Applicability of NMR spectroscopy to quantify microplastics across varying concentrations in polymer mixtures†

Abstract

Quantitative nuclear magnetic resonance (qNMR) spectroscopy could potentially be used for environmental microplastic analyses, provided the challenges posed by mixed polymer samples with varying concentrations and overlapping signals are understood. This study investigates the feasibility of qNMR as a reliable and cost-efficient method for quantifying synthetic polymers in mixtures of low and varying concentrations, addressing key challenges and limitations. Polymer mixtures were analysed using deuterated chloroform (CDCl₃) and deuterated tetrahydrofuran (THF-d₈) as solvents, with polystyrene (PS), polybutadiene-cis (PB), polyisoprene-cis (PI), polyvinyl chloride (PVC), polyurethane (PU), and polylactic acid (PLA) as selected polymers. Mixtures contained either low and high concentrations of each polymer or equal concentrations of all six polymers. Polymer concentrations were measured using the internal standard method. The method showed low relative errors for low concentrations of PS in CDCl₃ and PVC in THF-d₈, with values of −5% and 0%, respectively, while PB and PI in CDCl₃ show relative errors of +5% and −3%, respectively. We observe significant linearity between nominal and measured concentrations with R² values ranging from 0.9655 to 0.9981, except for PU, which had high relative errors and poor linearity (R² = 0.9548). Moreover, simultaneous quantification of six polymers in THF-d₈ proves effective at intermediate concentrations. However, overlapping proton signals are observed, causing high-concentration polymers to mask low-concentration ones. While this study demonstrates low limit of quantification (LOQ) and advances in simultaneous polymer quantification, further research is needed to improve qNMR accuracy for mixed polymer samples and environmentally relevant concentrations.