Chemical Equilibrium Theory, Part 7 - Heterogeneous Equilibria, Liquid Chromatography

Greetings,

This post shows how chemical equilibrium concepts can be used to explain the separation of compounds in liquid chromatography.

Liquid Chromatography Fundamentals

Chromatography gets its name from the phrase, "color-writing".  The first recorded use of chromatography was to separate plant pigments which is attributed to Mikhail Tswett, a great Russian scientist of the late 1800's and early 1900's.  Liquid-Solid Low-Pressure Column Chromatography Utilizes a Liquid Mobile Phase and a Solid Stationary Phase.  The mobile phase moves within and through the solid particles of the stationary phase.  The stationary phase is commonly silica gel, prepared by mixing mobile phase with enough dry, fine silica to produce a thick, yet pourable, slurry.  The slurry is next carefully transferred to a vertically-mounted glass tube equipped with a glass wool plug or a sintered glass bottom.  The tube (aka, 'column') should also be equipped with a drainage stopcock.  A suitable mobile phase for dye separation is a dilute aqueous iospropanol (aka, isopropyl alcohol) solution.  A sample mixture of three dyes can be prepared by combining equal weights with enough mobile phase to give a 10 % (w/v) solution.  A thin layer of purified sand is often applied to the silica gel top surface upon addition of the sample solution to prevent agitation of the starting stationary phase.  The sample solution can be applied via pipet to a thickness of 1-2 cm.

Phase Partitioning and Component Separation

As the sample mixture moves down the column equilibria are established between each sample component (the dyes in our case) and the stationary phase.  With a suitable mobile phase, the mobile to stationary phase equilibria will vary in position resulting in components spending different lengths of time in the stationary phase.  Or, in other words, the inter-phase partitioning of the sample components differs which leads to separate degrees of retention and consequently different component migration speeds.  The following figure provides a visual explanation.

Note the "band broadening" of each component depicted in the figure.  A good "chromatographer" will strive to minimize band broadening which can aid in the level of separation between eluting sample components.  Separation of sample components will be maximized by evenly distributing the stationary phase, application of a compact sample "slug" at the column top, minimum disruption of the stationary phase as mobile phase is added, and a judicious choice of mobile phase.

That's all for this post.  As always, thank you for reading! 

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