Doctoral Dissertations

Date of Award

8-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Harley H. Ross, George K. Schweitzer

Committee Members

Joseph R. Peterson

Abstract

Real-time monitoring of radiolabeled compounds in liquid chromatography eluates has been accomplished in recent years by the use of two types of flow-cell detectors, heterogeneous and homogeneous. Heterogeneous cells are packed with small irregular shaped solid scintillator particles; homogeneous systems mix liquid scintillator fluid with the eluate before it enters the flow cell. Heterogeneous cells are usually preferred because sample fractions can be easily recovered and continuous use of expensive liquid scintillator is avoided. One also eliminates the chemical quenching effects and peak broadening that often occur when the liquid scintillator is mixed with a liquid chromatography solvent.

A major disadvantage of heterogenous cells, however, is that they do not provide good efficiency for low energy beta emitters, such as tritium. This is due to total absorption of the lower energy betas in the liquid before they reach the scintillator particles. Also, partial energy loss of higher energy betas results in significantly reduced pulse heights available for measurement and, consequently, fewer pulses are detected. Smaller diameter particles have been used in an attempt to overcome the absorption problem by providing improved detection geometry. This, however, has led to problems of excessive back pressure, sorption of the samples on the scintillator, and even smaller pulse heights available for measurement by the photomultiplier tubes (PMTs) because of increased scattering and absorption of light in the cell.

A new type of heterogeneous flow cell has been designed and developed which is packed with aligned scintillator-fibers instead of the solid scintillator particles currently used in heterogeneous flow cells. The present fiber-scintillator flow-cell radioactivity detector was designed to overcome the light scattering difficulties of the conventional heterogeneous type detector. Improved back pressure and sample retention characteristics were also expected due to the alignment of the fibers.

The fiber-scintillator detector has shown good detection efficiency for most beta emitting isotopes when 0.1-mm diameter fibers were used. The fiber-cell has also demonstrated improved compound retention characteristics and a back pressure which is more than a factor of 50 less than that of typical commercially available particle-scintillator cells.

The experimental detection efficiency observed with the fiber cell was, in most cases, very close to the theoretical geometric efficiency predicted by a computer model which was also developed. It is concluded that the most significant factor which limits the efficiency for most beta emitters is cell geometry. Further improvements in fiber cell efficiency would require the use of smaller and/or more uniform diameter fibers. However, for very low energy beta emitters such as tritium, it appears that the low pulse height available for detection may be the primary limiting factor.

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