Structure Analysis by X-Ray Diffraction
The wavelength of X-rays is within the same order of magnitude as the distance of atoms or molecules in crystals, i.e. some tenths of a nanometer. In addition, XR interfere with electrons, and space in crystals is filled with electrons (and some nuclei).
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Structure Analysis by X-Ray Diffraction
8.1 Fundamentals
8.3 Organic Pigments
The wavelength of X-rays is within the same order of magnitude as the distance of atoms or molecules in crystals, i.e. some tenths of a nanometer. In addition, XR interfere with electrons, and space in crystals is filled with electrons (and some nuclei). Thus, XR traversing crystals suffer diffraction and interference. The interference patterns can be used for crystal structure elucidation; at the same time they are characteristic for the substance the crystals are made of. Most organic substances are composed of C,H,N,O and have few electrons compared with compounds containing heavy atoms. They are therefore poor scatterers; to obtain XR diagrams of CHNO crystals using conventional XR sources demands several hours of exposure times. The availability of the intense cyclotron radiation yields good diagrams in reasonably short times.
The problems with XRD of organic pigments can be summed up like this (Curry et aI., 10.2.3; with minor changes):
8.2 Inorganic Pigments and Fillers All inorganic pigments and fillers contain heavy atoms and are thus good scatterers. This is advantageous if pigmented paints or pigmented amorphous polymers (plastics, rubber) have to be analysed. Several books and publications contain data on XRD of pigments. A total of 71 XRD patterns (bar graphs) are found in the book of Scholl (l.c.); unfortunately, no spacings and intensities are given in numbers. More data may be found in the powder diffraction file of the International Centre for Diffraction Data (N.N., 10.1). Earlier though valuable sources are the books of Konig and Kittel (both 10.1). A useful source of reference in XRD data on synthetic dyes and pigments is the contribution by Whitaker in the book ofVenkataraman (10.1).
D. O. Hummel, Atlas of Plastics Additives © Springer-Verlag Berlin Heidelberg 2002
1. Organic pigments are poor scatterers. 2. Many organic OP, due to their intense coloration, are applied in low concentrations. Without separation, their XRD are weak and difficult to interpret. 3. The unit cells of OP are generally much larger than those of inorganic pigments. Thus, their diffraction lines are confined to lowe angles. Unfortunately, this region has, in the Debye-Scherrer technique, the highest background and may drown weak diffraction lines. 4. OP in paints may be non-crystalline and will then yield no XRD pattern. 5. Many OP may be derived from one parent compound, they belong then to a structurally similar family. Changes in chemical substitutions or inserted metal produce dramatic colour changes yet sometimes only subtle changes in XRD patterns.
Despite these problems the authors were able to produce valuable data for the identification of organic pigments in the forensic examination of paints. Debye-Scherrer powder photography was used both for OP and for casework paint flake specimens. Powdered pigments were loaded into 0.3 mm i.d. glass capillary tubes. The camera had 114.6 mm i.d., KODIREX or NO-SCREEN film was used. The sa
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