Correlative Electron Microscopy, High Resolution Ion Imaging and Secondary Ion Mass Spectrometry for High Resolution Nanoanalytics on Biological Tissue

Abstract

Until the early years of this century, applications featuring sub-20 nm-imaging and chemical analysis at the same time were only available for electron microscopy. However, especially for life science samples, chemical analysis by e.g. energy dispersive x-ray microanalysis (EDX) was tedious and detection limits were often insufficient to investigate trace elemental information in subcellular organelles. Within the past years however, several applications have been further developed that use a focused primary ion beam for imaging (by detecting secondary electrons, SE) and chemical analysis (by secondary ion mass spectrometry, SIMS) of elements, isotopes and small or large cluster molecules in both plastic embedded tissue sections and even cryo-preserved material [1]. By combining TEM and nanoscale SIMS on the same sample, high resolution imaging with quantitative EDX was correlated with high sensitivity SIMS with a probe size of about 50-100 nm[2]. The latter has advantageous sensitivity, mass resolution, detection limits and provides valuable information of the distribution of structural and trace elements. Although biological tissue sections have poor contrast in the SE images, secondary ion maps of e.g. endogenous CN-ion clusters show the same morphological fingerprints as EM images based on a chemical analysis. Meanwhile, correlative instruments are being developed that combine high lateral resolution ion microscopy and SIMS, e.g. the HIM-SIMS with < 1nm lateral resolution in SE imaging and sub-20 nm in SIMS mapping modes [3, 4]. Furthermore, a prototype integrated transmission electron microscope-SIMS (TEM-SIMS) allows direct switching between TEM and SIMS and therefore combines their respective benefits, e.g. for analysis of low Z elements including hydrogen and isotopes [5].