Hybrid SIMS: Secondary Ion Mass Spectrometry Imaging with High Mass Resolving Power

Secondary ion mass spectrometry (SIMS) is the method of choice when high lateral resolution mass spectrometry imaging with only minimal sample preparation is needed. The aim to characterize complex samples, e.g. biological tissue, increases the demand for imaging instrumentation with high resolving power, i.e. the capability to separate ion signals with a small difference in mass. Higher mass resolving power significantly reduces the risk of misleading ion distribution images due to mass interferences. On the other hand, to obtain robust and confident (molecular) assignments of detected ion species MS/MS techniques paired with a high mass accuracy are essential. Ideally, the mass accuracy should not be affected by surface charging, sample topography, and primary ion beam parameters. As a consequence researchers have driven forward the idea of developing SIMS instruments that (1) decouple ion generation and ion detection[1],[2] and/or (2) utilize mass analyzers with high resolving power[3],[6] and MS/MS[4],[5]. Even a combined tool with the ability to perform fast imaging and high resolution mass spectrometry including MS/MS capability was described[6], but never commercialized. A multi-national development project team[7] lead by NPL UK succeeded in 2016 in building a dedicated instrument for high resolution mass spectrometry analysis especially for biological, but also industrial applications[8]. This instrument combines a time-of-flight and an orbital trapping mass analyzer and is commercially available under the name “Hybrid SIMS”. The 2D/3D mass spectrometry imaging was performed on a Hybrid SIMS (IONTOF GmbH, prototype described in [8]), a combined TOF-SIMS Orbitrap mass spectrometer. 30 keV Bi3 ions were used for sub-micron surface imaging in combination with the TOF analyser, while 20 keV Arn (1000<n<4000) clusters served as primary ions for the Orbitrap analyser (Q Exactive HF, Thermo Fisher Scientific) and to sputter through the sample. For 3D analysis TOF-SIMS imaging (Bi) can be alternated with Ar cluster sputtering (non-interlaced mode). Ions from the sputter phase can be analysed in the Orbitrap analyzer and can selectively be subjected to MS/MS, based on an in-house developed algorithm. Acquisition was fully controlled by SurfaceLab 7 (IONTOF) employing Thermo's Exactive API. After introduction to the instrument concept and design, benefits and limitations of the two analyzers will be discussed and the key advantages of their combination will be exemplified on basis of industrial as well as biological applications. Certain restrictions related to saturation of the ion registration system, lateral resolution, speed of the analysis and higher complexity of spectral data will be mentioned and measures to reduce or overcome their impact will be discussed.