NIST Glassy Carbon Absolute Intensity SRM
NIST SRM 3600 – Absolute Intensity Calibration Standard for Small-Angle X-ray Scattering is the officially certified glassy carbon reference material for SAXS absolute intensity calibration. It serves as the formal replacement for the glassy carbon standards that were distributed locally in the past.
This standard was calibrated in part on the USAXS instrument and then validated through testing on additional instruments before certification. It is commercially available from NIST (listed price at the time referenced: ~ $800).
Users are strongly encouraged to obtain their own certified calibration standard for routine work and long-term consistency.
In practice, SRM 3600 can be used in the same manner as the earlier glassy carbon reference artifacts. It is the same general material class and based on the same calibration principles, but with more rigorous characterization, tighter controls, and formal certification by NIST.
Glassy Carbon samples measured using USAXS instrument
Jan Ilavsky, 2016-08-01 : I used to provide these samples (when available) for free but since now the NIST SRM is available for purchase, I encourage everyone to buy the NIST SRM 3600 (see above). AT this time there is about 175 samples of Glassy Carbon I supplied used by the SAS community worldwide.
Jan Ilavsky, 2011-05-27 : About 100 samples of Glassy carbon 1mm material are currently used throughout the World with SAXS instruments (desktop as well as synchrotron). Some are also used with SANS instruments, even though some interesting issues with dependence of sample transmission on neutron wavelength are still under investigation. If you need sample for your own use, please contact me via e-mail with your needs.
Anyone using my Glassy carbon standard is kindly asked to cite the following publication (which is Open Access on the journal web site!):
Calibrating using this standard
I get a lot of questions how to calibrate using this standard... Movie (~2015) is in my Youtube channel , here is short summary:
- Measure glassy carbon as sample for whatever time Tc you need and reduce it as sample using your reduction method. Set for now Calibration Coefficient (Cf) = 1. You need to know the thickness (=1mm) and transmission (~0.85 for Cu wavelength, but measure it!). Calculate constant Cf by diving MY dataI sent you by Your data. This is correction factor which puts your instrument on the absolute scale.
- The easiest way to get the scaling factor is by using "Data manipulation I" from Irena package, which has ability to "autoscale" two data sets using the curve between the cursors. USAXS data do contain some flat background, so they will likely deviate at high q. Select only part of data which has similar shape.
- Next calculate Calibration Constant Cf:
Cf= MyData/YourData
- Now measure your sample, reduce with its own thickness and own transmission with Cf from above to put this on absolute scale.
- If you need to measure sample at different times, you need to correct Cf for time:
Cf(new time) = (originalGlassy Carbon measurement time / new sample measurement time) * Cf
Absolute intensity conversion
For details of the conversion of raw intensity to absolute units of differential scattering cross-section per unit volume per solid angle, please consult:
Fan Zhang, Jan Ilavsky, Gabrielle Long, John Quintana, Andrew Allen, and Pete Jemian, "Glassy Carbon as an Absolute Intensity Calibration Standard for Small-Angle Scattering," Metallurgical and Materials Transactions A 41 (5), 1151-1158 (2010).
Pete Jemian's 1990 PhD thesis, http://www.jemian.org/pjthesis.pdf, page 51, equation 83.