Concepts common to small-angle X-ray scattering and small-angle neutron
scattering are described in the overarching lemma small-angle
scattering. Small-angle X-ray scattering is a
small-angle scattering technique where the elastic scattering of X-rays by a
sample which has inhomogeneities in the nm-range, is recorded at very low
angles. This angular range contains information about the shape and size of
macromolecules, characteristic distances of partially ordered materials, pore
sizes, and other data. 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 can resolve even larger dimensions.
SAXS and USAXS belong to a family of X-ray scattering techniques that are
used in the characterization of materials. In the case of biological
macromolecules such as proteins, the advantage of SAXS over crystallography
is that a crystalline sample is not needed. Nuclear magnetic resonance
spectroscopy methods encounter problems with macromolecules of higher molecular
mass. However, owing to the random orientation of dissolved or partially
ordered molecules, the spatial averaging leads to a loss of information in SAXS
compared to crystallography. Applications
SAXS is used for the determination of the microscale or nanoscale structure of
particle systems in terms of such parameters as averaged particle sizes,
shapes, distribution, and surface-to-volume ratio. The materials
can be solid or liquid and they can contain solid, liquid or gaseous domains
of the same or another material in any combination. Not only particles, but
also the structure of ordered systems like lamellae, and fractal-like
materials can be studied. The method is accurate, non-destructive and usually
requires only a minimum of sample preparation. Applications are very broad
and include colloids of all types, metals, cement, oil, polymers, plastics,
proteins, foods and pharmaceuticals and can be found in research as well as in
quality control. The X-ray source can be a laboratory source or synchrotron light
which provides a higher X-ray flux. SAXS instruments
In an SAXS instrument a monochromatic beam of X-rays is brought to a sample
from which some of the X-rays scatter, while most simply go through the sample
without interacting with it. The scattered X-rays form a scattering
pattern which is then detected at a detector which is typically a
2-dimensional flat X-ray detector situated behind the sample perpendicular
to the direction of the primary beam that initially hit the sample. The
scattering pattern contains the information on the structure of the
sample. The major problem that must be overcome in SAXS instrumentation is the
separation of the weak scattered intensity from the strong main beam. The
smaller the desired angle, the more difficult this becomes. The problem is
comparable to one encountered when trying to observe a weakly radiant
object close to the sun, like the sun's corona. Only if the moon blocks out the
main light source does the corona become visible. Likewise, in SAXS the
non-scattered beam that merely travels through the sample must be blocked,
without blocking the closely adjacent scattered radiation. Most available
X-ray sources produce divergent beams and this compounds the problem. In
principle the problem could be overcome by focusing the beam, but this is not
easy when dealing with X-rays and was previously not done except on
synchrotrons where large bent mirrors can be used. This is why most laboratory
small angle devices rely on collimation instead. Laboratory SAXS instruments can
be divided into two main groups: point-collimation and line-collimation
instruments: 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 and the wastefulness of the collimation process
— only those photons are allowed to pass that happen to fly in the right
direction — the scattered intensity is small and therefore the measurement time
is in the order of hours or days in case of very weak scatterers. If focusing
optics like bent mirrors or bent monochromator crystals or collimating
and monochromating optics like multilayers are used, measurement time
can be greatly reduced. Point-collimation allows the orientation
of non-isotropic systems to be determined.
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. A disadvantage is that the recorded pattern is essentially
an integrated superposition of many adjacent pinhole patterns. The resulting
smearing can be easily removed using model-free algorithms or deconvolution
methods based on Fourier transformation, but only if the system is isotropic.
Line collimation is of great benefit for any isotropic nanostructured materials,
e.g. proteins, surfactants, particle dispersion and emulsions.
See also Biological Small angle X-ray scattering
X-ray scattering techniques GISAXS
Fluctuation X-ray Scattering References
External links Your portal to SAXS nanostructure
analysis The Small Angle Scattering Portal
SAXS at a Synchrotron Looking At Nothing, a weblog about SAXS
with much supplementary and introductory material
A movie explaining the workings of a pinhole collimated SAXS apparatus
A movie explaining the workings of a slit collimated SAXS apparatus
A movie demonstrating small-angle scattering using laserlight on a hair
Small-angle scattering special interest group at the Advanced Photon Source,
Argonne National Laboratory, USA = List of SAXS beamlines =
Risø, SAXS beamline Roskilde, Denmark SAXS/WAXS Beamline, Australian
Synchrotron Melbourne, Australia SAXS/D, SSRL Beamline 4-2, SLAC USA
SAXS/WAXS-Non Crystalline Diffraction Beamline-Sincrotrón ALBA, Cerdanyola del
Vallès Spain SAXSGISAXS Beamline, Advanced Light
Source 7.3.3, LBNL USA SAXS endstation at the SIBYLS Beamline,
Advanced Light Source 12.3.1 USA SAXS1 and SAXS2 beamlines at Brazilian
Synchrotron Light Laboratory Brazil ID02 SAXSUSAXS beamline Grenoble,
France BM29 BioSAXS beamline Grenoble, France
SWING Beamline at Synchrotron SOLEIL Saint-Aubin, France
P12 Beamline at DESY Hamburg, Germany BL9 beamline of DELTA, Technical
University of Dortmund Germany SAXS Beamline at Elettra Trieste, Italy
cSAXS beamline, Swiss Light Source Villigen, Switzerland
I22 beamline at Diamond Light Source, Harwell Science & Innovation Campus
England, UK B21 beamline at Diamond Light Source,
Harwell Science & Innovation Campus England, UK
APS USAXS instrument, Advanced Photon Source, Argonne National Laboratory USA
BNL SAXS/WAXS X9 beamline, National Synchrotron Light Source, Brookhaven
National Laboratory USA D1 beamline, CHESS Cornell University
USA G1 beamline, CHESS Cornell University
USA SaxsWaxs, BM26B Dutch-Belgian beamline
at ESRF Grenoble, France 3C, 4C, and 9A beamlines at PAL,
POSTECH, Pohang, South Korea I911-SAXS beamline, MAX IV Laboratory
Lund, Sweden APS CMS group, Argonne National
Laboratory IL, USA SPring-8, RIKEN/JASRI Hyogo, Japan
KEK, High Energy Accelerator Research Organization Ibaraki, Japan
SAGA Light Source, Kyushu Synchrotron Light Research Center Saga, Japan
= SAXS Instrument Manufacturers = Anton Paar GmbH, Austria
Bruker AXS, Germany Hecus X-Ray Systems Graz, Austria
PANalytical. The Netherlands Rigaku Corporation, Japan
Xenocs, France Saxslab, Denmark