Small angle X-ray scattering (SAXS) is an increasingly important variation on X-ray diffraction (XRD) studies. The primary advantage of SAXS is that it can be used to probe larger-scale structures than XRD.

The general procedure in SAXS is quite similar to XRD. The sample is irradiated with X-rays, which produce a diffraction pattern as the wave nature of the X-rays interacts with the physical structure of the sample. An X-ray detector collects the diffracted X-rays, and the measured pattern is analyzed to reconstruct the sample’s physical configuration. The interaction is governed by Bragg’s Law, which relates the diffraction angles to the wavelength of the X-rays and the spacing between features in the sample. In the case of standard XRD, the typical angles that are involved correspond to spacings on the order of the interatomic spacings within a molecule, so that XRD can be used to probe molecular structure.

In SAXS, smaller diffraction angles are investigated, and these correspond to longer spacings. For macromolecules and proteins, SAXS can be used to study the global structure of the molecules (as opposed to the individual interatomic spacings). Other types of samples include nanoparticles, prepared surfaces of metal alloys or cement and various kinds of colloidal or polymer suspensions. Porosity, particle size, surface area and other material characteristics can also be inferred from SAXS data.

In principle, standard XRD systems can be used for SAXS studies. Dedicated and optimized systems are more suited for labs that are primarily interested in SAXS. For SAXS, the X-ray beam needs to be finely collimated as either a point source or a linear source. One of the disadvantages of SAXS is that the restrictions on the source may make analysis times quite long. The use of brighter sources and area detectors (as opposed to linear or point detectors) helps reduce measurement time. Another way to avoid the long analysis time is to use a synchrotron beamline as the X-ray source. Synchrotrons are generally found at national laboratory facilities, and adding a beamline alone may require the investment of a few million dollars at such a facility.

While SAXS originated primarily as a materials science tool, the technology has evolved so that life science applications—principally for the study of proteins—are becoming far more common. Thus, vendors have begun optimizing systems for life science research. Materials science applications span a very broad range of sample types, from nanoparticles to polymer suspensions and metallic alloys.

The major X-ray instrumentation suppliers compete in the SAXS market, either with options on XRD instrumentation or with dedicated systems. Bruker added to its SAXS offerings with the acquisition of Hecus (see IBO 1/31/12). Other market participants include Forvis, inel, Photon Production Laboratory and Xenocs. SAXS market demand amounted to about $20 million for lab systems in 2011.

SAXS at a Glance:

Leading Suppliers

• Bruker

• Rigaku

• PANalytical (Spectris)

Largest Markets

• Academia & Govt.

• Polymers

• Pharmaceutical

Instrument Cost

• $150,000 and up