Applications of Raman Spectroscopy in Analytical Chemistry

Abstract

This thesis investigated the applications of Raman spectroscopy in various fields of analytical chemistry. This involved using the technique for examining microplastic debris found on beaches around the Canterbury region, identifying pigments used in two unknown pieces of artwork from Auckland Museum, characterising shell composition of native marine organisms, imaging the microstructure of processed cheese, and using the low frequency region of Raman spectra to deduce crystallinity of pharmaceutical samples. Using Raman microscopy together with fluorescence-coupled microscopy, various microplastics were located and identified within beach sediment samples around Canterbury. After assembling a library of Raman spectra of common plastics, it was found that the majority of microplastic pollutants on these beaches consisted of polystyrene (54.8%), followed by polyethylene (20.5%), and polypropylene (11.0%). A further 13.7% of the suspected microplastics remained unidentified. The investigation of pigments on two fragments of unknown artworks utilised the high precision of Raman microscopy to isolate the different pigments present. Six different pigments were identified in total, including: Mars red, red lead, barium white, lead white, ivory black and lamp black. Eight different chiton species native to New Zealand were investigated with respect to their shell content. This involved taking linear maps from shells using Raman microscopy. The investigation revealed that their shells are made up of aragonite, a calcium carbonate polymorph. Towards the dorsal surface of the shell, there is typically protein and pigment present. Pigments were found to be various different carotenoids. Despite differences in the pigments present, it was found that each of the eight species present were relatively consistent with respect to shell component distribution. The investigation of processed cheese using Raman microscopy involved initially assembling a library of Raman spectra of cheese and the potential additives. Various controlled cheese formulations were made and analysed using two different Raman microscopes. Low resolution Raman images used spectra collected with a Senterra confocal Raman microscope and used either single band integrals or principal component analysis to generate Raman images. High resolution images used spectra collected with a WITec Alpha 300AR+ and used either single band integrals or band integral ratios to generate Raman images. This data showed that fat, protein, water, TSC, starch and paprika distribution could be imaged. Other additives were unable to be imaged due to low concentrations and limitations of the imaging methods. Low frequency Raman, FT-Raman, and ATR-IR spectroscopy were used to investigate the crystallinity of various mixtures of indomethacin, tryptophan and furosemide after ball milling. This involved ball-milling samples for set time periods and analysing the sample using the different techniques. Results revealed that the low frequency region of the Raman spectrum contains information inherently associated with the crystallinity of these samples, and that crystallinity decreased steadily as the samples were milled for longer periods of time. The rate of amorphisation was also found to increase if either indomethacin or furosemide was mixed in a 1:1 ratio with tryptophan. Principal component analysis also suggested that formation of a second polymorph may occur during milling of indomethacin samples.