X-Ray Diffraction Analysis (XRD)
XRD analysis allows both qualitative and quantitative salt analyses. When X-rays impinge on a crystalline material a series of "reflections" are produced that are characteristic for each phase, similar to a "fingerprint". It is a laboratory method and requires small samples or even very small ones if using special sample holders.
Basic principle: in a crystal the atoms are arranged at well-defined intervals and form a crystal lattice. The basic model of the defined crystal lattice is repeated periodically. The distances of these periods (lattice spacing) are in the range of 0.02 to 2.5 Å. The lattice spacings define the crystal lattice planes, and their position in the crystal is determined by the integer multiple of the crystal lattice spacing. The radiation is now diffracted on the crystal lattice planes at the corresponding wavelength (x- rays), comparable to the diffraction of light on a grating.
Incident x-rays on a crystals cause a number of reflections, which relate to the incident angle of the rays and the specific crystal lattice planes. Here the Bragg equation applies:
n * λ = 2 sin Θ
n = integer,
λ = wavelength of the incident x-rays,
d = interplanar spacing or spacing between lattice planes
Θ = glancing or grazing angle- the angle between the incident ray and the lattice planes.
If the wavelength of the x-rays is known, the crystalline phase can be determined from the position and the intensity of the diffraction reflections. X-ray diffraction cannot provide much information on amorphous, i.e. non-crystalline substances such as glass.
In practice it is most common to measure powder samples, because the micro-crystallites are oriented into all direction (statistically). Thus, it is possible to detect nearly all crystal lattice planes without moving the sample (sometimes the sample holder is rotated). With certain minerals such as the phyllosilicates, a preferred orientation is obtained on "normal" sample preparation but special measures may prevent this.
There are several possibilities for the detection of diffracted radiation. A photographic film with a suitable camera was in the past used for recording the reflections according to the diffraction angle and the intensity of the diffracted radiation (Debeye-Scherrer-, Guinier- and Gandolfi-camera). Today the reflections are usually detected with a counter tube or a solid state detector (Goniometer method). The detector determines the intensity of the reflections for each angle with a certain step width covering the desired angle range. The intensity of the x-rays as a function of the angle are recorded in the diffractogram. Today, tables with XRD data are kept in databases available for computer-assisted analysis. These provide a tool for the reliable determination of all crystalline organic and inorganic substances (approx. 50,000 sample plots, 35,000 of these for inorganic substances).
Advantage: The x-ray diffraction provides a qualitative, a semi-quantitative and in some cases also a quantitative determination of crystalline substances. In conservation and restoration, it delivers particularly good results for the study of pigments, salts, rock samples, corrosion products and ceramic materials. The material quantities needed for analysis, can vary between some milligrams, when using a powder diffractometer, down to a few microgram. Single crystal sample holders using the diffractometer method and Debye-Scherrer- or Guinier- and Gandolfi-method, can work with even less. The investigation method is non-destructive, i.e. the sample can be re-used for subsequent investigations.
Disadvantage: For phase mixtures only phases from a proportion of 1-5% can be detected.