Small Angle X-ray Scattering (SAXS)

 

 

Model: Nanostar

Manufacture: Broker

About SASX:

SAXS measurements typically are concerned with scattering angles Ø < 3 °. As dictated by Bragg's Law, the diffraction information about structures with large d-spacings lies in this region. Therefore the SAXS technique is commonly used for probing large length scale structures such as high molecular weight polymers, biological macromolecules (proteins, nucleic acids, etc.), and self-assembled superstructures (e.g. surfactant templated mesoporous materials).

SAXS measurements are technically challenging because of the small angular separation of the direct beam (which is very intense) and the scattered beam. High quality collimating optics is used to achieve good signal-to-noise ratio in the SAXS measurement.

SAXS calibrants are used to accurately determine the sample-to-detector distance, which allows one to compute the wave vector, q = (4p/l) sinØ, from the image of the area detector. l is the X-ray wavelength.

Periodic calibrants are the traditional choice: silver stearate (d = 48.68 Å) and silver behenate (d = 58.376 Å). For larger d-spacings, a common biophysical standard is rat collagen. The lattice constant d is given variously in the literature as 668 Å - 670 Å in wet condition. In the dry state it is around 647 Å (Tim Wess, Josef Orgel, "Changes in collagen structure: drying, dehydrothermal treatment and relation to long term deterioration", Thermochimica Acta 365, 119-128 (2000).

The silver stearate and silver behenate calibrants are easy to prepare. Fill a capillary at least 1 cm tall with the dry powder and seal it. (Flame sealing is easiest in this case, although the powder will burn if you do not leave at least little space). These samples must be stored in light-tight containers since they will slowly photo degrade. One solution is to wrap a test tube in black electrical tape and store the capillary within. To make the specimen to detector distance more precise, place a 0.1 or 0.2 mm filled capillary within a 1.0 or 1.5 mm capillary. Silver stearate and silver behenate are strong scatterers, so there will still be a sufficient signal.

 

 

Fig.1. Schema of SAXS Machine

Bruker AXS NanoStar SAXS instrument employs a pinhole SAXS camera with a sealed X-ray tube employing Cu anode (Ka wavelength of 1.542 Å) operating at 40 KV and 35 mA. The cross-coupled Göbel mirrors reflect a parallel beam of high intense X-rays on the sample. The beam diameter at the sample is about 200 mm. The sample-to-detector distance is adjustable and is kept around 100 cm to achieve small angle of scattering. The detector is a Bruker HI-STAR position sensitive area detector characterised by very low dark current (4-5 counts per second over 1024 x 1024 pixels). The whole beam path is under vacuum to avoid air scattering effects. The transmissions of samples are determined using Glassy Carbon as standard and the background subtracted scattered intensity is normalized to baseline. SAXS NT software is used for data acquisition and evaluation. Flood Field and Spatial corrections are carried out periodically to check inhomogeneities in detector sensitivity and photon counting.

Key features:

- High Brilliance X-rays of constant intensity
- Gobel Mirrors for parallel high intense X-ray beam
- High Resolution Pin-hole Camera: hence desmearing of data is not necessary.
- HI-Star 2-Dimnesional area detector
- High signal to noise ratio for SAXS pattern
- Heating and cooling stage (-50°C ~ 300°C).
- The range of scattering wave vector q: (0.1nm-1 to 4nm-1) or the range of particle size: 1 ~ 60nm
- Suitable for both liquids and solid samples
- High linearity for the detector (up to 1,000,000 counts per second)
- 2-D scanning of the sample (radiography)

Examples of Applications:

SAXS is the method of choice if your samples show any spacing in the range of some 10Å up to 1000Å . SAXS is the only non-destructive method which allows to investigate this length scale present in complex biological or pharmaceutical samples, polymers or metal alloys, or in every case where Transmission Electron Microscopy (TEM) is used right now.

It can be applied to measure fractal dimension, porosity and guinier plot, size and shape, ordering and intermolecular interactions of samples of various states, such as  solid, liquid, opaque and anisotropic samples.  It is  often applied to examine polymers, biomacromolecules, carbon nanotubes, surfactant assemblies, both soft and hard tissues.

(1) Analysis of Microstructures of the bone.

Fig. 2. Microstructure of Bone by Radiographic scanning

Above figure shows the microstructure of the bone resolved at micron level. The intensity bar gives information about the depth of the microstructures at a given point. From such analysis we can able distinguish healthy bone from diseased bone.

(2) Analysis of Microstructures of the vertebra.

 Fig. 3. Microstructure obtained from Transmission and Scattered Intensity from a Vertebra

These modes gives microscopic view of vertebra. These techniques can be use as an alternative to the microscopic techniques such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM).

(3) Analysis of Microstructures of the rat tail tendon:

The structural features of rat tail tendon is examine by radiography (figure 4a) and SAXS spectra (figure 4b).

Fig. 4a. Radiograph of rat tail tendon which contained collagen fibrils The regions indicated by integer 1….9 can be correlated with peaks in the SAXS spectra shown below

 

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Fig. 4b. SAXS of rat tail tendon which contained collagen fibrils

Therefore from the SAXS spectrum, it is possible to estimate the ordering in the collagen fibrils of rat tail tendon.

 

Fig. 5.  Dr. Sawant is loading the sample for SAXS experiment.