Strategies to enhance figures of merit in ICP-ToF-MS

Inductively coupled plasma with time-of-flight mass spectrometry (ICP-ToF-MS) is currently setting new benchmarks for the analysis of single particles (SP) and elemental mapping using laser ablation (LA). The rapid collection of the full elemental mass spectra promotes non-target approaches, fast imaging as well as inquiries of particle stoichiometry. However, one shortcoming often associated with ICP-ToF-MS is a lack of detection power due to lower duty cycles relative to sequentially operating mass analysers. The sensitivity of ICP-ToF-MS can be increased using two strategies, which are detailed in this study. First, instead of analysing full mass spectra, elements in the low and high mass range were excluded from analysis using a Bradbury-Nielsen gate. The resulting restricted mass range was acquired up to 5 times faster increasing duty cycles and sensitivity accordingly. Second, isotopes of polyisotopic elements recorded simultaneously were accumulated to increase signal to noise ratios. In a proof of concept, we applied SP ICP-ToF-MS for the first time for the characterisation of upconversion nanoparticles (UCNPs) which contained Gd and Yb. Both signal amplification strategies were combined and the consequences for detection limits and signal to noise ratios were considered and compared to a standard method. Sensitivities were increased up to factor 27 when accumulating all Gd and Yb isotopes at 177 kHz, and size detection limits decreased by a factor of approximately 3. Improved figures of merit promoted more accurate investigations of UCNPs, which were characterised regarding size distributions and composition. As second application, we demonstrated the utility of the described strategies in LA-ICP-ToF-MS. Mo and Se were targeted as relatively rare elements in rat brain tissue. Increased acquisition frequencies of 185 kHz and isotope accumulation resulted into drastically improved signal to noise ratios and enabled the mapping of both while still considering relevant neuroanatomical elements such as Fe and Zn.