Particle Size Analysis of Polystyrene Microplastics by Single Particle (sp) ICP-MS

Importance of the topic

The increasing accumulation of plastic debris in the environment leads to the formation of microplastics (MPs, 5 mm–1 µm) and nanoplastics (NPs, <1 µm), whose long-term ecological and health impacts remain poorly understood. Reliable analytical techniques to quantify particle size distributions and number concentrations are vital for assessing the fate, transport, and risks associated with plastic pollutants in water, soil, air, and biological systems.

Objectives and Study Overview

This study aimed to advance single particle inductively coupled plasma mass spectrometry (spICP-MS) for direct analysis of polystyrene (PS) microplastics by monitoring the 13C isotope. The goals were to:

• Calibrate the instrument response (counts per second) versus PS bead volume for sizes between 0.8 and 5 µm.
• Determine nebulization efficiency and detection limits for different particle sizes.
• Apply the method to track size and number changes of 5 µm PS MPs under simulated UV degradation.

Methodology and Instrumentation

PS microbeads of nominal diameters 0.8, 1.0, 1.8, 3.0, and 5.0 µm were suspended in deionized water at 2–50 mg/L. A Agilent 8900 Triple Quadrupole ICP-MS operated in spICP-MS MS/MS mode (Q1 = Q2 = m/z 13, no collision/reaction gas) measured 13C pulse signals. Key operating parameters included RF power 1600 W, nebulizer flow 0.78 L/min, sample flow 0.08 mL/min, dwell time 0.1 ms, and 60 s acquisition. Nebulization efficiency (η_neb) was calculated from known particle concentration, flow rate, and acquisition time. Size distribution and particle counts were processed using the single particle software module.

For UV degradation, 5 µm PS suspensions (10 mg/L) were irradiated under a 36 W UV lamp with stirring. Samples were taken at 0, 12, 16, and 20 hours for spICP-MS analysis of changing size distributions and number concentrations.

Instrumentation

• Agilent 8900 Triple Quadrupole ICP-MS with ORS4 collision/reaction cell
• Quartz spray chamber and quartz torch (1.0 mm i.d. injector)
• Glass concentric nebulizer and nickel interface cones
• PTFE sample introduction tubing (0.51 mm i.d.)
• 36 W UV lamp for degradation studies

Main Results and Discussion

A linear calibration (R2 = 0.9999) related 13C intensity (counts per second) to particle volume for PS beads from 0.8 to 5 µm. Nebulization efficiency ranged from 1.9% (0.8 µm) to 0.5% (5 µm), leading to particle number detection limits of 2.0 × 10^6 to 6.9 × 10^6 particles/L. Analysis of mixed bead samples reproduced size distributions consistent with SEM measurements. During UV exposure, the mean particle size decreased from 4.9 µm to 2.2 µm over 20 hours, with a marked increase in sub-1.8 µm fragments. Size-fractionated number concentrations revealed initial stability for >3 µm MPs, followed by a rapid rise in mid-size (1.8–3 µm) and small (0.8–1.8 µm) particles after 16 hours, demonstrating in situ tracking of degradation dynamics.

Benefits and Practical Applications

• Direct measurement of carbon-based MPs without metal labeling simplifies sample preparation.
• Quantitative size and number analysis across submicron to micrometer range supports environmental monitoring.
• High temporal resolution enables kinetic studies of MP fragmentation under stress conditions.
• Method can be adapted for diverse polymer types and complex matrices.

Future Trends and Applications

Advances may include coupling spICP-MS with molecular spectroscopic techniques for polymer identification, improved sample introduction systems for higher nebulization efficiency, and portable ICP-MS platforms for field measurements. Integration with automated data processing and size-fractionation workflows will enhance throughput for routine environmental and toxicological studies.

Conclusion

An spICP-MS method on an Agilent 8900 ICP-QQQ was developed for direct quantification of polystyrene microplastics by 13C detection. The approach achieved accurate size calibration (0.8–5 µm), known nebulization efficiencies, and sensitive particle counting. Applied to UV-induced degradation, the method tracked dynamic shifts in particle size distributions and number concentrations, providing insights into secondary microplastic formation. This technique offers a robust tool for environmental fate studies and risk assessments of plastic pollutants.

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