Online Microdroplet Calibration for Accurate Elemental Analysis via Single-Cell ICP-TOF-MS

In fifteen words or less, explain the significance of this contribution (Novel Aspect).: Enables precise single-cell elemental analysis using simultaneous microdroplet calibration in complex biological matrices

Abstract Text:

Essential metals such as Fe, Cu, Zn, Mn, P, and S exist at femtogram levels in individual cells, making their detection highly challenging. Inductively coupled plasma-mass spectrometry (ICP-MS) is a powerful tool for trace elemental analysis, but traditional bulk analysis averages signals across cell populations, masking important cell-to-cell variability. Single-cell ICP-MS (SC-ICP-MS), which introduces intact cells in suspension, offers high-throughput elemental analysis at the individual cell level, revealing population heterogeneity. [1]

However, SC-ICP-MS faces specific limitations when applied to mammalian cells, which typically require chemical fixation prior to analysis. [1] Common fixatives—such as methanol, glutaraldehyde, or paraformaldehyde (PF)—can alter intracellular elemental distributions. Additionally, introducing live cells often involves complex, high-salt matrices like phosphate-buffered saline (PBS), which cause matrix effects that compromise analytical accuracy.

This study explores an online microdroplet (MD) calibration strategy using the MicroDroplet Generator (MDG, Quantistar, Tofwerk AG, Switzerland) for accurate, matrix-matched single-cell elemental analysis. [2] Through a dual-inlet sample introduction system, calibration microdroplets and nebulized cells are simultaneously delivered into the ICP using a syringe pump. Because both droplets and cells experience identical matrix effects, this setup enables effective real-time calibration. The MDG is coupled with an ICP-Time-Of-Flight-MS (icpTOF-2R, Tofwerk AG), allowing simultaneous, multi-element detection in both droplets and cells.

Case studies include PF-fixed red blood cells (PF-RBCs), PF-fixed white blood cells (PF-WBCs), and diluted whole blood samples prior to cell isolation. Cells were analyzed in water and in PBS at varying concentrations (100%, 50%, 25%, 0%), as well as live-cell conditions in 100% PBS. Key elements analyzed include major components (Fe, P, S) and trace elements (Zn, Cu, Mn). Results from this matrix-matched calibration approach are compared to conventional SC-ICP-MS methods using external ionic standards and silver nanoparticles (AgNPs, 80 nm) for transport efficiency estimation. [3] This novel method offers improved quantification accuracy for single-cell elemental analysis under biologically relevant conditions and complex media.

References

[1] M. Corte-Rodríguez, et al.,  TrAC, 2020, 132, 116042-116058

[2] L. Hendriks, et al., J. Anal. At. Spectrom., 2019, 34, 716.

[3] R. Dejonghe, et al., Microchem. J., 2024, 207, 112013.