Speaker
Description
Small-angle X-ray scattering (SAXS) measurements of aqueous L-cysteine solutions were performed at the SPB/SFX instrument of the European XFEL using train-resolved acquisition with the AGIPD detector. Detecting weak sample-dependent signals under XFEL conditions is challenging because the measured contrast is often dominated by normalization uncertainties, beam fluctuations, detector effects, and liquid-jet variability. To address this problem, we developed a train-resolved analysis based on independent scale-plus-offset fitting of cysteine and water profiles, combined with transmission-matched water--water controls. This approach isolates residual scattering contributions that cannot be explained by global intensity variations and exceed the residual structure observed in matched water controls.
Top panel: Control-corrected residual SAXS profiles after train-resolved scale-plus-offset correction and subtraction of transmission-matched water controls.
Bottom panel: Corresponding statistical significance (ΔR/σΔR), demonstrating the statistical robustness of the recovered signal.
For 0.5 M aqueous L-cysteine, we observe a reproducible control-corrected residual SAXS signal whose amplitude increases strongly with XFEL transmission. The residual exhibits a characteristic sign-changing dependence on momentum transfer, with a positive contribution at low q and a negative contribution at higher q. Closely similar residual shapes are observed independently at the two highest transmission settings, consistent with a common fluence-dependent contribution. Bootstrap and convergence analyses show that the signal emerges progressively from the accumulation of many train pairs and exhibits uncertainty scaling close to the expected 1/√N behaviour.
Beyond the observation of a weak fluence-dependent residual SAXS signal, the work establishes a methodological result of broader relevance for XFEL solution scattering: high repetition rate alone is insufficient to achieve maximum sensitivity. Train-resolved analysis together with closely matched water controls proved critical for recovering statistically robust weak signals and distinguishing them from slow experimental drifts and common-mode fluctuations, a challenge that also arises in nonlinear X-ray measurements.