Question 1: Two chromatographic columns (GC) are being compared for use in an an

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• Question 1:
  Two chromatographic columns (GC) are being compared for use in an analysis. One is a 50 cm long packed column that is 5 mm in inside diameter (ID), with a stationary phase that is 1um thick on the support packing and has a total of 25,000 theoretical plates. The other column is 1 meter long, also 5 mm ID, with the same thickness of stationary phase on the support, and has 40,000 theoretical plates. Ignoring any other variables:

1.   a)      Which column would provide the best separation of sample peaks? Why?
2. b)      Which column would provide the sharpest peaks? Why?
3. c)If the 50 cm column had a stationary phase thickness of 2 um on the support, how would that affect the results? (Hint-

Explanation / Answer

In column chromatography, the stationary phase, a solid adsorbent, is placed in a vertical glass (usually) column. The mobile phase, a liquid, is added to the top and flows down through the column by either gravity or external pressure. Column chromatography is generally used as a purification technique: it isolates desired compounds from a mixture.

The mixture to be analyzed by column chromatrography is placed inside the top of the column. The liquid solvent (the eluent) is passed through the column by gravity or by the application of air pressure. An equilibrium is established between the solute adsorbed on the adsorbent and the eluting solvent flowing down through the column. Because the different components in the mixture have different interactions with the stationary and mobile phases, they will be carried along with the mobile phase to varying degrees and a separation will be achieved. The individual components, or elutants, are collected as the solvent drips from the bottom of the column.

Column chromatography is separated into two categories, depending on how the solvent flows down the column. If the solvent is allowed to flow down the column by gravity, or percolation, it is calledgravity column chromatography. If the solvent is forced down the column by positive air pressure, it is calledflash chromatography, a "state of the art" method currently used in organic chemistry research laboratories The term "flash chromatography" was coined by Professor W. Clark Still because it can be done in a flash.

Silica gel (SiO2) and alumina (Al2O3) are two adsorbents commonly used by the organic chemist for column chromatography. An example of each of these adsorbents is shown below.

These adsorbents are sold in different mesh sizes, as indicated by a number on the bottle label: "silica gel 60" or "silica gel 230-400" are a couple of examples. This number refers to the mesh of the sieve used to size the silica, specifically, the number of holes in the mesh or sieve through which the crude silica particle mixture is passed in the manufacturing process. If there are more holes per unit area, those holes are smaller, thus allowing only smaller silica particles go through the sieve. The larger the mesh size, the smaller the adsorbent particles. Adsorbent particle size affects how the solvent flows through the column. Smaller particles (higher mesh values) are used for flash chromatography, larger particles (lower mesh values) are used for gravity chromatography. For example, 70

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