Vitamin assay by means of ultraviolet reflectance spectroscopy

Typescript.Thesis (Ph. D.)--University of Hawaii, 1966.Bibliography: leaves [92]-94.ix, 94 l illus., tablesA procedure whereby ultraviolet reflectance spectroscopy can be employed for the analysis of substances resolved on thin-layer plates was developed with the use of aspirin-salicylic acid mixtures that had been separated on silica gel plates. This system was selected for study not only because its salicylic acid-silica gel G components was relatively stable, but also because no difficulties were encountered in locating the resolved compounds. Both appeared as yellowish-brown spots when the chromatop1ates were dried. The optimum range and maximum accuracy of such analyses were then deduced by applying two graphical methods to data obtained with the use of two systems--rhodamine B adsorbed on silica gel G, which absorbs in the visible, and aspirin adsorbed on silica gel G, which absorbs in the ultraviolet. Plots of experimental data were contrasted with plots that might be expected for an ideal system that conforms to the Kubelka-Munk equation. The results seemed to indicate that the minimum error to be expected is of the order of 6% per 1% reflectance reading error, and that the optimum range for analysis can be arrived at after plotting the reflectance data according to either of the methods discussed, regardless of whether the system in question conforms to the Kubelka-Munk equation or not. The procedure developed with the aspirin-salicylic acid mixtures was then applied to the analysis of five vitamins of the B group-thiamine hydrochloride, pyridoxine hydrochloride, nicotinic acid, nicotinamide and p-aminobenzoic acid. The suitability of two techniques devised to locate the resolved vitamins prior to analysis was investigated. One method involved the observation, under ultraviolet light, of chromatoplates prepared with adsorbent containing fluorescent material; the other involved scanning of the plates by means of a spectrophotometer set at an appropriate wavelength. All but two of vitamins could be identified by means of their reflectance spectra, with the two having identical spectra being distinguished with the aid of their Rf values. The procedure also provided quantitative data having a standard deviation of 0.3-0.4 reflectance unit for the vitamins studied. Finally the utility of the procedure in the analysis of relatively unstable substances was demonstrated by employing it in conjunction with the radioactive tracer technique to determine small amounts of ascorbic acid. Ascorbic acid which had been tagged with the radioactive compound was separated on thin-layer plates and then determined by reflectance spectrophotometric titration. By also measuring the radioactivity of the sample both before and "after the development of the chromatoplates, it was possible to estimate the amount of acid that was originally present. Because the reflectance and radioactivity measurements were carried out on the same analytical sample, which was held in a simple, windowless cell designed for this purpose, it was unnecessary to prepare a separate sample for the radioassay