Raman spectroscopy for analytical applications

Abstract

Raman spectroscopy was used in combination with chemometrics for applications requiring both bulk and micro scale analysis. The bulk analysis studies involved complex mixtures, with the specific studies looking at detection of counterfeit medicines and determining the composition of human gallstones. The studies involving Raman spectroscopy in micro scale domains included probing the chemical composition of hypomineralised lesions in human teeth and monitoring treatment of these lesions, an investigation into malformed sand tiger shark teeth and studying the chemical and physical properties of venomous snake fangs.

Genuine, generic and counterfeit Cialis® tablets were non-destructively measured using Raman spectroscopy and classified as genuine or counterfeit/generic using soft independent modeling of class analogy (SIMCA) classification. The Raman spectra were also used to identify and semi-quantify the active pharmaceutical ingredients (API) in the samples. The selection of appropriate preprocessing methods was found to be very important for the models predictive accuracy. Gallstones from 107 patients were analysed using Raman spectroscopy and multivariate curve resolution and major components key band integrals to determine the gallstones compositions. The compositions were used to classify the gallstones, with 82 (77 %) of the patients having cholesterol/mixed gallstones, 21 (20 %) patients had pigment type gallstones and 4 (4 %) patients had unassigned gallstone type. Age was found to correlate with gallstone type, with younger patients having a higher incidence of cholesterol/mixed stones and older patients having a higher incidence of pigment stones. There were no other strong correlations of stone type with parameters such as ethnicity, and aetiological factors. Hypomineralised lesions of varying severity on human teeth were mapped using Raman microscopy. These Raman maps showed lower levels of the mineral components of teeth in the hypomineralised lesion compared with the normal enamel. There were also changes in the mineral component in the hypomineralised regions, indicating increased disruption in the hydroxyapatite crystalline lattice of the hypomineralised regions compared with normal enamel. The resin treatment with various pre-treatment regimes were monitored and compared using Raman spectroscopy and the pre-treatment methods which involved both demineralisation and oxidation of organic constituents steps had the highest level of resin infiltration. Malformed sand tiger shark teeth from captive animals were studied to determine why their teeth had not formed correctly. The malformed teeth were found to contain little to no fluoroapatite in the enameloid layer of the teeth, the dentine regions were spectroscopically very similar to the control teeth. The malformation appears to be due to the lack of mineral component in the enameloid region, only the inorganic component of the enameloid were present. The Raman spectra of snake fangs from the cobra and adder species were analysed. It was found that the composition was similar to that of human dentine. This is consistent with the requirement of such fangs to be tough rather than hard.