Monitoring microplastics (MPs) in marine surface waters presents significant analytical difficulties. Particle concentrations may vary substantially, while collected particles are often mixed with biological matter, minerals, and other non-plastic materials.
For dependable characterization, chemical identification is therefore essential; visual classification alone is especially unreliable for small particles whose shapes are difficult to distinguish. FTIR imaging helps address this issue by producing spatially resolved chemical maps across complete filter surfaces.
With this approach, particles are detected and identified based exclusively on their spectral signatures. This enables polymer-specific classification while simultaneously providing particle-level information on size and morphology.
This article outlines the use of FTIR imaging to characterize microplastics in Southern Ocean seawater collected during the International Antarctic Coastal Circumnavigation Expedition (2024–2025), as described by Dr. J. Paulo Felizardo and Dr. C. Rodrigues of UFF Brazil.
Experimental
Surface seawater was collected from several stations throughout the Southern Ocean. At each location, approximately three liters of water were obtained with a Niskin bottle and immediately filtered in situ under vacuum through pre-muffled stainless steel meshes with a pore size of 25 μm.
After filtration, the loaded meshes were refrigerated and transported to the laboratory, where they were processed according to the extraction procedure described below.
Sample preparation was performed in a series of steps. Organic material was first digested with a 10% KOH solution at a 10:1 solution-to-sample volume ratio, then incubated at 40 °C for 72 hours. The treated samples were then passed through a stainless steel mesh again.
For density separation, each filter received 10–15 mL of NaI solution at 4.4 mol/L and a density of 1.8 g/cm3. The samples were sonicated for five minutes at 50 Hz, magnetically stirred for an additional five minutes, and centrifuged at 7000 rpm for two minutes.
The supernatant was subsequently vacuum-filtered onto 0.2 μm aluminum oxide (anodisc) filters. A procedural blank underwent the complete preparation process, with environmental water omitted, to assess potential contamination introduced during handling.
Figure 1. Exemplary data obtained from a Southern Ocean surface seawater sample. Left: transmitted-light mosaic of the anodisc filter. Right: corresponding hyperspectral µ-FTIR image acquired with the Bruker LUMOS II and evaluated with the Bruker MPID software. Image Credit: Dr. J. Paulo Felizardo and Dr. C. Rodrigues
Results
The three Southern Ocean samples contained between 9000 and 32,000 MPs per m3. Reported concentrations for Southern Ocean surface waters range from approximately 0.01 to 15,400 MPs per m3, demonstrating the substantial methodological differences among published studies.
The relatively high concentrations measured in this work are probably related to the lower effective particle-size threshold of the analytical workflow. FTIR imaging captures small particles that may not be retained, measured, or reported by conventional non-imaging methods and subsampling-based procedures.
Polymer composition differed among the sampling stations. Polypropylene represented the majority of identified particles at most locations, accounting for 68–81% of the total. At one station, poly(ethylene terephthalate) reached a relative abundance of approximately 52%.
Average particle sizes varied according to polymer type, with mean dimensions ranging from 40 to 160 μm. Only a minimal particulate load was detected in the procedural blank, indicating that background contamination from laboratory processing was negligible.
Figure 2. Exemplary MPID result obtained from a Southern Ocean subsurface seawater sample. Extensive particle information is given together with an interactive classification map. When a particle or particle class is selected, all respective information is directly displayed. Image Credit: Bruker Optics
Conclusion
The LUMOS II FTIR imaging microscope, together with automated MPID processing, supports the detection and polymer-specific identification of particles on anodisc filters. The workflow also retains particle-level information about size and morphology.
Particles measuring approximately 10 μm and larger can be detected and chemically identified, providing concentration estimates that more fully represent the small-particle fraction accessible through μ-FTIR imaging.
The application also demonstrated the utility of MPID software for automatically processing complete FTIR images of microplastic samples within a few minutes. The software generated comprehensive statistics for the particles present on each filter and classified them according to number, chemical identity, and size.
The Southern Ocean samples contained 9000 to 32,000 MPs per m3. These results should not, however, be compared directly with concentrations obtained using less sensitive collection methods, including 300 μm mesh nets.
Because such methods exclude a substantial portion of the smaller particles detected by FTIR imaging, the higher concentrations reported here mainly reflect differences in the measurable particle-size range rather than definitive evidence of increased environmental contamination.
Concentrations were calculated from three-liter water samples and extrapolated to 1 m3. Consequently, even modest changes in the number of counted particles can produce substantial differences in the extrapolated values. The relatively small volume analyzed may also fail to represent the full spatial variation in particle abundance.
These limitations should be taken into account when interpreting the findings or comparing them with other studies. Improved comparability will require harmonized sampling and analytical procedures, including standardized particle-size ranges and identification based on IR imaging.
Acknowledgments
Produced using materials originally authored by João Paulo Felizardo (PhD), Geochemistry Program, Federal Fluminense University; Camila Rodrigues e Silva (PhD), Geochemistry Program, Federal Fluminense University; and Alexander Staub (PhD), Product Manager IR Microscopy, Bruker Optics GmbH & Co. KG, Ettlingen.
This information has been sourced, reviewed, and adapted from materials provided by Bruker Optics.
For more information on this source, please visit Bruker Optics.