Small-angle X-ray scattering (SAXS) is an analytical technique to determine differences in density within a sample with a nanoscale resolution by analyzing the elastic scattering angles of X-rays.
What is the principle behind SAXS?
SAXS can determine the nanoscale structure of solids, liquids and gaseous particulate matter of almost any material, including colloids of all types. The method is non-destructive with minimal sample preparation, and may equally be applied to metals, polymers, proteins, oils, or ceramics, among others.
Typically X-rays, with a scattering angle between 0.1–1 and wavelength of between 0.7–0.2 nm, are used. The greater the electron density, the greater the scattering angle of the photon. The volume of a sample is inferred by the number of photons scattered and range of scattering angles, while the intensity of a detected signal is used to determine the density of a sample.
WeNMR Small Angle X-ray Scattering Animation
How do SAXS instruments work?
A monochromatic X-ray beam is emitted towards the sample under investigation, through which most X-rays pass without interaction.
A small percentage of the emitted X-ray photons strike electrons belonging to an atom within the sample, leading to the photon being scattered elastically.
Photons that are scattered elastically maintain their kinetic energy and wavelength while changing vector slightly. A flat X-ray detector located on the opposite side of the sample from the X-ray emitter then obtains a scattering pattern that can be used to infer information regarding the sample.
Since most of the X-ray photons pass through the sample without interaction, weak signals with low scattering angles may be hard to detect. Thus, intense X-ray beam consisting of non-scattered photons must be blocked before reaching the X-ray detector. This is most commonly done in two ways: point collimation or line collimation.
Point Collimation
Point collimation uses the diffraction of light through pin holes to shape the X-ray beam into a small circular spot that illuminates the sample.
A second point collimation filter behind the sample allows only photons of particular scattering angles to pass through to the detector, creating concentric circles of increasingly weaker signals at the detector.
Since many photons do not make it through the pin hole filter, this technique potentially takes several days of analysis time to collect sufficient data.
Line collimation
In line collimation, the beam is restricted in only one direction, creating a long but narrow beam of X-rays. A more complex scattering pattern is created at the detector using line collimation compared with point collimation, requiring intensive deconvolution. However, analysis time is in the order of seconds to minutes and much faster.
What types of materials can be analysed by SAXS?
A wide range of materials can be analysed using SAXS, including solid objects, powders, gels, liquid dispersions and gaseous particles. They may or may not be homogenous or crystalline.
SAXS is frequently used to determine the size, shape and monodispersity of nanoparticle dispersions or colloids, nanopowders and composites, polymers, surfactants, microemulsions, biomacromolecules, and mesoporous materials.
Additional information such as the composition of nanoparticles, surface to volume ratio, pore size distribution and aggregation behaviour may also be inferred by SAXS.
SAXS instrument manufacturers include Anton Paar, Bruker AXS, Hecus X-Ray Systems Graz, PANalytical, Rigaku Corporation, Xenocs and Saxslab.
Further Reading