Probing chemical reactions via X-ray emission spectroscopy using a colliding droplet mixer*

The high-intensity, high brightness X-ray pulses generated by X-ray free electron laser (XFEL) facilities are instrumental in studying fundamental processes with atomistic resolution. They allow  to study the electronic structure during transient reaction processes in the liquid phase at room temperature, in the absence of radiation damage.

By colliding two droplets acting as reagent carriers in air, a chemical reaction can be triggered via mixing and followed by time-delayed probe with X-ray spectroscopy. Colliding droplet mixing addresses two distinct challenges of other, more commonly used continuous flow sample delivery methods: First, two sufficiently small droplets (<50 pL) mix homogeneously within a few hundred µs, making reaction studies on the sub-millisecond time scale accessible. Second, by synchronizing the sample droplets with the X-ray source frequency (120 Hz), scarce samples such as metalloproteins can be measured. Here, we have characterized the mixing time scales of the reaction via fluorescence quenching reactions, and the stability of the droplet collisions via bright-field imaging. Also, we were able to follow an oxidation reaction of an iron model compound via element-specific Fe X-ray emission spectroscopy. We followed the changes in oxidation state over time, while retaining low sample consumption. Overall, this discontinuous flow mixing principle enables studies on a wide range of biologically and chemically relevant samples that are currently inaccessible.