The Denver X-ray conference was quite excellent, mostly becauase of the people to talk to during the coffee breaks and the dinners (as always, there should be more of these and longer!). Here’s my quick review of the conference: Read More
The long wait is over! A new publication is out, this one written by Martin Hollamby over the course of three tough years. For those who haven’t seen his one-minute intro video, Martin is investigating the behaviour of molecules in solvents. In his excellent paper, he shows how his range of molecules can self-assemble in (hydrophobic) solvents to form a variety of shapes and forms. Read More
It seems like this year has flown by at an amazing speed, yet quite a few things have come to pass in this year. Read More
One paper I managed to miss for my review paper on data corrections is a paper by M. H. J. Koch from Hamburg. The paper is written in a nice informal way, replete with good quotes, where he talks about his experiences in instrument development for SAXS and WAXS beamlines. In particular the paper details the development of delay line (wire) detectors, and may form a good introduction into this topic. It seems that wire detectors may still have their uses: their rapid response to incoming photons still puts them among the fastest 2D detectors out there. Read More
By the way, my topical review paper on SAXS data collection and correction has been published and is available open access here!
Ensuring that a measurement gets you the right values is quite important for any scientist. One way of finding out what the quality of your machine is, is to compare your results for a standard sample with those measured by others on their instruments.
Exactly this has been done by Adrian R. Rennie and coworkers: their publication (also open-access arXiv version or institutional repository version) details the results from analysis of a sample of polystyrene measured at a variety of neutron scattering instruments as well as an X-ray scattering instrument. The analysis and conclusions reveal a few telling issues with small-angle scattering measurements and instrumentation. Read More
Reading in the detector data, and programming methods for that, is one of the more tedious tasks of any data reduction program. Those of you who write their own data correction programs know this all too well. Many detector systems store their data a little bit differently than the others, despite the availability of decent standards for storing images and their metadata (fortunately, some manufacturers are now using standard data storage formats). For example, the NIKA manual shows a rather lengthy list of formats for which support had to be written. Read More
Following my previous work in progress detailing the data correction steps to obtain good data, I finally had the chance to write this down in a review article. This review article (open access) has been submitted on Monday. After it has been reviewed and (hopefully) published in the journal, I will ensure that that latest version is available as an open access paper (thanks to funds from NIMS/ICYS). Until then, please enjoy the pre-submission version and as always feel free to comment!
[Sep. 22 edit: the ArXiv link has been replaced with a link to the journal, where the paper is available under an open-access license]
After browsing through a recent Journal of Applied Crystallography, I came across a paper by Deschamps. It indicates to me that there is a slight lack of information communication in some aspects of SAXS.
Firstly, it mentions in the introduction that the advanced (Fourier-transform-based) SAXS analysis methods cannot “extract simultaneously the precipitate form factor and the precipitate size distribution”. Indeed they cannot, but neither can the classical methods (see for example page 147 of ). It is only when we assume a shape of the scatterer in the classical methods, that the number of possible size distributions reduces to a single solution. Inversely, if we assume a size distribution, there is only one general form factor which will match. This leads to the erroneous conclusion that the classical methods can result in a simultaneous determination of size and polydispersity.
My main point, however, is the research on the behaviour of the Guinier method in polydisperse systems. I, too, have been looking at this and found out that others have tread this path before me. My results here are in agreement with the results of others that the radius of gyration of the Guinier method in polydisperse systems follows the volume-squared weighted radius of gyration . The limits of applicability shift accordingly.
The work by Deschamps on the Porod behaviour in polydisperse systems is to my knowledge unique, but I have not looked in detail into this.
 O. Glatter and O. Kratky, Small angle X-Ray Scattering, (Academic Press, 1982), available online
 G. Beaucage, H. Kammler, and S. Pratsinis, Journal of Applied Crystallography 37, 523 (Jan 2004).
Last week-end, while visiting friends nearby, a copy of the book “bad science” by Ben Goldacre was dropped in my lap. Having read the occasional post on his weblog (http://www.badscience.net/), I had already planned to get it. So I started reading the book with rather high expectations. (This copy, if I did not misunderstand my friend, happens to be an unofficial “homeless” book. The idea is that they are passed along after you have read them. An idea I like!)
…well, his famous SAXS analysis method.
This documentGuinier_short, copyright Brian Pauw
gives a short description and review of the applicability of the Guinier method to polydisperse systems.
It also shows, through analysis of simulated data, what q-range should be measured for the Guinier method to be valid. In short, the rule of qmax=1.3/Rg still holds, but Rg in polydisperse systems is the volume-squared weighted Rg of the distribution.
This then implies that the Guinier method for polydisperse systems quickly becomes unusable as the required qmax cannot be reached with anything but USAXS systems for polydisperse samples.
This text (the linked PDF) is released under copyright (copyright by Brian R. Pauw, 2011) as I may want to include some of this in a later publication. I hope you understand…