BL19U2

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Beamline introduction
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The beamline BL19U2 is the first beamline dedicated to biological material small-angle X-ray scattering (BioSAXS) in SSRF to meet the rapidly growing demands of crystallographers, biochemists and structural biologists. The X-ray of BL19U2 beamline has the characteristics of high flux, low divergence angle and good collimation, and the station is equipped with automatic solution sampling device, rapid data acquisition detector and automatic data analysis processing program, can achieve high throughput measurement, mainly used in 1-200 nm scale research for  nucleic acid, protein, nucleic acid-protein complex, virus particles, membrane protein, and sugar, lipid structure evolution dynamics, also applies to colloid, liquid crystal, polymer and molecular, foam, emulsion, gel, granular material, glass and other system of microscopic structure characterization.
Beamline Layout
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The electrons come from an undulator IVU20 which can provide high brilliance for the BL19U2 end stations. A double flat silicon crystal (111) monochromator is used in BL19U2, with a tunable monochromatic photon energy ranging from 7 to 15 keV. The focusing element of the beamline is composed of two mirrors. One is the horizontal focusing mirror situated 31.2 m from the source. The other is the vertical focusing mirror situated 34 m downstream from the source. 

 

 Optical layout of the BL19U2 BioSAXS beamline
Abbreviations: beam-position monitor (BPM), double-crystal monochromator (DCM), deflection mirror (DM), horizontally focusing mirror (HFM), vertically  focusing mirror (VFM), photon shutter (PS), scatterless slit (S-Slit)
Beamline specification
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Energy range(keV):7~15
Energy resolution(ΔE/E): 1.97×10-4
Focusing beam size（HxV) (μm2):330×50
Flux（phs/s):3.9×1012 (@12keV, 260mA)
Beam divergence（HxV) (μrad2):46×26
Experimental methods
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1、High-output solution small-angle X-ray scattering
A flow cell made of a cylindrical quartz capillary with a diameter of 1.5 mm and a wall thickness of 10 μm is used in the BL19U2. This cell is embedded in a temperature-controlled copper holder block and sealed by O-rings. Two motorized x–z stages are employed to serve as the sample cell unit holder and sample storage plate. A syringe pump is used to pump samples and cleaning solutions into the cell. Typically, samples with a volume between 50 and 60 μl are delivered to the capillary for continuous-flow SAXS measurements, which can significantly reduce the effects of X-ray radiation damage. 
2、BioSAXS combined inline sample purification system
A commercial high-performance liquid chromatographyand multi-angle and dynamic light scattering detection devices were installed inline on BL19U2. The combination of multi-angle light scattering analysis with SAXS provides an effective way to assess mass fraction or molar mass and helps to obtain more biophysical parameters for eluting samples. Moreover, the DLS detector can be selectively linked into the system to help acquire the hydrodynamic radius and evaluate the homogeneity of samples. Data collection is triggered after sample injection. The eluted solution continuously flows through the exposure cell, thus effectively reducing the risk of radiation damage.
Data acquisition and analysis system
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The beamline control and data acquisition software at the BL19U2 beamline follows the protocols developed by the SSRF. This software is completely adapted to the experimentalist’s needs with a userfriendly graphical user interface (GUI). This GUI displays several important beamline parameters such as the incident flux and the transmitted flux measured at the pin-diode mounted in the beamstop. Relevant parameters such as exposure time, image file name and sample description can also be set in the GUI and transmitted to the Pilatus 2M operating program, which triggers the data acquisition by opening the experimental shutter.

Beamline control and data acquisition software, allowing automated data acquisition. A, Sample description input; B, exposure time definition; C, image data saving path definition; D, storage ring real-time state illustration.

A fully automated and highthroughput SAXS data analysis pipeline, SAS-cam was developed primarily based on the SASTBX package. Five modules are included in SAS-cam, encompassing the data analysis process from data reduction to model interpretation. The model parameters are extracted from SAXS profiles and stored in an HTML summary file, ready for online visualization using a web browser.