Field evaporation and atom probe tomography of pure water tips

Abstract

Measuring biological samples by atom probe tomography (APT) in their natural environment, i.e. aqueous solution, would take this analytical method, which is currently well established for metals, semi-conductive materials and non-metals, to a new level. It would give information about the 3D chemical structure of biological systems, which could enable unprecedented insights into biological systems and processes, such as virus protein interactions. For this future aim, we present as a first essential step the APT analysis of pure water (Milli-Q) which is the main component of biological systems. After Cryo-preparation, nanometric water tips are field evaporated with assistance by short laser pulses. The obtained data sets of several tens of millions of atoms reveal a complex evaporation behavior. Understanding the field evaporation process of water is fundamental for the measurement of more complex biological systems. For the identification of the individual signals in the mass spectrum, DFT calculations were performed to prove the stability of the detected molecules.

3D Sub-Nanometer Analysis of Glucose in an Aqueous Solution by Cryo-Atom Probe Tomography

“…The mass spectrum of the saturated glucose solution (Fig. 1) reveals more than 140 mass peaks, a clear increase compared to the pure water spectra 15 . Peak identi cation is ambiguous due to the large number of possible combinations of charge states and their elemental combinations.…”

“…Peak identi cation is ambiguous due to the large number of possible combinations of charge states and their elemental combinations. We generally favoured the simplest explanation for a speci c mass signal, using the lowest possible charge state option to assign a molecular identity to each signal, since the effective evaporation eld for water is low 15,16 and thus lower charge states are more likely to be observed. Different approaches to assign individual peaks will be discussed below and compared to the expected ratio of water to glucose molecules.…”

“…While single atoms typically evaporate from metallic samples, molecular ions of varying size [24][25][26] manifesting in a large variety are typically observed for non-metallic samples, with just a small fraction of atomic ions sometimes arising from the dissociation of metastable molecular ions during the ight. Earlier measurements of thin water layers 17,27,28 and recent observations from thick ice layers 15,16 show a strong tendency in the creation of protonated water clusters of varying size. Atomic hydrogen and oxygen are rarely detected and mostly fragments from the dissociation of molecular ions [29][30][31] .…”

“…Depending on the controlled removal of individual atoms from a sharp needle-like emitter in the form of ions, this destructive method transforms the studied sample volume into a 3D computer model with individual ions characterized chemically. One of the current trends in APT is to gain a deeper understanding of soft matter in general and liquids in particular [7][8][9][10][11][12][13][14][15][16] . While thin layers of ice, created by condensed vapour on metallic tips, have been studied previously [17][18][19][20][21] , the newly available cryo-transfer systems 22,23 facilitate the preparation and transport of APT specimens from frozen liquids with large sampled volumes.…”

“…Equipped with the background knowledge from the prior analysis of pure Mili-Q water 15 , we focus in this study on the differences in the eld evaporation behaviour between a saturated glucose solution, bulk glucose and pure water to broaden the discussion basis and advance the understanding and map the possibilities and limitations inherent to the application of state-of-the-art cryo-APT to the analysis of solutions, molecules and biological materials.…”