Evaluation of the performance of Nanoparticle Tracking Analysis (NTA) for the measurement of nanoplastics

In recent years, microplastics have been detected in fresh- and seawater, the atmosphere, sediments, soils, sewage sludge, biota, and food. However, these microplastics can degrade into even smaller plastic particles in the sub-micron range, referred to as nanoplastics. Due to their smaller dimensions and colloidal properties, nanoplastics could pose an increased hazard to the environment, biota and humans. While there are methods for detecting microplastics, the reliable detection and quantification of size and particle number concentrations of plastic particles less than a micrometre in size are still difficult. In this study, we describe the development and validation of a method for detecting nanoplastics using nanoparticle tracking analysis (NTA) and discuss the limitations of the method in the analysis of nanoplastics. Our method proved accurate and precise for 102 nm polystyrene Nanospheres, with a linear concentration ranging from 5.0 × 10⁶ to 2.0 × 10⁹ particles/mL, and a particle size range from 46 to over 350 nm. However, several limitations of the NTA method became apparent. When measuring polydisperse particle mixtures, NTA tends to underestimate the presence of smaller particles due to lower scattering intensity and overlapping signals. Moreover, NTA cannot chemically discriminate between nanoplastics and other types of nanoparticles, such as natural organic matter, leading to potential overestimation of plastic particle concentrations. Despite these weaknesses, the validated method was applied to eight brands of bottled mineral water. We detected particles with mean particle sizes in the range of 110 to 170 nm, and particle number concentrations between 1.0 × 10⁶ to 2.2 × 10⁷ particles/mL. Particle size distributions showed a particle size range of 50 to 500 nm. However, due to the limitations of NTA, it was not possible to conclusively confirm that the detected particles were nanoplastics, and therefore, the reported concentrations refer to the total particle content rather than specifically to nanoplastics.