Standardization of micro-FTIR methods and applicability for the detection and identification of microplastics in environmental matrices

The ubiquity of microplastics (MPs) in the various environment is of increasing concern. Although micro Fourier Transform Infrared Spectroscopy (μ-FTIR) represents an ideal method for the detection of MPs, this technique lacks a standardized mode to be followed for MPs in diverse environmental matrices. The study focused on the optimization, application, and validation of μ-FTIR techniques for the identification of smaller-sized MPs (20 μm-1 mm). In order to assess the validity of the various detection modes in μ-FTIR (reflection and transmission), a confirmatory test with known standard polymers, viz., polyethylene (PE), polypropylene (PP), polystyrene (PS), polyamide (PA), and polyvinyl chloride (PVC) were executed. Subsequently, for the validation and accuracy of the method, the polymer spectra of standard polymers acquired in μ-FTIR (smaller-sized) were compared with spectra of larger-sized particles (same standards) in FTIR-ATR (Attenuated Total Reflectance). The spectra were comparable and highlighted the similar pattern of the polymeric composition. The spectral quality and matching score (>60 %) with the reference library was taken into account to accentuate the authenticity of the different methods. This study highlighted the reflection mode (particularly diffuse reflection) as more effective for the quantification of smaller-sized MPs in complex environmental samples. The same method was successfully applied to a representative environmental sample (sand), supplied by the EURO-QCHARM for inter-laboratory study. Two polymers (PE and PET) were correctly identified out of three spiked polymers (PE, PET, and PS) in the given sample. Similarly, in terms of matching algorithms, the results for diffuse reflection (PE-71.7 % and PET-89.1 %) were found satisfactory as compared to micro-ATR (PE-67 % and PET-63.2 %) reflection mode. Overall, this study illustrates an extensive perspective of different μ-FTIR techniques, recommending the most reliable, easy, and non-destructive method to unambiguously characterize diverse types of polymers of smaller MPs in complex environmental matrices.