Ice Deposition of Water Secondary Ion Mass Spectrometry at Cryogenic Temperatures

Abstract

Abstract

Water gas cluster ion beams (GCIBs) are increasingly used in cryogenic secondary ion mass spectrometry (cryo-SIMS) to improve the detection of biomolecules while preserving their molecular integrity. However, the effects of water cluster primary ion (PI) beams on cryogenically cooled surfaces are not yet fully understood. Here, we demonstrate that continuous bombardment with (H 2 O) n + GCIBs can induce ice deposition, which competes with sputtering and can ultimately obscure the underlying sample, thus compromising depth profiling and molecular analysis. Using deuterated (D 2 O) n + clusters and subsequent C60 + depth profiling, we confirmed that the deposited ice originates predominantly from water molecules introduced by the primary ion beam. Systematic investigation of cluster size and kinetic energy identified the energy per water molecule (E/n) as the key parameter governing the balance between net sputtering and ice deposition. Significant ice deposition occurred at E/n 5 eV. Measurements of the deposited ice cross-sections indicated that up to 40% of the incoming water molecules could remain on the surface under deposition-dominated conditions. Substrate temperature also influenced deposition, with ice accumulation observed below approximately 188 K for the investigated beam conditions. Experiments on copper, organic deposits, and mouse brain tissue further demonstrated that this phenomenon is not restricted to metallic substrates and that the deposition rate varies with surface composition. These findings establish beam energy per molecule, substrate temperature, and material-dependent sputtering behaviour as important factors controlling ice accumulation during cryo-SIMS using (H 2 O) n + GCIBs. Careful optimisation of GCIB parameters is therefore essential to minimise deposition artefacts and maintain reliable molecular depth profiling, particularly when cryogenic sample preservation is required. Lastly, the ability to deposit known amounts of ice in localised areas could prove useful for preparing samples with well-defined and controlled characteristics.