Quantitative Raman analysis of microplastics in water using peak area ratios for concentration determination

Microplastics (MPs), defined as plastic particles ranging from 1 µm to 5 mm, pose a significant environmental and health concern. Their ubiquitous presence in various environments, from oceans and freshwater systems to terrestrial ecosystems and even the human food chain, has prompted extensive research into detection and quantification methods. Current techniques like optical methods and thermal analysis have limitations in accurately distinguishing MPs from other particles or in analyzing smaller particles. Optical methods lack specificity, while thermal methods are destructive and limited to larger particles. Vibrational spectroscopy, particularly Raman spectroscopy, offers a promising alternative due to its ability to provide both qualitative and quantitative information with minimal interference from water.

Existing literature highlights various methods for analyzing microplastics in water. Optical methods offer a rapid, cost-effective approach, but lack specificity. Thermal analysis techniques, such as TGA-MS and pyrolysis-GC/MS, provide highly sensitive and accurate measurements but are limited to larger particles (>500 µm) and are destructive. Vibrational spectroscopy, specifically FTIR and Raman spectroscopy, are widely used for identifying and quantifying microplastics due to their ability to generate unique spectral fingerprints. However, FTIR is significantly affected by water, whereas Raman spectroscopy shows less interference. While Raman spectroscopy has been used to analyze the composition and crystallinity of MPs, its application for quantitative analysis in water samples, especially using peak area ratios, remains under-explored. Previous studies have utilized peak intensity or area to quantify chemicals but intensities are susceptible to measurement conditions, limiting their use for absolute concentration determination.

This study employed Raman spectroscopy to quantify PE and PVC MPs in DI water. Spherical PE (40–48 µm) and PVC (40–100 µm) particles from Sigma-Aldrich were used. Calibration samples were prepared by mixing PE or PVC separately with DI water at concentrations ranging from 0.1 wt% to 1.0 wt%. For validation, mixtures of PE and PVC in DI water were prepared at the same concentration range. Samples were stirred for 30 minutes to ensure particle dispersion. Raman spectra were obtained using a confocal Raman spectrometer (XperRam C series, Nanobase Inc.) with a 5X magnification lens, a 30 mW, 532 nm laser, and a scanning area of 800 x 800 µm. Each spectrum was the average of 20 spectra, each collected over 25s, using a Gaussian method. The peak area ratios of the most intense peaks (1295 cm⁻¹ for PE and 637 cm⁻¹ for PVC) relative to the H₂O peak were used to construct calibration curves via linear fitting in Origin software. The calibration model was validated by comparing the predicted concentrations (from the calibration curves) with the true concentrations of the mixed PE and PVC samples. The accuracy of the model was evaluated using standard error of calibration (SEC) and relative standard error of calibration (%RSEC). Quality control measures included Raman analysis of deionized water and the use of glass equipment to avoid contamination.

Raman spectra showed increasing peak intensities with increasing MP concentration (Fig. 3). Calibration curves (Fig. 4) demonstrated high linearity between the peak area ratio and concentration for both PE (R² = 0.98537, y = 0.25539x - 0.02187) and PVC (R² = 0.99511, y = 0.37139x - 0.01717). Validation using mixed PE and PVC samples (Fig. 5 and 6) showed good agreement between predicted and true concentrations. The SEC and %RSEC values were 0.0723214 and 0.48052 for PE, and 0.0985576 and 0.52645 for PVC respectively (Table 2). These low values indicate high accuracy and reliability of the proposed method.

The high R² values and low SEC and %RSEC values demonstrate the accuracy and reliability of the proposed method for quantifying PE and PVC microplastics in water using Raman spectroscopy and peak area ratios. This approach effectively mitigates the influence of varying measurement conditions on peak intensity, providing more robust and consistent concentration measurements compared to methods relying solely on peak intensity. The ability to simultaneously identify and quantify multiple MP types in a single measurement enhances its practical application in real-world environmental monitoring. The method's simplicity and relatively short measurement time suggest its potential for high-throughput analysis and real-time monitoring of microplastic pollution in diverse aquatic systems.

This study successfully developed and validated a novel Raman spectroscopy-based method for quantifying PE and PVC microplastics in water using peak area ratios. The high accuracy and reliability of this method, demonstrated by the high R² values and low SEC and %RSEC values, make it a promising tool for environmental monitoring. Future studies could explore the method's applicability to a wider range of MP types and matrices, as well as investigate the impact of different water chemistries on the accuracy of the measurements.

The current study focused on spherical PE and PVC particles. The method's applicability to other MP shapes, sizes, and polymers needs further investigation. The study was conducted using deionized water; the presence of other substances in real water samples could affect the accuracy of the measurements. Further research is needed to assess the influence of turbidity, suspended particles, and other interfering substances on the results. The accuracy of the analysis might also depend on the quality and resolution of the Raman spectrometer utilized.