Small angle X-ray scattering (SAXS)

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SAXS (small-angle X-ray scattering) is a small-angle scattering (SAS) technique where the elastic scattering of X-rays (wavelength 0.1 ... 0.2 nm) by a sample which has inhomogeneities in the nm-range, is recorded at very low angles (typically 0.1 - 10°). In this angular range, information about the shape and size of macromolecules, characteristic distances of partially ordered materials, pore sizes and the like is contained. SAXS is capable of delivering structural information of macromolecules between 5 and 25 nm, of repeat distances in partially ordered systems of up to 150 nm. USAXS (ultra-small angle X-ray scattering) can resolve even larger dimensions.

The advantage over crystallography is that the samples need not be crystalline, the measurement is non-destructive and NMR methods encounter problems with macromolecules of higher molecular mass (> 30000-40000). However, owing to the random orientation of dissolved or partially ordered molecules there occurs spatial averaging which leads to a loss of information.

[edit] Applications

SAXS is used for the determination of the structure of particle systems in terms of averaged particle sizes or shapes. The materials can be solid or liquid and they can contain solid, liquid or gaseous domains (so-called particles) of the same or another material in any combination. The method is accurate, non-destructive and usually requires only a minimum of sample preparation. Applications are very broad and include colloids of all type, metals, cement, oil, polymers, plastics, proteins, foods and pharmaceuticals and can be found in research as well as in quality control. Furthermore the method allows to determine parameters, e.g., inner structure and surface-to-volume ratio. The X-ray source can be a laboratory source or Synchrotron-light which provides a higher X-ray flux.

[edit] SAXS instruments

SAXS instruments can be divided into two main groups: point-collimation and line-collimation instruments:

1) point-collimation instruments have pinholes that shape the X-ray beam to a small circular or elliptical spot that illuminates the sample. Thus the scattering is centro-symmetrically distributed around the primary X-ray beam and the scattering pattern in the detection plane consists of circles around the primary beam. Owing to the small illuminated sample volume the scattered intensity is small and therefore the measurement time is in the order of hours or days. Point-collimation allows to determine the orientation of non-isotropic systems (fibres, sheared liquids).

2) line-collimation instruments confine the beam only in one dimension so that the beam profile is a long but narrow line. The illuminated sample volume is much larger compared to point-collimation and the scattered intensity at the same flux density is proportionally larger. Thus measuring times with line-collimation SAXS instruments are much shorter compared to point-collimation and are in the range of minutes to hours.

[edit] External links and references

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