Quantification of Microplastics in Soil and Sediment Using Dry-Ice-Assisted Fractionation

Importance of the topic

Microplastics in soils and sediments are an emerging environmental and public‑health concern due to their persistence, potential to alter soil function, and routes of human exposure (for example via agriculture). Reliable, reproducible quantification across diverse matrices is essential to support monitoring, risk assessment, and mitigation measures. The described method addresses key bottlenecks in extraction efficiency, matrix interference, and automated polymer identification to improve comparability and routine applicability of terrestrial microplastics analysis.

Objectives and study overview

The application note reports development, validation, and environmental application of a dry‑ice‑assisted foam fractionation extraction coupled with Agilent 8700 LDIR chemical imaging for automated particle‑based microplastics quantification in soils and sediments. Main aims were to (1) improve particle isolation from complex matrices using vigorous dry‑ice‑generated foam with surfactant, (2) integrate a particle internal standard to monitor per‑sample recovery, and (3) demonstrate analytical performance (repeatability, spike recoveries) and applicability to field samples from Victoria, Australia.

Methodology and sample processing

Key workflow steps:

• Sample preparation: Freeze‑dried aliquots (5 g) of soil/sediment.
• Dry‑ice foam fractionation: Addition of ~3.5 g dry ice and 0.1% Triton X‑100 to generate vigorous bubbling and stable foam that concentrates microplastics. A saturated CaCl2/hexane mixture aids density partitioning across polymer densities while overflow collection captures foam and solvent layers via a glass vial with an overflow spout.
• Organic removal: Collected surfactant/solvent layers treated with 30% H2O2 (60 mL) and incubated (55 °C, 120 rpm) for ~16 h to oxidize coextracted organics.
• Secondary density separation: CaCl2‑based separation with shaking, sonication, and centrifugation; particles recovered onto polycarbonate (PC) filters (10 µm filtration reported earlier for initial cleanup).
• Transfer to IR slides: Ethanol‑assisted transfer from PC filter to Kevley Mirr IR slides by gentle pressing and peeling technique to maximize particle transfer for LDIR analysis.
• Internal standard: Green polyethylene (PE) beads (210–250 µm) added to every sample as a particle‑based process check; recoveries monitored but not used to correct reported counts.

Instrumentation used

Primary analytical platform: Agilent 8700 LDIR chemical imaging system with Agilent Clarity software (version 1.7.17). Key operational details:

• Spectral window: ~975–1800 cm⁻¹ for polymer identification.
• Particle detection range used: 10–1000 µm (note study defines microplastics as 1–<1000 µm; instrument lower limit here is 10 µm).
• Identification: Automated particle analysis using a modified microplastics library; acceptance thresholds: medium 0.80–0.90, high 0.90–0.99; polyamide threshold raised (>0.85) to reduce false positives.
• Environmental controls: Continuous N2 purge (15–20 L/min) and temperature‑controlled lab (22 ± 0.5 °C) to limit humidity and contamination.
• Typical acquisition settings: automated focus/background and fast scan/sweep modes to prioritize throughput.

Results and discussion

Quality control and repeatability:

• Contamination control: Laminar flow handling, pre‑washed equipment, blanks (method, field) and laboratory control samples (LCS) used routinely.
• Instrument repeatability: Low‑count and moderate‑load tests showed consistent detection and sizing across repeated runs. Example: two single particles (PP and PTFE) measured at 100 ± 0.4 µm and 97 ± 0.3 µm across five runs; multi‑particle slides reproduced counts with low CVs. Environmental slide repeats yielded overall particle counts with CV ≈ 4% for total particles and CV ≈ 11% for microplastics above match quality 0.80.

Internal standard performance:

• PE bead ISTD recoveries (n = 74) ranged 48–100% with a mean ~81% (reported mean 81 ± 12%). Recovery distributions varied by sample type (soil 84%, sediment 82%, LCS 58%, blanks 73% mean shown), demonstrating the value of per‑sample process checks. The ISTD was not applied as a numeric correction factor.

Spike recovery validation:

• Polymers spiked: polypropylene (PP) fibers, polyethylene terephthalate (PET) fragments, and polytetrafluoroethylene (PTFE) fragments at low/medium/high loads.
• Acceptance window: 60–140% recovery.
• Results: Medium and high spikes generally met acceptance criteria for both soil and sediment. Example soil means — medium: PET 84 ± 8%, PP 86 ± 8%, PTFE 72 ± 8%; high: PET 75 ± 4%, PP 76 ± 8%, PTFE 70 ± 4%. At low spikes recoveries were more variable and PTFE often fell below 60% in both matrices, reflecting increased loss susceptibility of small, dense fragments and the sensitivity of particle‑based recovery at low absolute counts.
• Observations: PET and PTFE (higher density) tended to have lower recoveries than lower‑density fibers (PP), consistent with density‑dependent losses during transfer. PTFE particles also discolored during extraction, hindering visual confirmation and making PTFE a less optimal visual spike standard.

Environmental application:

• Applied to soils (chromosol, dermosol, hydrosol) and freshwater sediments from Victoria, Australia.
• Reported concentrations: soils 4,360–102,000 microplastics kg⁻¹; sediments 41,400–127,000 microplastics kg⁻¹.
• Dominant polymers: acrylonitrile butadiene styrene and polyamide were frequently detected in soils; sediment loads were dominated by fragments in the 10–50 µm size range.

Benefits and practical applications

• Improved extraction efficiency: Dry‑ice‑assisted foam fractionation provides vigorous, reproducible mobilization of particles and effective concentration in the foam/solvent layer for subsequent recovery.
• Broad density coverage: The combined foam approach with saturated CaCl2 and hexane supports isolation of diverse polymer densities without aggressive chemical digestion that might damage particles.
• Automated, particle‑resolved quantification: The 8700 LDIR delivers consistent particle location, polymer ID, sizing, and counting suitable for higher‑throughput laboratory workflows and routine monitoring.
• Per‑sample QC: Inclusion of a particle ISTD enables sample‑specific verification of extraction and transfer performance, addressing a common gap in microplastics workflows.

Limitations and practical caveats

• Lower detection bound: The implemented LDIR particle range started at 10 µm; particles below this size were not quantified though the study’s broader microplastic definition extended to 1 µm.
• Low‑count sensitivity: Spike recovery at very low particle counts is inherently variable; absolute particle losses have large proportional effects.
• ISTD considerations: PE beads are process checks but may fragment or behave differently in low‑matrix samples and therefore should be excluded from final counts.
• Polymer alteration: PTFE discoloration during extraction reduces visual traceability for spike checks and suggests careful selection of spike materials depending on the workflow.

Future trends and applications

• Standardization: Wider adoption of particle‑based ISTDs and automated imaging systems can help harmonize terrestrial microplastic data and support interlaboratory comparability.
• Method extension: Adapting foam fractionation for a broader set of matrices (e.g., organic‑rich soils, sludge) and extending IR sensitivity toward smaller particles (sub‑10 µm) will expand applicability.
• Complementary analytics: Combining LDIR with complementary techniques (e.g., Raman imaging or pyrolysis‑GC/MS) for polymer confirmation and mass estimation could strengthen quantitative mass‑based reporting.
• Field monitoring and regulatory use: Robust automated workflows with per‑sample QC can underpin routine monitoring programs and inform regulatory thresholds and remediation strategies.

Conclusion

The dry‑ice‑assisted foam fractionation protocol coupled to the Agilent 8700 LDIR chemical imaging system provides a practical, reproducible approach for particle‑based microplastics quantification in soils and sediments. Validation using polymer spikes and an integrated particle ISTD demonstrated acceptable recoveries at typical environmental loads, good instrument repeatability, and applicability to environmental samples. The method offers a balance between extraction efficiency, polymer coverage, and automated identification needed for routine laboratory monitoring while highlighting areas for further optimization for low‑count and small particle analysis.

References

1. An L.; Liu Q.; Deng Y.; Wu W.; Gao Y.; Ling W. Sources of Microplastic in the Environment. In: He D., Luo Y. (eds) Microplastics in Terrestrial Environments. The Handbook of Environmental Chemistry, vol. 95. Springer, Cham., 2020. DOI: 10.1007/698_2020_449
2. Möller J. N.; Löder M. G. J.; Laforsch C. Finding Microplastics in Soils: A Review of Analytical Methods. Environmental Science & Technology 2020, 54(4), 2078–2090. DOI: 10.1021/acs.est.9b04618
3. He D.; Zhang X.; Hu J. Methods for Separating Microplastics from Complex Solid Matrices: Comparative Analysis. Journal of Hazardous Materials 2021, 409, 124640. DOI: 10.1016/j.jhazmat.2020.124640
4. Möller J. N.; Heisel I.; Satzger A.; Vizsolyi E. C.; Oster S. J.; Agarwal S.; Laforsch C.; Löder M. G. Tackling the Challenge of Extracting Microplastics From Soils: a Protocol to Purify Soil Samples for Spectroscopic Analysis. Environmental Toxicology and Chemistry 2022, 41(4), 844–857.
5. Nabi I.; Zhang L. A Review on Microplastics Separation Techniques from Environmental Media. Journal of Cleaner Production 2022, 337, 130458.
6. Agilent Technologies. Best Practice for On‑Filter Analysis of Microplastics Using the Agilent 8700 Laser Direct Infrared (LDIR) Chemical Imaging System. Agilent Technologies, 2023.
7. Renner G.; Nellessen A.; Schwiers A.; Wenzel M.; Schmidt T. C.; Schram J. TrAC Trends in Analytical Chemistry, 2019, 111, 229–238.
8. Samandra S.; Marchiandi J.; Alwan W.; Ellis A. V.; Clarke B. O. Quantification of Microplastics in Soil and Sediments Using Dry Ice Assisted Fractionation With an Agilent 8700 Laser Direct Infrared Chemical Imaging System. Analytical Methods 2026.