Date of Award


Document Type

Master Thesis

Degree Name

Masters of Science (Research)


Applied Physics & Instrumentation

First Advisor

Dr. Niall Smith


One of the major challenges facing the pharmaceutical industry is the control of the amount of trace contaminants during the manufacture of organic substances. Lowering the level of contamination improves the quality of the product and reduces possible side effects.

One of the main techniques used to detect trace contaminants is UV-VIS spectroscopy, where the unique absorption fingerprint of a chemical reveals its presence. UV-VIS absorption is based upon the Beer-Lambert Law. The critical point to note about this law is that the absorption is exponentially dependent on both the concentration of the absorber and the pathlength traversed by the light passing through the absorber. Since this thesis is concerned with the development of a novel detection system for UV-VIS spectroscopy in which the minimum detectable concentration of a contaminant is reduced, we examined methods for increasing the pathlength..

Current UV-VIS systems generally use single-path absorption techniques. Multi-pass absorption cells do exist, and these increase the pathlength by reflecting light many times through the sample volume. However, all are expensive and complex to construct. They are also sensitive to movement.

In this thesis, we describe novel designs for a multi-pass absorption cell. The design criteria required the cells to be inexpensive, simple to construct, and robust in operation so that they could potentially be used in a real-life scenario and not just in a laboratory. The first design consisted of two parallel flat mirrors. Extensive simulations were performed to determine how well we might expect the configuration to perform. The results of the simulations revealed that it is possible to substantially increase the pathlength with two parallel mirrors, but only if they are parallel to a very high tolerance.

To circumvent the strict parallelism required by the first design, a second design was developed. This design, consisting of a cube of six internally reflecting mirrors, is unique. Light is coupled into and out of the cell by two small apertures. Light entering the cell is reflected within many times, each time passing through the cuvette and the absorbing sample. Consequently, the pathlength is increased. The smaller the apertures, the greater the increase in pathlength. Tests with the cell initially showed very encouraging results, but after some problems with the CCD detection system it was not possible to generate the same results. Nevertheless, the unique design of the cell shows promise and requires further work in order to characterise and optimise its performance.


Presented to H.E.T.A.C. for the award of Masters Degree in Science in Applied Physics and Instrumentation

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