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Investigation and Correction of Size-Dependent Transport Efficiencies of Microparticles in SP ICP-MS Analysis
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Size-dependent transport effects were identified and corrected for in microplastic sampling methodologies, showing that standard collection nets and filters underrepresent certain size fractions. Addressing these biases is necessary for generating accurate estimates of microplastic abundance across size ranges.
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 including microplastics (MPs). The accuracy of determining 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 a reference technique which allowed a quantitative counting of microparticles. We evaluated two nebuliser/spray chamber combinations, a conventional Scott-type and a total consumption (TC) 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 results demonstrated a pronounced decrease in TE as MP sizes increased. 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 number-based calibrations, we propose a correction method based on a sigmoidal TE model that retrospectively adjusts histogram frequencies affected by size-dependent losses.
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