Quantification of microplastics in soil and sediments using dry ice assisted fractionation with an Agilent 8700 laser direct infrared chemical imaging system

This study reports a novel protocol for the extraction and quantification of 14 different polymers in soil and sediments. The method used in this work follows the conventional drying, density separation, and analysis process, but offers improved efficiencies through the unique use of dry ice foam fractionation to isolate microplastic particles from soil and sediments. To validate the method, recovery tests were conducted by spiking a known amount of purple polypropylene fibres (100-1000 µm), blue polyethylene terephthalate and red polytetrafluorethylene fragments (10-100 µm) at three different levels (high, medium, and low). The acceptable recovery range was determined to be 60 and 140%. The mean recoveries of PET and PP for both matrices were well within the acceptable range. However, the low concentration recoveries of PTFE fell marginally below 60% but for medium and high concentrations, recoveries were within 60-140%. More importantly, to the knowledge of the authors, this is the first particle-based study to have used an internal standard, polyethylene beads (210-250 µm), to verify the efficiency of the method for each sample. All samples in the study contained a known number of PE beads, which ranged between 20 and 34 PE particles, with an average recovery of 81 ± 12% (range: 48 to 100%), and all but one sample were within the defined acceptable range of 50% to 150%. The integration of an ISTD within a particle-based workflow will be a significant shift in microplastic quantification as it allows each sample to be monitored. The method was then successfully applied to soil (n = 3) and sediments (n = 3) from three different locations in Victoria, Australia. Microplastic levels for soil and sediments ranged from 4360-102 000 microplastics per kg and 41 400-127 000 microplastics per kg, respectively.