencies and large plate heights such systems have the following characteristics: broad peaks poor limits of detection long separation times columns can only tolerate low operating pressures i gravity flow or peristaltic pump to apply mobile phase to column
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Solvent reservoir Column head Column Column packing Porous glass plate Column chromatography – an example of the equipment used in low-performance liquid chromatography Sample is usually applied directly to the top of the column. Detection is by fraction collection with later analysis of each fraction
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Low-performance liquid chromatography advantages : – simple system requirements low cost popular in sample purification used in the removal of interferences from samples – used in some analytical applications not common due to low efficiency, long analysis times and poor limits of detection
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– LC methods that use small, uniform, rigid support material particles < 40 μ m in diameter usually 3-10 μ m in practice good system efficiencies and small plate heights such systems have the following characteristics: narrow peaks
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This note was uploaded on 05/09/2010 for the course CHEM 221 taught by Professor Dr.robertpowers during the Fall '07 term at San Diego.

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421-821-chapter-28 - Liquid Chromatography A.)...

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