Premium
SAXS experiments on absolute scale with Kratky systems using water as a secondary standard
Author(s) -
Orthaber Doris,
Bergmann Alexander,
Glatter Otto
Publication year - 2000
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889899015216
Subject(s) - scattering , absolute scale , small angle x ray scattering , calibration , collimated light , intensity (physics) , small angle scattering , calibration curve , small angle neutron scattering , chemistry , neutron scattering , optics , computational physics , physics , atomic physics , chromatography , laser , quantum mechanics , detection limit
For small‐angle scattering, of X‐rays (SAXS) and neutrons (SANS), the importance of absolute calibration has been recognized since the inception of the technique. The work reported here focuses on SAXS measurements using Kratky slit systems. In former days, only molecular weights or scattering per particle were determined, but today absolute calibration implies the use of the unit of cm −1 for the scattering curve. It is necessary to measure the so‐called absolute intensity, which is the ratio of the scattering intensity to the primary intensity P 0 . Basically there are two possible ways to determine the absolute intensity. The first one is the direct method, which involves the mechanical attenuation of the primary beam by a rotating disc or a moving slit. The second is the indirect method, using secondary standards. Water is well suited as a calibration standard because of the angle‐independent scattering. The essential advantage is that the scattering of water only depends on the physical property of isothermal compressibility. Before presenting an example of the practical performance of this method, the most important theoretical equations for an SAS experiment on the absolute scale are summarized. With the slit collimation system, the scattering curve of water can be measured with high enough statistical accuracy. As a first example, the scattering curve of the protein lysozyme on the absolute scale is presented. The second example is the determination of the aggregation number of a triblock copolymer P94 (EO 17 –PO 42 –EO 17 ). Taking into account that at least 10% of the polymer sample consists of diblocks, the accuracy of around 10% for the determined aggregation number is rather good. The data of P94 are also considered on the particle scale in order to obtain the radial scattering‐length density distribution.