Capillary electrochromatographic separations of polycyclic aromatic hydrocarbons

One of the limitations of capillary electrophoresis (CE) is the inability to separate neutral analytes. This is particularly significant in environmental analysis since many pollutants and toxins are neutral species. One set of analytes of interest are polycyclic aromatic hydrocarbons (PAHs). CE can be expanded to separate neutrals in two ways.

The first is to pack the capillaries with a stationary phase. Neutral solutes can then partition between the mobile and stationary phases the same as in high performance liquid chromatography (HPLC). This technique, dubbed capillary electrochromatography (CEC), is very similar to HPLC except in CEC an electric field is used to pump the solvent through the capillary. The work presented herein on CEC focuses on the use of non-aqueous solvents (acetonitrile modified with methylene chloride or tetrahydrofuran) for the separation of PAHs and fullerenes. Non-aqueous CEC was shown to provide separations of PAHs with efficiencies as high as 160,000 plates/m. The effects of mobile phase composition on such factor as electroosmotic flow, plate height, and capacity factor (k’) are reported.

The second way is to introduce an electrophoretically mediated phase to the running buffer. Solutes can then associate with this phase and thus acquire an electrophoretic mobility. The phase system presented here uses native (neutral) and derivatized (charged) cyclodextrins (CDs). CDs are cylindrically shaped macrocyclic sugar molecules that possess a hydrophobic cavity and a hydrophilic exterior. In this technique, dubbed cyclodextrin distribution capillary chromatography (CDCE), solutes are separated based on their differential distribution between neutral and charged CDs. CDCE was shown to provide unique selectivity and good resolution of methyl substituted anthracenes. Control of retention is possible through varying the concentrations and types of CDs employed. Laser-induced fluorescence provided detection limits in the low-to-subparts per billion range. Field strength and total CD concentration exert a substantial influence on efficiency. Analysis of derivatized CDs with CE reveals information about their composition (range and degree of substitution). Molecular modeling is also employed to investigate the effect of substitution on the CD shape. The effects of CD cavity size on the elution order of various PAHs are also reported.