Batch Mode Experiments

The B21 sample environment is derived from the Arinax BIOSAXS liquid sample-handling robot (SHR) (Round et al., 2015) that provides a sample viewing camera perpendicular to the X-ray beam and holds the temperature-controlled sample exposure unit (SEU). The SEU holds a sample exposure pod which is a modular component that can be readily exchanged. Samples are loaded into the SEU from the sample storage unit (SSU), a temperature-controlled (4 to 40°C) environment where samples are arrayed in either 96-well plates, PCR strips or 1.5 ml microfuge tubes. Samples can then be transferred from the SSU to the SEU, which has two types of exposure mode, static mode and flow mode.

Flow mode moves the sample through the capillary at a speed of 1 µl per second during the exposure, minimizing radiation damage, and is suitable for sample volumes between 30 and 100 µl with larger volumes allowing for longer exposure times.

In static mode samples are loaded and exposed without moving, and this enables data collections from samples with volumes as low as 15 µl. In this mode radiation damage is evident in protein samples in phosphate-buffered saline after 1s, although this buffer system has been found to be particularly prone to radiation damage.

The SEU can hold samples at temperatures ranging from 2 to 60°C for temperature-based exposures. In addition, the SEU maintains the sample capillary in a vacuum environment that removes the background due to atmospheric scatter. Static mode enables high-throughput data collection and well defined sample compositions for titrations of buffer components such as ligand concentrations etc. (McMahon et al., 2020) or for time-dependent measurements (Kelly et al., 2020). Typically, measuring a sample will take around 2 minutes in static mode, which includes 20–30s for measurement and around 120s for loading the sample and washing, rinsing and drying the sample cell. A 96-well plate full of samples can thus be measured in around 4 hours.