The solid phase degradation of L-ascorbic acid formulations.

Shephard, A.B., 1996. The solid phase degradation of L-ascorbic acid formulations. PhD, Nottingham Trent University.

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The basis of the thesis is the hypothesis that the reactivities of chemicals in the solid phase are significantly different from that in solution. The aim of the study was to investigate the chemical kinetics, degradation and means of stabilizing solid formulations of L-ascorbic acid.

Initial work investigated non-chromatographic methods for the determination of L-ascorbic acid. Due to their lack of specificity and proneness to interferences by potential degradation products chromatographic methods of analysis were developed. Samples were quantitatively determined for L-ascorbic acid using suppressed ionization reverse-phase high performance liquid chromatography. An automated precolumn sample preparation system was used for the determination of dehydroascorbic acid. Gradient elution systems were developed and used to chromatograph any degradation products formed in solution or solid formulations. The reagent 2,4-dinitrophenylhydrazine was used to monitor for any carbonyl containing degradation products.

Capillary gas chromatography failed to detect any degradation products in severely discoloured solid samples of L-ascorbic acid degraded by moisture alone although it did show different degradation patterns in solid samples degraded by basic and oxidative reagents. Normal-phase thin layer chromatography resolved coloured degradation products from L-ascorbic acid.

Chemical incompatibility between L-ascorbic acid and tablet excipients was determined using differential scanning calorimetry.

Moisture alone was found to degrade L-ascorbic acid in the solid phase and the degradation was always characterized by discolouration. Tristimulus colorimetry was used to measure the extent of colour change and results compared favourably with a quantitative assay for L-ascorbic acid in a kinetic study. Activation energies of 7.96 and 14.2 kcal/mol were obtained by tristimulus colorimetry and quantitative assay respectively. The reaction was first-order. Differences were shown between degradation in solution and in the solid phase. The coloured degradation products were isolated by gradient elution high performance chromatography. Eight different coloured degradation products were detected. Elemental analysis of the main coloured degradation product showed it to consist of 46.26% carbon, 5.42% hydrogen and 48.32% oxygen. Evidence was provided by pyrolysis of this material that it consisted of polymeric furan compounds.

Item Type: Thesis
Creators: Shephard, A.B.
Date: 1996
ISBN: 9781369313451
Divisions: Schools > School of Science and Technology
Record created by: Jeremy Silvester
Date Added: 04 Sep 2020 08:35
Last Modified: 28 Jun 2023 08:56

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