Synchrotron-based
X-ray fluorescence microscopy (XFM) analysis
is a powerful technique that can be used to visualize elemental distributions
across a broad range of sample types. Compared to conventional mapping
techniques such as laser ablation inductively coupled plasma mass
spectrometry or benchtop XFM, synchrotron-based XFM provides faster
and more sensitive analyses. However, access to synchrotron XFM beamlines
is highly competitive, and as a result, these beamlines are often
oversubscribed. Therefore, XFM experiments that require many large
samples to be scanned can penalize beamline throughput. Our study
was largely driven by the need to scan large gels (170 cm
2
) using XFM without decreasing beamline throughput. We describe a
novel approach for acquiring two sets of XFM data using two fluorescence
detectors in tandem; essentially performing two separate experiments
simultaneously. We measured the effects of tandem scanning on beam
quality by analyzing a range of contrasting samples downstream while
simultaneously scanning different gel materials upstream. The upstream
gels were thin (<200 μm) diffusive gradients in thin-film
(DGT) binding gels. DGTs are passive samplers that are deployed in
water, soil, and sediment to measure the concentration and distribution
of potentially bioavailable nutrients and contaminants. When deployed
on soil, DGTs are typically small (2.5 cm
2
), so we developed
large DGTs (170 cm
2
), which can be used to provide extensive
maps to visualize the diffusion of fertilizers in soil. Of the DGT
gel materials tested (
bis
-acrylamide, polyacrylamide,
and polyurethane), polyurethane gels were most suitable for XFM analysis,
having favorable handling, drying, and analytical properties. This
gel type enabled quantitative (>99%) transmittance with minimal
(<3%)
flux variation during raster scanning, whereas the other gels had
a substantial effect on the beam focus. For the first time, we have
(1) used XFM for mapping analytes in large DGTs and (2) developed
a tandem probe analysis mode for synchrotron-based XFM, effectively
doubling throughput. The novel tandem probe analysis mode described
here is of broad applicability across many XFM beamlines as it could
be used for future experiments where any uniform, highly transmissive
sample could be analyzed upstream in the “background”
of downstream samples.
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