High-Resolution EDS Mapping of Intracellular Chemical Variations

Elemental analysis of biological specimens using SEM EDS is inherently challenging, not only due to low X-ray yield and spectral overlap, but also because of pronounced beam sensitivity. In this study, Deinococcus radiodurans was analyzed using SEM EDS with the XFlash^® FlatQUAD 2 detector.

Challenges

As shown in Figure 1, D. radiodurans undergoes rapid beam-induced deformation at 5 kV, progressively flattening and losing structural definition with increasing exposure. This places strict limits on acquisition time and dose, requiring high-sensitivity detection to obtain meaningful chemical information before structural integrity is compromised.

In addition, reliable detection of light elements remains challenging due to low signal intensity and spectral interferences introduced by supporting substrates and preparation methods. Contributions from Au/Pd coatings, Ni, Cu, Mo TEM grids, carbon support films, and heavy-element stains (e.g., Os, Pb, U) can dominate the low-energy region of the spectrum. In particular, biologically relevant elements such as phosphorus and sulfur overlap with Au-M, Os-M, and Pb-M lines, requiring high count rates and robust spectral deconvolution for reliable separation (Figure 2 and 3).

Deinococcus radiodurans is known for its exceptional resistance to ionizing radiation, largely attributed to efficient DNA protection and repair mechanisms. Elemental mapping of phosphorus and sulfur provides insight into the spatial organization of nucleic acids, proteins, and protective biomolecules within the cell.

Results

EDS maps acquired at 5 kV and 1.2 nA probe current reveal clear biochemical compartmentalization, with phosphorus-rich domains embedded within a sulfur-rich matrix. This spatial separation reflects localized concentrations of phosphorus- and sulfur-containing biomolecules that are associated with nucleic acids, proteins, and cellular defense mechanisms.

Despite strong peak intensities from the Au/Pd-coated Cu TEM grid, high-count rate (~900,000 cps) and high spectral resolution can be achieved with the XFlash^®FlatQUAD 2L detector. This allows reliable deconvolution of overlapping peaks (Figure 4) for the accurate identification of intrinsic biological signals. In this example, we demonstrate that the detector unique combination of high detection efficiency and optimized geometry ensures robust signal acquisition even under low-dose conditions required for beam-sensitive samples.

This approach enables rapid acquisition (~60 s) of high-quality chemical maps, resolving sub-micron features while drastically minimizing beam-induced damage. It proves the capability of advanced EDS technology to probe functional chemical organization in biological systems under realistic SEM conditions.

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Figure 1: Sequential SEM images acquired at 5 kV and 1.2 nA probe current with beam exposures of (a) ~30 s, (b) ~120 s and (c) ~300 s. The D. radiodurans cell undergoes clear beam-induced deformation, progressively flattening and losing structural definition. This highlights that high-sensitivity detection is essential to obtain meaningful elemental information before beam-induced deformation alters the specimen.

Figure 2: SEM image (left) and EDS elemental maps of Deinococcus radiodurans showing phosphorus (P, yellow) and sulfur (S, purple) distributions. The maps reveal spatially distinct P- and S-rich regions, indicating sub‑micron chemical heterogeneity. High count rate acquisition (~900,000 cps, 60 s) at 5 kV and 1.2 nA probe current using XFlash^® FlatQUAD 2L enables reliable separation of P and S signals despite overlap from Au coated TEM grid contributions.

Figure 3: SEM image (left) and corresponding EDS elemental maps of Deinococcus radiodurans showing distributions of P, S, Na, O, N and Mg. The maps reveal spatially heterogeneous elemental distributions at the sub‑micron scale, consistent with intracellular chemical variation. Acquired at 5 kV and 1.2 nA using XFlash^® FlatQUAD 2L, enabling high-sensitivity mapping of light elements in biological material despite low signal levels and spectral overlap.

Figure 4: EDS spectrum of Deinococcus radiodurans showing clear peak deconvolution of biologically relevant P and S signals from the Au/Pd-coated Cu TEM grid substrate. Despite strong overlap with Au (2.123 kV), the P (2.013 keV) and S (~2.307 keV) peaks are reliably resolved, confirming intrinsic cellular chemistry. This separation is critical for accurate interpretation of phosphorus- and sulfur-driven biochemical compartmentalization in radiodurans, without substrate-induced misassignment.

Further Information

Acknowledgements

Thanks to Dr Brian Caffrey of the Rosalind Franklin Institute (RFI), UK, for the sample.

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High-Resolution EDS Mapping of Intracellular Chemical Variations

Challenges

Results

Further Information

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