Sequential combination of micro-FTIR imaging spectroscopy and pyrolysis-GC/MS for microplastic quantification. Application to river sediments

Several studies have focused on quantifying microplastics (MP) in the environment using μ-FTIR and Py-GC/MS, the most common analytical methods. However, their application to complex matrices like sediments is affected by interferences specific to each method. In this study, we developed a protocol combining μ-FTIR and Py-GC/MS for sequential analysis of MP (10-500 μm) in 16 river sediment samples, targeting PE, PP, and PS polymers. Mass concentrations were estimated from the particle volume in μ-FTIR and measured directly by Py-GC/MS using internal calibration. Results show consistency between the two methods across different sites, with variability of two orders of magnitude in concentration ranges from 0.3 to 50 items g-1 and 0.2 to 17 μg g-1 for μ-FTIR, and 0.8 to 21 μg g-1 for Py-GC/MS. Replicate analyses (2 to 6 per site) revealed that intra-site variability was mainly influenced by sample preparation and, to a lesser extent, by the measurement technique. While estimated and measured concentrations were similar, discrepancies were observed in polymer proportions: PP predominated in μ-FTIR, while PS was more prevalent in Py-GC/MS. These differences are explained by the specific limitations of each method, especially the limited detection of synthetic fibres and tyre or road abrasion particles by μ-FTIR, which are detected as MP by Py-GC/MS. This comparative study provides recommendations for evaluating compatibility between studies using either technique and offers guidelines for selecting the most appropriate method based on research interests.