Structural polymorph characterisation using fibre-optic linked FT-Raman-DSC.
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
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.