Date Approved

2009

Degree Type

Open Access Senior Honors Thesis

Department or School

Chemistry

Abstract

Tunable selectivity provides a relatively simple and inexpensive way to manipulate peak positions and gain resolution in chromatographic separations. Length tuning utilizes two columns of different polarities connected in series. Selectivity is manipulated by changing the relative lengths of the two columns. However, a direct correlation is not seen between relative length and effective contribution due to gas compression effects. Rather, a direct correlation is observed between the carrier gas transport time through a segment of column (relative to the total carrier gas transport time) and the effective contribution of that segment. This relationship has been used to predict retention data for analytes in a target mixture, and to determine the combination of columns that would result in the best resolution overall. Further examination reveals that at a given length fraction, the effective contributions of the columns in series are independent of inlet pressure. The relative resolution, a measure of peak separation independent of peak width, is thus constant. In contrast, the resolution calculated using the Purnell Equation does depend on inlet pressure, in accordance with the plate height measured for each individual column and its fractional contribution to a tandem-column separation. Retention factors, peak widths, and plate heights for aromatic molecules of varying functionality, and homologous series of alkanes and alcohols were measured over a range of pressures using both individual columns and several different tandem-column combinations. Measured values of overall plate height and resolution closely matched theoretical predictions and it was determined that theoretical surface plots could be used to accurately predict optimal length fractions for separation.

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