Improving laboratory X-ray spectrometers with MYTHEN

The pursuit of high-resolution spectroscopy has never been easier. The winning combination of very brilliant synchrotron beam and high-quality monochromator resulted in numerous spectroscopy beamlines, some of which exploit HPC technology to further improve the data quality and acquisition times [1, 2, 3]. Even the smallest and the most affordable HPC detector, MYTHEN, is used as a part of the setup in e.g. the Johansson spectrometer at Swiss Light Source [4], or the RIXS beamline at PETRA III.

On the other hand, the pursuit of high resolution in a laboratory was never easy. Apart form the obvious challenges, flux and acquisition time, the developments have to face many more: cost restraints, maintenance-efficiency, flexibility, and scanning mechanisms of data collection. However, two recent developments managed to overcome these issues – by employing a smart design and the DECTRIS MYTHEN¹ detector.

Polychromatic simultaneous wavelength-dispersive X-ray fluorescence (PS-WDXRF)

Sato and co-workers approached the problem elegantly [5]. By integrating the MYTHEN 1K in the Shimadzu² MX-2400WD-XRF spectrometer, they exploited the 1280 detector strips to simultaneously collect data diffracted by an analyzer crystal under a range of angles (θ_low-θ_high, see Fig. 1). As this directly translates to a range of energies (E_high-E_low), the new setup was named polychromatic simultaneous wavelength dispersive fluorescence (PS-WDXRF [5]). This scan-free spectrometer covers an energy interval of 1.24 keV, with an energy resolution of 3.9 eV (FWHM of Fe Kα₁) and a sampling width of 0.98 eV per strip. The first results are more than encouraging. Although collected in only 300 s, the data show that both peak position shift and intensities can be used to distinguish between Mn(II) and Mn(VII). As similar results were obtained for Cr(III) and Cr(VI), the application is expected to be used for a range of transition metals – a game-changer for understanding redox processes in batteries.

Polychromatic simultaneous wavelength-dispersive X-ray fluorescence (PS-WDXRF)

Figure 1. PS-WDXRF setup proposed by Sato et al. [5]

Von Hámos spectrometer for X-ray absorption (XAS) and X-ray emission spectroscopy (XES)

Pushing the energy resolution and flexibility of the spectrometer a notch further did not allow for shortcuts – the instrument had to be designed from a scratch. Németh and co-workers decided for a geometry rarely used in the laboratory – von Hámos – and combined it with a segmented crystal [6] and the MYTHEN 1K (Figure 2). Their design [7] did not only enable a range of energies to be collected scan-free, but it also allowed for an easy switch between XAS to XES setups. And all this at high resolution.

The XAS setup features an energy resolution of incredible 2 eV, with a detector contribution of only 0.25 eV. A spectrum covering 360 eV is collected in one shot, whose high-quality data relies on the background suppression using optimized threshold values. The counting mode of the detector is actively used to calculate the statistical error of the measurement, i.e. the detection limit, which is as low as 0.33% for the intensities recorded in one hour including the absorption by the sample. The first published results show that structural variations and stability constants of the complexes formed by a tertiary system Ni(II)-EDTA-CN^- can be determined from the same measurement series [8]. This proves that the laboratory XAS can be considered as a multipurpose analytical tool.

The XES setup is achieved by a simple readjustment of the breadboards. Energy resolution is defined by slitting the sample emission, and it is theoretically limited by the analyzer. Although such slitting decreases the photon flux, the signal intensity can be compensated with long exposure times because the detector accumulates no noise.

Figure 2. XAS setup (easily realigned into XES setup) proposed by Németh et al. [7]

Can the MYTHEN detector transform X-ray spectrometry as it did diffraction? Without any doubt, both developments hugely benefitted from detector features: high spatial resolution, background suppression, wide active area, noise-free performance, counting mode, sensitivity, price and maintenance-free design. But, it is the commercial potential of these spectrometers that could really make a difference. We can imagine these spectrometers in every lab. Can you?

Footnotes:

These developments are based on the MYTHEN 1K detector. Since 2014 DECTRIS offers the MYTHEN2 detector series, featuring additionally a compact module with 640 strips.
Shimadzu offers X-ray diffractometers equipped with MYTHEN2 R 1K systems.

References:
[1] Micelli, A. (2009), JINST 4, P03024.
[2] Pacold, J. I. et al. (2012), J. Synch. Rad. 19, 245-251.
[3] Bitter, M. et al. (2014) Rev. Sci. Instrum. 85(11), 11D627.
[4] Kleimenov, E. et al. (2009) J. Phys. Conf. Ser. 190, 012035.
[5] Sato, K. et al. (2017) X-Ray Spectrom. 46, 330-335.
[6] Szlachetko, J. et al. (2012) Rev. Sci. Instrum. 83, 103105.
[7] Németh, Z., et al. (2016) Rev. Sci. Instrum. 87, 103105.
[8] Bajnóczi, E.G., Németh, Z., Vankó, G. (2017) Inorg. Chem. 56, 14220-14226.