Improving Laboratory X-Ray Spectrometers

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

At the same time, the pursuit of high resolution in a laboratory has never been easy. In addition to the apparent challenges, flux, and acquisition time, the developments have to face many more issues like flexibility, maintenance-efficiency, cost restraints, and scanning mechanisms of data collection. However, these problems were solved by two recent developments, which employed a smart design and the DECTRIS MYTHEN¹ detector.

Polychromatic Simultaneous Wavelength-Dispersive X-Ray Fluorescence (PS-WDXRF)

The problem was elegantly approached by Sato and co-workers [5]. They incorporated the MYTHEN 1K in the Shimadzu² MX-2400WD-XRF spectrometer and exploited the 1280 detector strips to simultaneously collect data diffracted by an analyzer crystal under a range of angles (θ_low-θ_high, see Figure 1). The new setup was named polychromatic simultaneous wavelength-dispersive fluorescence (PS-WDXRF [5]) since it directly translates to a range of energies (E_high-E_low). With this scan-free spectrometer, an energy range of 1.24 keV is covered with an energy resolution of 3.9 eV (FWHM of Fe Kα1) and a sampling width of 0.98 keV per strip.

The first results are more than heartening. Even though collected in just 300 seconds, the data shows that peak position shift as well as intensities can be employed to differentiate between Mn(II) and Mn(VII). Since similar results were achieved for both Cr(III) and Cr(VI), the application is expected to be used for a variety of transition metals – a game-changer for understanding redox processes in batteries.

Figure 1. PS-WDXRF setup proposed by Sato et al. [5]. Image credit: Dectris Ltd.

Von Hámos Spectrometer for X-Ray Absorption (XAS) and X-Ray Emission Spectroscopy (XES)

Pushing the flexibility and energy resolution of the spectrometer a notch further did not allow for shortcuts – the instrument had to be built from a scratch. Németh and team decided to use von Hámos⌠—a geometry which is rarely used in the laboratory —and combined it with the MYTHEN 1K and a segmented crystal [6] (Figure 2). Their design [7] allowed a range of energies to be collected scan-free and also enabled an easy switch from XAS to XES setups. All these were done at high resolution.

The XAS setup features an incredible energy resolution of 2 eV, with a detector contribution of just 0.25 eV. A spectrum covering 360 eV is gathered in a single shot, whose high-quality data depends on the background suppression using optimized threshold values. The detector’s counting mode is actively employed to compute 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 stability constants and structural variations of the complexes formed by a tertiary system Ni(II)-EDTA-CN- can be established from the same measurement range [8]. This proves that the laboratory XAS can be regarded as a multipurpose analytical tool.

The XES setup can be achieved by simply readjusting the breadboards. Slitting the sample emission makes it to possible to define the energy resolution, and this is theoretically restricted by the analyzer. Although the photon flux is decreased by such slitting, the signal intensity can be off-set with extended exposure times because the detector does not accumulate any noise.

Figure 2. XAS setup (easily realigned into XES setup) proposed by Németh et al. [7]. Image credit: Dectris Ltd.

Is it possible for the MYTHEN detector to transform X-ray spectrometry as it did diffraction? Undoubtedly, both developments have been largely benefited from detector features: wide active area, high spatial resolution, background suppression, counting mode, sensitivity, noise-free performance, price and maintenance-free design. However, the commercial potential of these spectrometers could really make a difference. These spectrometers can well be imagined in every laboratory.

Footnotes

1. These developments are based on the MYTHEN 1K detector. DECTRIS offers the MYTHEN2 detector series since 2014, featuring an additional compact module with 640 strips.
2. Shimadzu provides 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.

This information has been sourced, reviewed and adapted from materials provided by Dectris Ltd.

For more information on this source, please visit Dectris Ltd.