Performance of analytical techniques for microplastic and nanoplastic quantification in the presence of clay

Accurate quantification of microplastics (MPs) and nanoplastics (NPs) in laboratory and environmental samples remains challenging, particularly in complex matrices such as clay. While ultraviolet-visible (UV-Vis) and fluorescence spectrophotometry (FS) are widely used due to their simplicity and high throughput, their reliability diminishes in the presence of clay. Existing separation methods are often ineffective for MP/NPs and may introduce additional errors during the measurement process. Advanced analytical techniques have shown high accuracy for monodisperse MP/NP solutions, yet their reliability in clay-rich environments remains untested. This study evaluates two such techniques, flow cytometry (FCM) and nanoparticle tracking analysis (NTA) - in both fluorescence and non-Fluorescence modes, alongside UV-Vis and FS, assessing their performance in measuring polystyrene (PS) MP/NPs (0.1-5 µm) at varying concentrations, in the presence of kaolinite, montmorillonite and bentonite clays. Tests with clay-free monodisperse MP solutions indicated that UV-vis and FS can measure wide range of MP size and concentrations, while NTA and FCM showed effective detection within narrower size range, 〈 0.5 µm and 〉 0.5 µm, respectively. The presence of clay adversely affected all methods to varying extents. UV-Vis was the most susceptible, with errors exceeding 10 % at PS/clay mass ratios above ∼ 1. In contrast, FCM was the least affected due to its ability to distinguish particles based on size, shape, and granularity. Fluorescence-based detection offered a clear advantage in separating MPs from clay interference, allowing FS, NTA, and FCM to show improved measurement accuracy compared to non-fluorescent based measurements. However, FS accuracy declined as PS/clay ratio dropped below 0.01 due to autofluorescence from clays. Overall, FCM emerged as the most suitable method for rapid and reliable quantification of MP/NPs in clay-rich matrices, using both fluorescent and non-fluorescent detection modes. These findings provide direct practical advantage for laboratory-scale investigations and a foundation for developing protocols for fast, accurate MP/NP quantification in clay rich environmental matrices.