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Chromatography puzzling May 22, 2008

Posted by fetzthechemist in Speculation.

Certain reversed-phase separatons of isomers puzzle me. The current theory of the mechanism just does not seem good enough.

The fullerene isomers of C78 separate readily into three peaks in non-aqueous reversed phase. So do certain isomer pairs of polycyclic aromatic hydrocarbons of nine through twelve rings. These isomers vary in dimensionality by a few picometers.

Current theory, first proposed by Lane Sander and Steve Wise at NIST, says that the reversed phases used separate by size and shape. These phases use a dichloro or trichlorosilane, rather than the usual monochlorosilane, to bond the C18 groups to the silica surface. The silane reacts more than once, creating a short polysiloxane backbone with C18 groups off of it – like a comb with teeth. These are so large and bulky that they arrange themselves, orienting into parallel planes of lipophilic phase. The analytes permeate between these planes.

For the separation of the isomeric fullerenes or PAHs to result requires a very, very narrow distribution of pore spaces. These C18 planes move and are squishy, so their distribution veries by several Angstroms, tenths of a nanometer. The analytes differ by ahundred times smaller sizes.

So how do these molecules separate so efficiently? Is there a secondary effect based on other interactions?



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