Figure 2.

Apparatus and method for obtaining SAXS profiles from solution samples at 100 K. (a) A SAXS beamline was configured with a cryostream providing a steady flow of T = 100 K N₂ gas at the sample position. Monitors for incident intensity (ion chamber) and transmitted intensity (PIN diode) allowed measurement of the x-ray transmission factor for each sample and normalization of scattering profiles. X-ray windows of 200 nm thick Si₃N₄ on the upstream and downstream flight tubes minimized background scattering. The 500 μm square aperture of the upstream window functioned as a second guard slit. The window-free sample holder was made from a 1.8 mm section of thin-wall polyimide tubing. A ∼1 μL sample was injected into the tubing and then vitrified by cooling in the gas stream. The sample cell was oriented so that the x-ray beam passed along the central axis of the tubing. The photograph shows a sample in the cryostream. (b) Cryo-SAXS profiles obtained from vitrified solutions of 2 mg/ml GI and its matching buffer, as well as for the instrumental background with the sample removed. Slight differences in the x-ray path lengths through the GI and buffer samples contributed to differences in their scattering. (c) Measurement of the x-ray transmission factor, T, and normalization by the path-length, log(1/T), provided accurate background subtraction using the data in (b) to obtain GI’s SAXS profile.