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Title: Structural polymorph characterisation using fibre-optic linked FT-Raman-DSC.
Author: Sprunt, John.
ISNI:       0000 0001 3475 7149
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 1998
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A low-cost fibre-optic probe was constructed, and used to provide an optical link between an FT-Raman spectrometer and a Differential Scanning Calorimeter (DSC). The resulting Simultaneous Raman-DSC (SRD) method combines the vibrational structural information of FT-Raman spectroscopy with the thermal data of DSC, which is particularly useful for characterisation of samples exhibiting temperature- and thermal conditioning-dependent polymorphic (i. e. structural) changes. The samples chosen for method development and structural study were ammonium nitrate (NH4NO3), the triglyceride sn-1,3-distearoyl-2-oleoyl glycerol (SOS), cocoa butter (which is chiefly composed of a mixture of monounsaturated triglycerides), and some chocolates of differing composition. The bifurcated `6-around-i' parallel fibres probe design allowed Raman spectra to be collected from unencapsulateds amplesi n the normal DSC sample position without extensive mechanical modifications or the use of extra optical components. Method development using the above samples showed that the combined SRD sampling has various effects on collected data. With this probe design, a "glass background" spectrum is superimposed on the Raman spectrum, and must be subtracted using the spectrometer software. At raised temperatures, a thermal emission feature appears in the Raman spectrum at high wavenumber shift. The incident laser light was found to raise sample temperatures by around 1.5-3K. Raman spectra were therefore collected with samples held around 10K below transition onset temperatures. The necessary lack of encapsulation resulted in broadened thermal transitions in DSC heating curves, and potential heat loss due to radiative emission at raised temperatures. However, for samples analysed near room temperature, onset melting points for transitions with the laser off were not significantly altered by SRD sampling. Ammonium nitrate is often irreproducible in it's exact phase transition thermal behaviour. SRD analysis showed the expected phases IV, III, II and I at room temperature and above at normal atmospheric pressure. Raman spectra could then be unambiguously assigned to the respective phases. Comparison with previous literature work allows the spectral changes seen to be interpreted using symmetry-based rationalisations. The triglyceride SOS was found to conform well to the literature thermal behaviour. SRD analysis showed DSC heating curves with onset melting points and features identifying the polymorphs produced as a, , y, ß', ß2 and [il. A literature survey allows the corresponding Raman spectra to be interpreted in the light of structural packing suggestions made by workers using various other analytical techniques. Cocoa butter was successfully prepared in states 3,4,5 and 6 of the six polymorphs known to exist. DSC melting behaviour and heating curve shapes were used to unambiguously relate Raman spectra to the individual polymorphs. Interpretation of Raman spectra was based on the SOS triglyceride SRD results, together with structural packing suggestions from the literature. A selection of chocolate samples were also analysed by SRD. Those with a high cocoa butter content were found to approximate well to the thermal and structural behaviour of pure cocoa butter.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Molecular vibration; Spectroscopy