Water (Absolute Intensity Calibrant)

Water is in principle the best a-priori standard we have for absolute intensity calibration, and there are several articles demonstrating and exploring this. Indeed there are two nice things to say about water as an absolute intensity calibrant: 1)  water’s compressibility directly dictates the absolute scattering to be 0.0163 cm^-1 and 2) water seems to have a flat curve in the SAXS regime.

Well that’s where the nice stuff stops. Water scatters very very little… to the point where 1 mm of water scatters about the same amount as a glass capillary with 30 micron walls. And unless you have an intense X-ray source, do be prepared to wait a while to get reasonable statistics. And waiting builds up cosmic background in your image, which creates havoc with any simple data reduction routines… so in order to use it correctly, you have to take really good care of your water measurement, your empty container measurement, your background, and your transmissions.

And as if that wasn’t enough… the scattering from water is NOT flat, and compressibility depends strongly upon temperature, so be sure you know your sample temperature and have the data to extrapolate your results to q = 0. All in all a nightmarish standard.

To learn more do take a look at this truly nice article by C. Huang, et al. (Proc Natl Acad Sci U S A. 2009; 106(36): 15214–15218) and/or SAXSLAB’s much less ambitious technical note on the matter.