Impact of Soil Matrix on Microplastics Analysis by Py-GC/MS

Significance of the topic

Microplastic contamination in soils is an emerging environmental and analytical challenge. Accurate identification and quantification of polymer types in complex solid matrices are essential for source attribution, risk assessment, and remediation strategies. Pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS) offers a rapid, polymer-specific analytical route, but soil matrices can strongly influence pyrolysis chemistry and create interferences that bias both qualitative identification and quantitative results. The work summarized here assesses matrix effects from real soils and evaluates strategies to improve quantitative accuracy and reduce false positives in Py-GC/MS microplastic analysis.

Objectives and overview of the study

• Assess how soil matrix composition affects Py-GC/MS identification and quantitation of common microplastics.
• Compare external standard calibration with matrix-matched (spiked) calibration to evaluate recovery improvements.
• Highlight qualitative identification challenges caused by co-pyrolysis products and library-match ambiguities.
• Propose analytical best practices and future directions to improve reliability of Py-GC/MS for environmental soils.

Methodology

• Samples: Three distinct soil samples were collected from different locations and homogenized; sample locations and sampling context were documented.
• Sample preparation: From each homogenized soil, 200 g was dried in a vial for 3 hours at 40 °C; an aliquot of 2 mg (or specified calibration masses) was placed into pyrolysis cups for analysis.
• Standards and calibration: A commercial low-level microplastics calibration standard (Frontier low calibration mix) containing nine target polymers (PE, PP, PS, ABS, PMMA, PC, PVC, Nylon-6 (N6), and Nylon-6,6 (N66)) was used. A five-point calibration (0.2, 0.4, 0.8, 2.0, 4.0 mg) was prepared in triplicate using a CaCO3 diluent. Matrix-matched calibration was prepared by spiking one soil (Sample B) at the same calibration levels.
• Recovery tests: Soils were spiked at 2 mg of the microplastic standard (triplicate) to assess recoveries with both external and matrix-matched calibration curves.
• Data processing: Pyrolysis thermolysis products were analyzed by GC/MS, peaks were assigned to polymer-specific marker compounds, and identifications evaluated by spectral library matching. Quantification was performed using the calibration curves to report mass in µg per analyzed aliquot.

Used instrumentation

• Pyrolysis–GC/MS system (Py-GC/MS) as the analytical platform; schematic and instrument photos were referenced.
• Software: F-search MP 2.1 (Shimadzu) or equivalent library-search tools used for automated identification and scoring of pyrolysis products.
• Standards: Frontier low microplastic calibration mix with CaCO3 diluent for weighing and preparing calibration levels.

Main results and discussion

• Analytical performance: Calibration curves exhibited strong linearity across the evaluated mass range for all target polymers (reported R2 values generally >0.99; the overall dataset includes values such as 0.9927–0.9996 depending on polymer and marker), and short-term repeatability at the lowest level (0.2 mg) showed acceptable %RSD for many polymers.
• Matrix effects: Using a standard (external) calibration often produced unsatisfactory recoveries when applied to soil samples. Matrix-matched calibration (spiking Sample B) improved recovery percentages for multiple polymers, demonstrating the importance of accounting for soil-specific effects in quantitation.
• PVC-specific observations: PVC quantitation relied on naphthalene as a marker in this work. High library-match scores (>90%) were obtained for PVC in several samples when using that marker, and PVC recoveries were notably high. However, library search diagnostics indicated that naphthalene peaks can originate from non-PVC sources in soils; one example showed a library quality score of 43.5 for the 10.04 min peak in Sample B, suggesting naphthalene there was not from PVC. Other PVC-related pyrolysis markers (HCl fragments, benzene, toluene, styrene) exist and should be used in conjunction to avoid false positives.
• Co-pyrolysis interferences: Lignin-rich or organic-rich soils generate pyrolysis products that overlap marker ions for PS, PVC and other polymers (for example, aromatic compounds), complicating both spectral matching and quantitative peak integration. Sample C exhibited a higher background across total ion chromatograms, reducing signal-to-noise and likely biasing quantitation.
• Polymer detection: Some target polymers were not detected in the environmental samples. Recoveries and measured masses varied by polymer and by soil; the poster presents per-polymer recovery plots and example measured masses (e.g., PE, PP, PS, ABS, PMMA, PC, PVC, N6, N66) for the spiked experiments.
• Identification ambiguity and marker selection: The study highlights that single-marker approaches (e.g., relying solely on naphthalene for PVC) can lead to false positives. Using multiple complementary markers and improving spectral libraries can reduce misidentification from matrix-derived products.

Benefits and practical applications of the method

• Py-GC/MS delivers polymer-resolved information rapidly from small sample aliquots, enabling screening of soils and sediments for microplastic content without extensive pre-cleanup.
• Matrix-matched calibration is shown to substantially improve quantitative accuracy in soils, so laboratories aiming at accurate mass estimates should incorporate spiked soil standards or surrogate matrices resembling sample composition.
• Understanding matrix-specific pyrolysis chemistry helps laboratories interpret ambiguous library matches and avoid false positives, improving data quality for environmental monitoring and research use.

Future trends and applications

• Development of composite multi-marker identification schemes tailored to each polymer class to reduce false positives from matrix co-pyrolysis products.
• Expansion and curation of pyrolysis spectral libraries with matrix-aware entries, including spectra from lignin-rich and other high-background soils, to improve automated matching algorithms.
• Standardization of matrix-matched calibration procedures and interlaboratory comparisons to harmonize quantitation approaches for soils and sediments.
• Investigation of optimized sample-preparation workflows that balance matrix removal with minimization of microplastic loss or alteration, including evaluation for diverse environmental and potential clinical sample types.
• Advanced data processing (deconvolution, chemometrics) to separate overlapping pyrolysis signals from matrix and polymer sources.

Conclusions

The study demonstrates that soil matrices actively influence pyrolysis products and thus can significantly bias both identification and quantification of microplastics by Py-GC/MS. While calibration linearity and repeatability are good under controlled conditions, external calibration may not be adequate for environmental soils. Matrix-matched calibration (spiking) improves recoveries and quantitative accuracy. Reliance on single pyrolysis markers (e.g., naphthalene for PVC) can produce false positives because many matrix components generate the same pyrolysis fragments. The work recommends multi-marker strategies, improved libraries, and careful consideration of sample preparation to produce robust microplastic data from soils.

References

• Matos Mejías C., Hiramatsu Y. Impact of Soil Matrix on Microplastics Analysis by Py-GC/MS. Shimadzu Scientific Instruments, Inc., poster and application note materials. Columbia, MD.