Improvement of biological time-of-flight-secondary ion mass spectrometry imaging with a bismuth cluster ion source

TLDR

The authors fitted a bismuth liquid metal ion gun to a TOF‑SIMS and evaluated its cluster beams on rat brain sections, comparing secondary ion yields, cross sections, efficiencies, and lateral resolution across m/z 23–750 while accounting for primary ion current to optimize acquisition times. The bismuth LMIG produces Binq+ (n = 1–7, q = 1,2) clusters that deliver higher intensities and efficiencies than Au3+, yielding improved mass and lateral resolution; Bi3+ and Bi52+ are the most effective primary ions, with Bi3+ enabling large‑area imaging and Bi52+ providing high sensitivity and sub‑400‑nm resolution for small‑area images.

Abstract

A new liquid metal ion gun (LMIG) filled with bismuth has been fitted to a time-of-flight—secondary ion mass spectrometer (TOF-SIMS). This source provides beams of Binq+ clusters with n = 1–7 and q = 1 and 2. The appropriate clusters have much better intensities and efficiencies than the Au3+ gold clusters recently used in TOF-SIMS imaging, and allow better lateral and mass resolution. The different beams delivered by this ion source have been tested for biological imaging of rat brain sections. The results show a great improvement of the imaging capabilities in terms of accessible mass range and useful lateral resolution. Secondary ion yields Y, disappearance cross sections σ, efficiencies E = Y/σ;, and useful lateral resolutions ΔL have been compared using the different bismuth clusters, directly onto the surface of rat brain sections and for several positive and negative secondary ions with m/z ranging from 23 up to more than 750. The efficiency and the imaging capabilities of the different primary ions are compared by taking into account the primary ion current for reasonable acquisition times. The two best primary ions are Bi3+ and Bi52+. The Bi3+ ion beam has a current at least five times larger than Au3+ and therefore is an excellent beam for large-area imaging. Bi52+ ions exhibit large secondary ions yields and a reasonable intensity making them suitable for small-area images with an excellent sensitivity and a possible useful lateral resolution <400 nm.