# Discrete Entity Analysis via Microwave-Induced Nitrogen Plasma–Mass Spectrometry in Single-Event Mode

## TL;DR

This paper introduces a new method using nitrogen plasma mass spectrometry to analyze tiny particles like nanoparticles and cells with high accuracy.

## Contribution

The first use of single-event MINP-MS for discrete entity analysis, overcoming Ar-based interferences and enabling ultra-trace element detection.

## Key findings

- Single-event MINP-MS achieved an Fe detection limit of 8.6 ag, significantly lower than ICP-MS.
- The method accurately sized Fe2O3 and Se nanoparticles, and Se-enriched yeast cells using external calibration.
- Polystyrene and PTFE microplastics were accurately sized, showing suitability for polymeric materials.

## Abstract

In this work, single-event microwave-induced
nitrogen
plasma–mass
spectrometry (single-event MINP-MS) was evaluated for the first time
for the analysis of discrete entities such as nanoparticles, biological
cells, and microplastics. Nitrogen (N2) effectively overcomes
Ar-based polyatomic interferences, enabling (ultra)­trace element determination
of Fe and Se using their most abundant isotopes, 56Fe (91.66%)
and 80Se (49.82%). Iron oxide nanoparticles (Fe2O3 NPs) ranging from 20 to 70 nm were accurately characterized,
with excellent agreement with established sizing techniques, such
as transmission electron microscopy (TEM) and dynamic light scattering
(DLS). A limit of detection (LoD) of 8.6 ag for Feequivalent
to an LoDsize of 19 nm for Fe2O3was
achieved, which is significantly lower than recent values reported
for high-end quadrupole-based ICP-MS. Selenium nanoparticles (SeNPs)
of 150 and 250 nm were also accurately characterized, without the
N2-based plasma experiencing issues handling relatively
large metallic NPs (linearity, R
2 = 0.9994).
Se-enriched yeast cells (SELM-1 certified reference material) were
successfully analyzed via single-cell MINP-MS using external calibration
based on SeNPs and a transport efficiency-independent approach. In
addition, 2–3 μm polystyrene (PS) and polytetrafluoroethylene
(PTFE) were accurately sized by monitoring 12C+, confirming the method’s suitability for handling micrometer-sized
polymeric materials (microplastics). The average duration of individual
events (680 ± 160 μs) suggests that the digestion of individual
entities in N2-based plasmas is comparable to that in Ar-based
plasmas. These results open new avenues for this instrumentation as
an alternative to ICP ionization sources, also in the context of discrete
entity analysis.