MMSN -- Techniques

The past 100 years or so has seen the evolution of a very powerful set of analytical tools for the determination of molecular and solid state structure. Some of these tools can be accommodated in conventional laboratories (so called 'home labs'), whereas increasingly more powerful probes require major infrastructure facilities such as the Australian Synchrotron being built in Victoria and the Replacement Research Reactor being built in New South Wales. Techniques used both in conventional labs and at major facilities include:

  • X-ray diffraction from single crystals; this is the most widely used technique for accurately and precisely determining structure at atomic resolution. Sophisticated procedures make the growth of crystals of materials of interest increasingly routine, in fact robots are now being used. Crystals are comprised of regular arrangements of atoms (often in discrete molecules) and illuminating a crystal with X-ray light results in the light being scattered in all directions, from the electrons surrounding the atoms, forming a characteristic diffraction pattern. The diffraction process is determined by the nature of the atoms (ie the number of electrons around each atom) and their three dimensional arrangement. Measuring the diffraction pattern can then provide the atomic structure.
  • X-ray diffraction from powders; when single crystals cannot be grown structural information may nonetheless be gleaned from grinding the crystalline material of interest into a micro-crystalline powder. X-ray diffraction from the powder can be sufficiently characteristic to enable a complete structure determination. The method is currently less effective than using single crystal diffraction and the development of structure determination methods using powder diffraction is an area of very active research.
  • Small Angle and Wide Angle X-ray Scattering (SAXS/WAXS); provides a structural information from materials that do not have the highly ordered arrangements of component atoms found in crystals. The SAX technique gives size and shape structural information at scales of 500 to 0.1 nanometers (0.000000001 of a meter), whereas WAX provides information to atomic resolution.
  • X-ray absorption spectroscopy (XAS); measuring the absorption of X-rays (rather than scattering) as the light wavelength (energy) is varied reveals characteristic absorption maxima called absorption edges that occur when the light has sufficient energy to move an electron in the material to a higher 'energy level'. There is characteristic fine structure near the absorption edge as the light energy increases as the level of absorption declines. Extended X-ray Absorption Fine Structure (EXAFS) provides structural information such as atom to atom distances and the number of atoms 'bonding' to any metal atoms that may be present. X-ray Absorption Near Edge Structure (XANES) yields information about the geometrical arrangement of atoms and the oxidation state of atoms. X-ray absorption spectroscopy does not require crystalline material and liquid solutions are often studied. Biological cells can be studied directly, as can soil samples for environmental analysis. At longer wavelengths X-ray absorption spectroscopy can be used to study surfaces and thin films.
  • X-ray Photoelectron Spectroscopy (XPS); is an analytical technique that depends upon the measurement of the energies of photoelectrons that are ejected from atoms when they are irradiated by low energy X-ray light. Used to study surfaces, XPS has a number of attributes including a high (and variable) range of sensitivities to structures on the outermost surface, an ability to identify such structures chemically, and a reasonable capacity for elemental quantification and structure thickness.
  • X-ray microtomography is an imaging technique that uses computed tomography to construct high resolution 2 and 3 dimensional images of small objects. The high contrast and microtomography capabilities proposed for the Australian Synchrotron will be exploited to great effect in the areas of materials science, non-destructive testing and mineralogy.

Synchrotron sources provide light many orders of magnitude brighter than can be obtained in a conventional laboratory, and the wavelength of this light can be tuned to enable experiments and measurements otherwise impossible.

Neutron diffraction and scattering provides structural information that complementing that gained from the scattering and absorption of light. Neutrons interact with the nucleus of an atom, rather than the electrons surrounding the nucleus, and these interactions can locate atoms positions more accurately than can X-ray interactions with electrons, which may be distributed between atoms.

The powerful instrumentation used in structurally characterising molecules and solid state materials is inherently expensive to purchase or construct. This is obviously true for instruments at major facilities, and it is also true of uniquely capable instruments housed in conventional laboratories. Such equipment is inevitably too costly to replicate and maintain in multiple locations. Practitioners currently travel in teams to use instrumentation not locally available; this is both costly and time inefficient. Fortunately recent technological developments make automation and the provision of Internet access to remote instrumentation a realisable possibility.

ARC -- Australian Research Council