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Investigation and correction of size-dependent transport efficiencies of microparticles in SP ICP-MS analysis.
Summary
Researchers systematically measured how single-particle ICP-MS transport efficiency declines sharply for microplastics above 3 µm in diameter — a size range where conventional constant-TE assumptions break down — and developed a sigmoidal correction model using flow cytometry as an independent reference, showing that without this correction both particle size distributions and number concentrations are systematically biased toward smaller particles.
Single particle inductively coupled plasma-mass spectrometry (SP ICP-MS) has emerged as a powerful technique for characterising nanoparticles (NPs) and is increasingly used to target microparticles (MPs) including microplastics. The accuracy of determined size distributions and number concentrations in SP ICP-MS is critically dependent on a precise knowledge of transport efficiency (TE), which describes the fraction of particles transferred from a suspension into the plasma. While for NPs TE can be assumed to be a constant, a sharp decline can be noted as sizes of particles increase. In this study, we systematically investigated the size dependency of TE by analysing polystyrene (PS) MPs within a range from 1 μm to 20 μm using SP ICP-MS and flow cytometry (FC). The latter was employed as an independent technique, which allowed counting of microparticles to determine particle number concentrations. We evaluated two nebuliser/spray chamber combinations, a conventional Scott-type and a total consumption setup. To improve the comparison of the same standard suspensions across the different setups, a novel multi-particle event filter was implemented to identify overlapping event signals and to improve particle counting accuracy. The shift of the TE from a constant to a size-variable parameter has significant repercussions for the accurate determination of both particle number concentrations and size distributions. Using the Scott-type spray chamber and the total consumption setup, the aerosol-based TEs were 5.5% and 47.3%, respectively. When targeting MPs with a size of 3 μm, TEs decreased to 1.3% and 30.0%. For larger MPs of 10 μm it further decreased to 0.1% and 3.3%, respectively. To address the size dependency of the TE and the resulting overrepresentation of small particles in size and frequency-based calibrations, we propose a correction method based on a sigmoidal TE model that retrospectively corrects size histograms using bin-by-bin frequency adjustment.