mirror-based x-ray fluorescence microprobes at the advanced photon

MIRROR-BASED X-RAY FLUORESCENCE MICROPROBES AT THE ADVANCED PHOTON SOURCE AND THE NATIONAL SYNCHROTRON

LIGHT SOURCE

S. R. Sutton1,2, M. Newville2, P. Eng2, M. Rivers1,2 and A. Lanzirotti2

1Department of Geophysical Sciences and 2Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637

ABSTRACT

High-energy synchrotrons are valuable sources of highly collimated, intense X-ray radiation for use in X-ray microprobe analysis including trace element quantification (X-ray fluorescence), chemical speciation determinations (X-ray absorption fine structure spectroscopy) and phase identification (X-ray diffraction). Kirkpatrick-Baez mirrors are being increasingly utilized for production of X-ray microbeams (~ µm) because of their achromaticity, photon density gains in excess of 104, and long working distances (centimeters). Two such instruments reside at the GeoSoilEnviroCARS sector at the Advanced Photon Source (Argonne, IL, USA) and the X26A beamline at the National Synchrotron Light Source (Upton, NY, USA). These instruments are utilized primarily for research on problems in low temperature geochemistry, environmental science, igneous petrology, planetary science, and other earth science related topics but are also seeing application in other areas. The science is driven by the requirement for determination of the compositions, structures, oxidation states, and bonding characteristics of chemical species in materials with trace element sensitivity and micrometer spatial resolution. Example applications a given.

INTRODUCTION

X-ray microprobes utilize photons for excitation and photons for detection for spatially resolved materials analysis based on techniques such as X-ray fluorescence (XRF), X-ray absorption fine structure (XAFS) spectroscopy, X-ray diffraction (XRD) and fluorescence microtomography (CMT). The key to the practicality of the X-ray microprobe is the availability of synchrotron X-ray sources. The high brightness and brilliance of these sources allows small, intense X-ray beams to be produced leading to high sensitivity and spatial resolution. The sensitivity of the technique is further enhanced by the polarization properties of the synchrotron radiation that allows the geometry of the experiment to be optimized for highest signal to noise ratios. In this way, the capabilities of an X-ray microprobe are enhanced by orders of magnitude when used with a synchrotron source as compared to what is achievable with a laboratory tube source. Other discussions of synchrotron-based microprobe techniques can be found in references [1]-[9]. Hard X-ray (> few keV) microprobes are currently in use at most high energy synchrotron facilities and these instruments are complementary in terms of spatial resolution, sensitivity, flexibility and availability. The focus of this paper is on two microprobes, one operating at the National Synchrotron Light Source (Beamline X26A) and the other at the Advanced Photon Source (GeoSoilEnviroCARS Sector 13). The X26A Microprobe Facility (http://www.bnl.gov/x26a) is the currently the only high-energy X-ray microprobe at NSLS and resides on a dedicated bending magnet beam line. At the APS (http://www.aps.anl.gov), there are several microprobes on

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undulator beam lines. The one at GeoSoilEnviroCARS (13-ID; http://gsecars.org) is dedicated to earth and environmental science research with 90% of the beam time allocated to the scientific community through a proposal based system but the microprobe receives only 20% of the available beam time (sharing with other techniques such as high pressure diffraction). The APS undulator source, in combination with Kirkpatrick-Baez (KB) mirrors, is an excellent X-ray microprobe source, delivering ~ 1 µm X-ray beams with fluxes that are factors of 102-103 greater than those on other sources. The NSLS microprobe, also using KB mirrors, has the advantage of residing on a dedicated beam line so that more beam time is typically available for particular experiments. This microprobe has somewhat poorer sensitivity and spatial resolution than the APS instrument but is very well suited for many experiments. The combination of the two represents a powerful approach to materials characterization where initial characterization work is done at the NSLS and then the APS instrument is brought into play when its capabilities are required, such as the need for higher sensitivity or better spatial resolution. In this paper, we describe these two instruments, their capabilities and summarize some of the recent applications. INSTRUMENTATION The incident X-ray beam paths consist of the following components (running downstream from the X-ray source): monochromator, intensity monitor, microbeam optics and sample stage. The sample stage is typically mounted in a 45°/45° geometry. A visible light microscope is mounted normal to the sample surface for front surface viewing and the X-ray detector is positioned at 90° to the incident beam and within the horizontal plane of the synchrotron. This arrangement allows thick samples to be analyzed and minimizes background from scattered radiation, a consequence of the beam polarization. Si(111) monochromators are used for both instruments. The X26A microprobe has a water cooled channel cut whereas the APS system utilizes a fixed offset, double crystal device that is cryogenically cooled. The operating ranges are 4-30 and 4-50 keV, respectively, with bandwidths (dE/E) ~10-4, i.e., ~1 eV. The crystals also transmit higher harmonics that are typically well rejected by the high-energy cut-off of the microfocusing system. In the case of the double crystal device at the APS, harmonic rejection can also be achieved by detuning the second crystal. X-ray microbeams can be produced using techniques that rely on collimation, refraction, diffraction, or reflection [10]. These microprobes utilize Kirkpatrick-Baez (KB) microfocusing mirrors [11]-[14]. The KB mirrors have the advantages of achromaticity (all energies are focused to the same spot), achievable gains in flux density in excess of 104, and long working distances (centimeters) to accommodate an array of ancillary instruments (microscopes, detectors, etc.). The KB mirror system consists of two mirrors, one in the horizontal plane and one in the vertical plane where the flat mirrors are bent mechanically to elliptical shapes to perform point (source)-to-point (sample) focusing [12]. The mirrors are highly polished, single crystal silicon coated with several hundred Å of Rh. The “double-bounce” focusing system provides excellent harmonic rejection capabilities for microXAFS applications. These mirrors can demagnify the beam up to a factor of about 300 [12] which practically translates to microfocused beam sizes of about 1 µm at APS and 5 µm at NSLS.


Various complementary detectors are useful for the microprobe. For collecting X-ray fluorescence spectra (i.e., for XRF, XAFS and fluorescence CMT), the solid state, energy-dispersive (EDS) detector is the workhorse (~100 eV resolution). Both instruments have germanium array detectors produced by Canberra Industries, Inc. (Meriden, CT), coupled with digital signal processing electronics by X-ray Instrumentation Associates (Newark, CA). Another useful fluorescence detector for higher energy resolution (~10 eV) is the wavelength dispersive spectrometer and both microprobes have WDX spectrometers by Oxford Instruments (Fremont, CA). For microdiffraction, X26A has a Bruker 1500 CCD driven by the SMART software. A useful device for performing analyses at elevated temperature is the Linkam 1500 heating stage (Linkam Scientific Instruments, Ltd, UK). This device was designed for diffraction experiments at Daresbury and has been used by us at APS for XRF and XAFS analyses of individual fluid inclusions at temperatures up to ~500 °C (e.g.,[15]). The X-ray microprobe at APS has a microXRF detection limit near 0.01 mg kg-1 (10 ppb) in 30 pg samples which translates into ~ 100 ag (=10-16g) or ~ 106 atoms. At X26A, the detection limit is about 1 ppm. For microXANES, a concentration of ~ 10 mg kg-1 is required and 100-1000 mg kg-1 for microEXAFS (100 mg kg-1 and 0.1-1% for X26A, respectively). Fluorescence CMT is more time consuming than conventional transmission CMT because of the need for using a first-generation (focused beam) approach. Typical collection times at the APS are a few hours for a single 2D slice with element sensitivities in the 10 mg kg-1 range and spatial resolutions of a few µm. APPLICATIONS Synchrotron-based microanalytical techniques offer distinct advantages over other analytical techniques by allowing analyses in situ, an important example being the ability to determine chemical speciation of a wide variety of toxic elements in moist soils, waste-forms, and biological specimens with little or no chemical pretreatment at detection limits that typically exceed those of conventional methods by several orders-of-magnitude. The availability of these quantitative, high-sensitivity, high-spatial resolution, microanalytical techniques have led to major advances in our understanding of fluid/mineral interactions and geochemical siting and behavior of contaminants. In particular, microXAFS allows one to quantify and map oxidation state ratios in heterogeneous earth materials and individual mineral grains. Such information is crucial in understanding the toxicity, mobility and containment of contaminating metals in the environment, mechanisms of trace element partitioning, and paths of strategic metal enrichment in nature. Examples of applications of these techniques since 2001 are given in the sections that follow. Microbial Processes and Biomineralization: Interest in environmental microbiology and geomicrobiology has arisen in the past decade because of the importance of microbial-mineral interactions in controlling critical biogeochemical processes and because of the growing interest in bioremediation of contaminated sites. Microprobe research has included studies of arsenic speciation and reactivity in poultry litter [16], uranium reduction by Shewanella putrefaciens


bacteria [17], and sorption and biomineralization of Pb by Burkholderia cepacia biofilms [18], [19]. Plant Processes: Chemical reactions occurring in plant rhizospheres and processes affecting contaminate uptake by plants are of environmental and agricultural interest, particularly in terms of phytoremediation strategies. Studies in this area have included sequestration of arsenic by ferric iron plaque on Typha latifolia roots [20],[21], uptake of radionuclides by Nicotiana, Tagetes, and Amaranthus [22], chromium chemistry in clover [23], use of trace elements in dendroanalysis [24], [25], and Cd-challenged corn roots [26]. Contaminated Environments: Many experiments conducted using the microprobes focus on the chemistry of contaminants in various environments including sediment remobilization in contaminated wetlands [27], the impact of aquatic plants in the distribution of mine-waste contaminants [28],[29], lakes as natural sinks for heavy metals from mining activity [30], and arsenic microdistributions and speciation in gold mine tailings [31]. Vadose Zone Processes: Processes in the vadose zone are important controls on contaminant transport. Some of the studies in this area include residence time effects on surface-correlated arsenate [32],[33], plutonium speciation and sorption on minerals and tuff [34],[35], co-precipitation of uranium with iron oxides [36], identification of extractable soil manganese [37], uranium chemistry in organic rich sediments [38], and diffusion and reduction of chromate in soil aggregates and dependence on microbial activity [39],[40]. Hydrothermal Fluids and Seawater: Fluid inclusions hold the keys to understanding the transport of metals in hydrothermal fluids and subsequent ore formation. Synchrotron microprobes coupled with XAFS can provide information on metal speciation in individual fluid inclusions, especially when the analyses are performed above the homogenization temperature. In addition, the compositions of fossil corals are useful for paleothermometric reconstruction of past sea surface temperatures. Several studies have been conducted on these problems including copper complexation as neutral species in vapor inclusions from the Mole Granite (Australia) [15], hydrothermal leaching rates of metamict zircon [41], oceanic and porphyry-copper hydrothermal fluid compositions [42], and strontium heterogeneity and speciation in coral and relationships with growth structure [43]-[47]. Crystal Chemistry: Information on the processes by which elements are incorporated into minerals can lead to better understanding on element cycling in the Earth and the physical and chemical conditions under which minerals and rocks form. Laboratory studies of these mechanisms have included sector zoning of rare earth elements and uranium in fluorite [48],[49] and apatite [50],[51], site-specific adsorption complexes of copper and zinc [52] and coprecipitation of uranium [53],[54] in calcite, crystal chemistry of clay-Mn oxide associations in tuff [55], iron oxidation states in rock-forming minerals [56]-[62], volatiles in biotite [63], and Fe and Ni impurities in synthetic diamonds [64]. Art and Archeology: Microprobe studies can be useful in fingerprinting material compositions in pieces of art and identifying sources for materials used in archeological artifacts. Two such


studies were focused on glass production processes in the post-Roman period [65] and pigment compositions of ancient Grecian artifacts [66]. Extraterrestrial Materials: Extraterrestrial materials (e.g., meteorites and lunar samples) can be analyzed non-destructively with synchrotron microprobe techniques. Compositional information can be used to help identify the parent bodies of meteorites and constrain their physical and chemical histories. Studies of these rare objects include examinations of meteorites from Mars involving the compositions of carbonate globules thought to be of hydrothermal origin [67]-[70], oxygen fugacity of formation of a calcium-aluminum inclusion in a carbonaceous chondrite [71], and evidence for ancient water on the Vesta asteroid [72]. Biology and Medicine: Trace element microanalyses of biological tissues can provide valuable insights into disease mechanisms. Research was conducted on the influence of zinc levels in the brain on memory deficits [73], human prostate cancer chemistry [74], identification of metal ions in purified protein [75], and technique development [76],[77]. Materials Development: The microprobes are valuable in materials development. Projects included passivation thresholds in Fe-Cr alloys [78] and the effectiveness of chromate conversion coatings [79]. FUTURE DIRECTIONS Spatial resolution currently achievable with hard X-ray microbeam techniques, down to the 100 nm range, is quite satisfactory. In many cases, spatial resolution at the beam size cannot be realized because of the properties of the sample, notable thickness (because of the penetrating nature of the X-rays, 100 nm resolution can only be achieved in samples <100 nm thick). Improvements in sensitivity can be achieved in two principal ways, increase in incident flux or improvement in detection efficiency. Major increases in incident flux will be impractical in many cases because of the radiation sensitivity of the earth and environmental samples of interest. Improvements in fluorescence detection efficiency will be a more fruitful avenue because current detection schemes use very small solid angles. Energy dispersive detectors that intercept large solid angles would be a major advance in sensitivity enhancement. Because of the interest in analyzing intact specimens with the minimum of sample preparation, fluorescence microtomography will likely be a technique used more frequently in the future. Improvements in data collection speed will be needed to make this technique more routinely applicable. ACKNOWLEDGEMENTS GSECARS is supported by NSF - Earth Sciences, DOE - Geosciences, W.M. Keck Foundation, USDA and the State of Illinois. Beamline X26A is supported in part by DOE-Geosciences. The Advanced Photon Source and National Synchrotron Light Source are supported by the U.S. DOE-BES under Contract No. W-31-109-Eng-38 and DE-AC02-98CH10886, respectively. REFERENCES [1] Bertsch, P. M., and D. B. Hunter (2001) Chem. Rev., 101 (6), 1809 -1842.


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mirror-based x-ray fluorescence microprobes at the advanced photon

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