Quick Fluorescence XAFS: A Technique for Recording Fluorescence X-ray Absorption Spectra during Chemical/Biochemical Reactions

Abstract

The Quick EXAFS (QuEXAFS) technique (1,2) provides an alternative way of recording X-ray absorption fine structure (XAFS) data where scan time is minimised by continuous scanning of the monochromator. In contrast to the dispersive technique, QuEXAFS is capable of obtaining data in fluorescence mode as well as in transmission and is therefore suitable for dilute samples. The reduction in data collection time makes it feasible to follow some biochemical or chemical reactions at room temperature with the XAFS technique. We have recently commissioned a QuEXAFS experimental set up on station 9.3 on the 5T wiggler at Daresbury SRS. This station is equipped with a vertically focussing mirror, which in addition to providing extra flux also performs harmonic rejection due to the critical angle cutoff off the mirror reflectivity. The use of the mirror for some harmonic rejection is complemented by the use of a very stable two crystal water cooled monochromator. This provides additional harmonic rejection yet is stable enough to be used without a dynamic harmonic rejection servo, an important consideration for QuEXAFS scans. The monochromator Bragg angle is driven by a dc motor system encoded by a 0.1mdeg encoder. This system can be optimised for constant angular velocity scans and thus is well suited for QuEXAFS and avoids vibrations associated with stepper motors driven at speed. The high flux on the sample together with the use of a high count rate 13 element solid state fluorescence detector (3) have allowed us to obtain quality data on a 5mM solution in less than a minute. Studies have also been carried out on a dilute solution of a transferrin intermediate. We note that this is the first fluorescence QuEXAFS reported and is the lowest concentration for which a XAFS spectrum has been collected in less than a minute. Improvements iii the sample geometry and in the scanning protocol will allow faster data collection on more dilute systems.