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Multi-purpose X-ray diffractometer equipped with Kα₁ optical system and ab initio powder crystal structure analysis

Summer 2012, Volume 28, No. 2
05-10
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Akimitsu Nedu

The method of single crystal structure determination has generally been applied to characterize crystalline material. However, due to the poor quality of crystal, micro crystals or twin crystals, data collection and/or the single crystal structure analysis cannot be employed on some samples. Furthermore, even if single crystals with the good properties are obtained, there may be some restrictions to the measurement environment.  For example, the crystals might have to be kept at low temperature or in the mother liquor. There is another method for determining single crystal structures, which uses powder diffraction data. Generally, it is easier to obtain a well-behaved powder sample than a single one.  Structure determination from powder diffraction data has been applied to powder samples of single crystals.  This has been accelerated by the rapid progress in instruments and software. At the same time, however, the powder crystal structure determination has an intrinsic disadvantage. The problem is that diffraction peaks in a whole pattern profile observed from a powder crystal are sometimes difficult to be resolved because of overlapping of more than two reflections. One source of overlapping reflections is the use of Kα₁–Kα₂ radiation.  Overlapping reflections can be caused by the X-ray wave length used or the optics. In other words, it is a key point to observe the whole pattern profile with less overlapping so that the structure determination from powder diffraction data can be successful.

A molecular complex containing more than two chemical compounds in a unit cell is called “cocrystal”. Presently, a cocrystal which includes an Active Pharmaceutical Ingredient (API) is often an ingredient for new pharmaceuticals. The physicochemical properties, such as the solubility, the rate of solution and the hygroscopicity of API itself, of the cocrystal are suggested are designed to be superior to the native API. Thus, pharmaceutical companies have been trying to make a variety of cocrystals. It is necessary to determine the pharmaceutical properties for each of the chemical compounds contained in the cocrystals.  Moreover, since a cocrystal may be difficult to produce in single crystal form, ab initio powder crystal structure determination is a necessary tool.

The separation of the characteristic Kα radiation into the Kα₁ and Kα₂ components is often difficult because of the low angular resolution of the optics utilized in general laboratory instruments used in the powder diffraction measurement. As a result, the presence of profiles observed by Kα₁ and Kα₂ lines makes the structure determination from powder diffraction data more difficult. In the present technical note, a multi-purpose X-ray diffractometer equipped with a high-resolution multilayer mirror, eliminating the Kα₂ component, is introduced. Moreover, an application of ab inito power crystal structure analysis is also introduced, utilizing the above mentioned instrument.  The high-resolution multilayer mirror system is “Kα₁ optical system”.

 

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