Mercury Lα1 High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: A Versatile Speciation Probe for Mercury

Abstract

Mercury is in some sense an enigmatic element. The element and some of its compounds are a natural part of the biogeochemical cycle; while many of these can be deadly poisons at higher levels, environmental levels in the absence of anthropogenic contributions would generally be below the threshold for concern. However, mercury pollution, particularly from burning fossil fuels such as coal, is providing dramatic and increasing emissions into the environment. Because of this, the environmental chemistry and toxicology of mercury are of growing importance, with the fate of mercury being vitally dependent upon its speciation. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation, but is severely limited by poor spectroscopic energy resolution. Here, we provide a systematic examination of mercury Lα1 high energy resolution fluorescence detected XAS (HERFD–XAS) as an approach for chemical speciation of mercury, in quantitative comparison with conventional Hg LIII-edge XAS. We show that, unlike some lighter elements, chemical shifts in the Lα1 X-ray fluorescence energy can be safely neglected, so that mercury Lα1 HERFD–XAS can be treated simply as a high-resolution version of conventional XAS. We present spectra of a range of mercury compounds that may be relevant to the environmental and life science research and show that density functional theory can produce adequate simulations of the spectra. We discuss strengths and limitations of the method and quantitatively demonstrate improvements both in speciation for complex mixtures and in background rejection for low concentrations.

Molecular Fates of Organometallic Mercury in Human Brain

“…We also examined the possibility of other mercury chemical species, in particular methylmercury l selenocysteinate, which when incorporated into the linear combination fitting schemes did show subtly improved fits in some cases. As we have previously noted, , the Hg Lα1 HERFD-XAS spectra of methylmercury l -selenocysteinate and methylmercury l -cysteinate are very similar, but the features of the Hg Lα1 HERFD-XAS for the selenium congener are shifted slightly to lower energy. , Such coordination might originate through inhibition of essential selenoenzymes. We have recently shown that methylmercury inhibits the essential selenoenzyme thioredoxin reductase through covalent binding of the selenolate of the active site selenocysteine residue .…”

“…As we have previously noted, 14,30 the Hg Lα1 HERFD-XAS spectra of methylmercury L-selenocysteinate and methylmercury Lcysteinate are very similar, but the features of the Hg Lα1 HERFD-XAS for the selenium congener are shifted slightly to lower energy. 27,30 Such coordination might originate through inhibition of essential selenoenzymes. We have recently shown that methylmercury inhibits the essential selenoenzyme thioredoxin reductase through covalent binding of the selenolate of the active site selenocysteine residue.…”

“…Some initial refinements did include both two-and three-coordinate Hg(II) selenolate complexes, bis-(benzeneselenolato)mercury-(II) and the tris(benzeneselenolato)mercury(II) anion, respectively, 30 but no convincing improvements in the quality of fit were obtained, possibly because these aryl complexes show different spectra from the biologically relevant alkyl species. 30 We note that the lack of a bona fide [Hg(SeCys) 4 ] 2− standard is a limitation of the present study.…”

“…In previous work, we have used conventional X-ray absorption spectroscopy (XAS) to examine brain tissue from individuals having different types of mercury exposure, including tissues from chronic low-level and short-term high-level organometallic mercury poisoning . Recently, high energy resolution fluorescence detected XAS (HERFD-XAS) has been exploited at both the Se K edge ,, and the Hg L III edge ,,,, in order to provide significantly improved speciation in dilute systems. HERFD-XAS depends upon measurement of the X-ray fluorescence with narrower energy resolution than the natural fluorescence linewidth, the fluorescence lines of choice being the Se Kα1 and the Hg Lα1 for the Se K and Hg L III edges, respectively.…”

Challenges and Opportunities for Applications of Advanced X-ray Spectroscopy in Catalysis Research

“…In 1991, Hämälainen et al demonstrated the dramatic sharpening of HERFD XAS technique on the L 3 edge of dysprosium, laying the foundation for the increasingly popular HERFD XAS approach. Today, the technique is being widely applied across the periodic table for a variety of elements, including the first, ,− second, − and third − row transition metals, main group elements, − halogens, lanthanides, − and actinides. − …”