Doctoral Dissertations

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Georges A. Guiochon

Abstract

Direct on-column detection is a beneficial method to study the performance of liquid chromatography columns. Extra column contributions from column tubing,column end fittings, and detector effects are eliminated. The research presented here presents a novel detection method for making direct on-column measurement of column performance. By matching the refractive indices of the mobile phase,the stationary phase and the material of the bed enclosure, one can render transparent to the eye the chromatographic column which is normally opaque in nature. As a result, band visualization is readily obtained. High definition oncidium detection becomes feasible by using a photographic detector instead of the conventional post-column, on-line (UVA/is or similar) detector. Quantitative Information regarding the concentration distribution in the band is obtained by utilizing optical scanners to obtain a digital image and computer imaging software.The processes of data collection and image analysis are discussed in detail and are illustrated by obsen/ing the concept of the infinite diameter column following a central point injection. The performance of the photographic detection methods compared to that of regular detection procedures. A calibration procedure is illustrated and suitable corrections are introduced to account for the transverse variation of the optical path length, as a result of using a cylindrical detector cell (the column itself), and for the refraction of light due to the differences between the refractive indices of the glass wall and the liquids involved. An error analysis is also reported. It shows that the method can reliably produce results with a precision of a few percent, allowing on-column evaluation of column performance and the derivation of the radial distributions of the column efficiency, the migration velocity of the zone, and the sample distribution at the head of the column.A number of columns, as similar as possible, were fitted with different 2pm and 10 pm-porosity stainless-steel frits of similar and varied sizes. Subsequentanalysis of the digitized band images revealed irregularities in the band shape resulting from frit contributions to band dispersion. The 2 pm frits produced more dramatic effects overall than the coarser frits. The smallest diameter fruit induced severely parabolic sample distributions at the column inlet compared to the larger fruit. Local axial dispersion coefficient values, expressed as local reduced plate height, were calculated. The results demonstrate the possibly damaging effects of the frit on the band shape.The study also shows that, under conditions where viscous fingering is prevalent, the design of the inlet header has little influence on the outcome of the viscous fingers. Two types of headers were studied. The first contained a small diameter inlet frit, which localized the majority of the sample in or near the central region of the column. The second header contained a wide frit and produced amore uniform radial distribution of the sample. In both cases, the extent of viscous fingering was essentially the same.

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