Expanding time-resolved ICP-MS capabilities for characterizing cells and biogenic nanoparticles in hypersaline media

The analysis of halophilic organisms using time-resolved inductively coupled plasma mass spectrometry (TRA ICP-MS) is challenging due to severe matrix effects caused by the high total dissolved solid concentration (200 g L^-1) in their culture media. This work introduces a methodology that overcomes these issues by using Haloferax mediterranei as a model organism. The developed method preserves cell integrity and minimizes matrix effects, which enabled us to investigate Pb bioaccumulation and selenium nanoparticle (SeNP) formation in this archaeon by means of time-resolved inductively coupled plasma time-of-flight mass spectrometry (TRA ICP-TOFMS).

To preserve H. mediterranei cells and mitigate matrix effects from high total dissolved solids, TRA ICP-MS working conditions were carefully optimized. A High-efficiency nebulizer and double-pass spray chamber were selected for sample introduction, as this configuration ensured high sensitivity and reduced matrix load into the plasma. Additionally, on-line sample dilution using a T-connector to achieve a 1:10³ ratio and aerosol dilution were implemented to further reduce matrix load and mitigate cell osmotic stress. The method was used to monitor Pb bioaccumulation and detect biogenic SeNPs within individual cells using ICP-TOFMS. Lead and SeNPs detection limits were, respectively, 15 ag cell^-1 and 42 nm. Single cell Pb uptake was highly heterogeneous, ranging from 20 to 300 ag cell^-1 while SeNP sizes were between 47 and 73 nm. Co-detected events of Pb and Se suggests that SeNPs are located intracellularly rather than being expelled and freely suspended in the medium.

This methodology represents a significant advancement, as it is the first TRA ICP-MS approach to accurately analyze single halophilic cell in hypersaline media without the need for extensive sample preparation. This development allows for the quantitative assessment of heavy metal bioaccumulation and NP formation in extremophiles, thereby extending TRA ICP-MS applicability.