The difference between the patterns is immediately obvious. The resulting diffraction patterns of the two samples are shown below. The underground sample you would be able to distinguish individual grain not only by feel but sight as well. It would resemble a flour such that if you were to rub the sample between your fingers you would not be able to feel the individual grains. The well ground sample has a particles size of at most 44 microns. The figure below illustrates this relationship for a single phase sample prepared properly, then poorly. Particle size is inversely related to both the degree of randomness of the crystallites and the measured intensity. The best way to make sure these conditions are met is to finely powder your sample.Įrrors Caused By Materials and Improper Preparation Sufficient diffraction intensity to meet satisfy counting statistics.Sufficient number of crystallites to get a representative intensity distribution for the sample.Total randomness of crystallite orientations.Ideally, you need to achieve three conditions in order to have good data: If you do not put in the effort to properly prepare your sample you can introduce errors that make phase identification difficult to impossible and estimates of abundances and crystallinity erroneous. However, since phase separation has occurred, the solid nanosuspensions would be expected to exhibit a greater tendency for physical instability under a given stress, that is, crystallization, than would a miscible system.Proper sample preparation is essential to getting highly quality XRD data. Such systems would be expected to have properties intermediate to those observed for miscible and macroscopically phase separated amorphous dispersions. Since DSC can not detect two T(g) values when phase separation produces amorphous domains with sizes less than approximately 30 nm, it is concluded that the trehalose-dextran system is a phase separated mixture with a structure equivalent to a solid nanosuspension having nanosize domains. In the case of the trehalose-dextran mixture, where only one T(g) value was detected, however, PDF analysis clearly revealed phase separation. From the PDF analysis, indomethacin-PVP was shown to be completely miscible in agreement with the single T(g) value measured for the mixture. In agreement with DSC measurements that detected two independent T(g) values for the dextran-PVP mixture, the PDF profiles of the mixture matched very well indicating a phase separated system. A lack of agreement of the PDF profiles indicates that the mixture with a unique PDF is miscible. Immiscibility is detected when the PDF profiles of each individual component taken in proportion to their compositions in the mixture agree with the PDF of the mixture, indicating phase separation into independent amorphous phases. The mixtures chosen were: dextran-poly(vinylpyrrolidone) (PVP) and trehalose-dextran, both prepared by lyophilization and indomethacin-PVP, prepared by evaporation from organic solvent. Recognizing limitations with the standard method of determining whether an amorphous API-polymer mixture is miscible based on the number of glass transition temperatures (T(g)) using differential scanning calorimetry (DSC) measurements, we have developed an X-ray powder diffraction (XRPD) method coupled with computation of pair distribution functions (PDF), to more fully assess miscibility in such systems.
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