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Department of Chemistry and Biochemistry

Small-Angle X-Ray Scattering (SAXS)

Fundamental Principles

Small‑angle X‑ray scattering (SAXS) is a powerful method to gather quantitative nanoscale information from a diverse range of samples from liquids to pastes, powders, and films.  The typical length scales probed range from ~1‑100 nm, corresponding well to the typical feature sizes of nanoparticles, block copolymers, and proteins.  This versatility enables characterizations of diverse samples including nanoparticle solutions, nanocomposites, block copolymers, mesoporous materials, and protein solutions.  Most SAXS experiments are carried out in transmission mode with the incident X-­ray beam pointed directly at the sample.  The scattered beam is then detected on the other side using a 2D area detector.  The beam is sensitive to spatial variations in electron density and can scatter from any interface that has a change in electron density, such as polymer‑polymer, polymer‑solvent, protein‑solvent, or solid‑pore interfaces.  The higher the contrast in electron density, the greater is the intensity of the scattered beam.  Beyond the frequency of these interfaces, the scattering profile also corresponds to the spatial arrangement of the interfaces and thus gives significant morphological details.  For example, the conformation of proteins in solution and the size and shape of nanoparticles may be determined quantitatively.  The results from a SAXS experiment correspond to the complete measured ensemble, typically about 0.1‑1 mm3 which corresponds to ~1018 measured samples of a 100 nm motif.

Grazing Incidence

For thin film samples, the incident X‑ray beam is directed at an angle of grazing incidence (GISAXS) to increase signal and eliminate the need for an X‑ray transparent substrate.  In GISAXS, the incident beam has an increased interaction with the surface and the angle can be used to modulate the probed depth from ~10 nm up to the full film thickness.  This control of sampling depth enables the surface structure to be measured separately and be compared to the interior structure.  Additionally, the scattered intensity at different azimuthal angles on the 2D detector corresponds to two different sample vectors.  Horizontal intensity modulations result from the in-plane film structure, whereas vertical intensity modulations result from the out‑of‐plane film structure.  The analysis of GISAXS data is more complicated than transmission SAXS because the scattered intensity is convoluted with the intensity from reflected beams.  There are a number of software packages available to aid in the data analysis.

Helpful Links

Introduction Slides from Scott Barton

SAXSGUI is the same general purpose data reduction package that is used on our instrument

Scatter is a modern SAXS package with both data reduction and fitting capabilities for 2D and 1D data

DPDAK package for analysis of large SAS datasets

ScatterBrain is a data reduction package developed at the Australian Synchrotron

SASView is used for reciprocal space fitting and can be downloaded from SourceForge

The NIST Small-Angle Scattering Portal has lots of links to software packages

ATSAS is used for analysis of biological macromolecules

Irena enables the fitting of complex systems with size distributions, hierarchical structures, and diffraction peaks - and requires the separate purchase of Igor

SASFit is used for calculating the radius of gyration, scattering invariant, or Porod constant and can be downloaded from SourceForge

ScAtter is a JAVA program for light-weight Kratky, Porod, and Guinier analysis

Free eBooks

Glatter and Kratky, 1982 .Small-Angle X-ray Scattering

Feigin and Svergun, 1987. Structure Analysis by Small-Angle X-ray and Neutron Scattering