A Hierarchical Etched Grid for Correlative Chemical Imaging.

Correlative microscopy linking synchrotron X-ray fluorescence (SXRF) with optical imaging is valuable for contextualizing chemical element distributions in biology. The spatial correlation necessary to achieve this presents fundamental challenges and can be a significant constraint on accuracy and data interpretation. We present a technical solution based on a finder grid concept, optimized for SXRF correlative studies of metals in biological tissues, with scope for wider adaptation and application. A hierarchically patterned fiducial system was directly etched onto spectroscopically clean quartz substrates via femtosecond laser ablation. This design enables improved correlation among SXRF, optical imaging, and histological staining over a greater range of length scales than conventional registration methods such as the use of tissue architecture from serial sections and the use of electron-microscopy-resolution finder grids and applied fiduciary markers that can introduce XRF-signal-dominating levels of elements such as copper, nickel, gold, and titanium. We present two quartz finder grid formats: a microgrid and a nanogrid design. We demonstrate their utility for rapid ROI relocalization and same-section correlative workflows using human brain tissue. The etched quartz finder grid approach facilitates rapid and reproducible ROI relocalization and alignment across instruments, particularly where integral fiducial markers are sparse or ambiguous.