Identification of crystalline and amorphous regions in low molecular weight materials using microthermal analysis

P. G. Royall, V. L. Kett, C. S. Andrews, D.Q.M Craig

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The use of microthermal analysis as a means of differentiating between amorphous and crystalline forms of a model low molecular weight solid has been investigated, with a view to establishing the strengths and limitations of the technique as a means of identifying different physical forms of the same substance within a single sample. Samples of the drug indometacin were prepared in amorphous, crystalline, and partially crystalline forms and the samples studied using microthermal analysis (MTA) and modulated temperature differential scanning calorimetry (MTDSC). MTDSC studies revealed a simple melting process for the crystalline material corresponding to the ? form of the drug, while a glass transition, recrystallization and subsequent melting was seen for the amorphous indometacin; in addition, the sample demonstrated a shift in the phase angle at approximately 65 °C which has been previously associated with flow of the sample in the DSC pan. MTA studies indicated that the thermal conductivity is dominated by the surface topology and no clear differentiation between amorphous and crystalline regions was obtained in this mode. A discontinuity was seen in the localized thermal analysis (LTA) profile for the crystalline sample at 152 °C corresponding to the melting of the material. Similarly, the amorphous material showed a shift in probe position at 64 °C, these responses being ascribed to sample softening. Topographical examination of the sample following LTA studies indicated the presence of a “crater” approximately 10-20 µm in diameter, giving an indication of the scale of scrutiny of the experiment. Studies using different scanning rates (2 to 20 °C/s) did not demonstrate significant changes to the temperatures of transition and crater size. Studies on partially crystalline samples clearly showed that the technique was able to differentiate between the two physical regions in LTA mode. The technique may therefore have considerable potential as a means of identifying distinct physical regions in a single sample. However, consideration needs to be given to the experimental and interpretative issues raised in this investigation.
Original languageEnglish
Pages (from-to)7021-7026
Number of pages6
JournalThe Journal of Physical Chemistry B
Issue number29
Publication statusPublished - 29 Jun 2001

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