Overview of methods and challenges for microplastic analysis Jes Vollertsen, Professor of Environmental Engineering, Aalborg University
A major challenge Citation from BASEMAN: Although microplastics (MP) are recognized as an emerging contaminant in the environment, currently neither sampling, extraction, purification nor identification approaches are standardized, making the increasing numbers of MP studies hardly if at all comparable. Error in microplastic determination Time The scientific community works hard to reach valid methods but we are not there yet
Standardized and trustworthy analytical methods are needed Without proper analytical methods we cannot: Assess the amount of microplastic in the environment Distinguish which are the most important sources Quantify impacts of microplastic We (the scientific community) do not (yet) have the final answer to how microplastic should be analyzed Over the later years, certain methods have shown promising results, while others have been deemed unsatisfactory
Lack of Standardized Operation Protocols Analyzing for microplastic in the environment, there are many ways of doing: Experimental design where and how to look for MP? Sample collection Mesh sizes? Sample sizes? Sample purification How to get rid of irrelevant substances without biasing the analysis?
Lack of Standardized Operation Protocols Analyzing for microplastic in the environment, there are many ways of doing: Microplastic identification How to safely distinguish artificial polymers from naturally occurring substances? How to report results Particle sizes: What is the size of a particle? Particle mass: How best to quantify the mass of a particle? Document the validity of the analysis this is often forgotten Document uncertainties this is often forgotten
Size why is it important? The traditional wisdom is that microplastic toxicity increases with decreasing size So size matters
Size why is it problematic? Microplastic degrades in the environment, continuously creating smaller particles One Big Particle becomes Many Small Particles So what does a particle number really tell you? 1 particle of 1000 x 1000 x 1000 μm Because 100 3 = 1,000,000 What dimension is it we report when we say size? 1,000,000 particles of 10 x 10 x 10 μm There is no clear consensus on this
Size why is it problematic? Any sample preparation applies forces on the particles Sample preparation causes large particles to break up into smaller ones the extent hereof is unknown Particle number is not a conserved unit there is no law of particle number conservation Hence particle numbers and sizes cannot be used to establish balances like which source is the more important
Mass why is it important? Mass is a consistent measure There does exist a law of mass conservation!!! Only mineralization will affect this measure Estimates on plastic loads to the environment must be made in units of mass (particle numbers make no sense here) Mass must be measured to allow this
Mass why is it problematic? Plastic is not one thing Measuring plastic mass requires measuring the mass of many different polymer particles No analytical method can actually detect all polymer types μft-ir imaging Yields a mass estimates with unknown accuracy Cannot measure car tire rubber TDU-Pyr-GC/MS; TED-GC-MS Can measure many but not all polymers (PVC?) (But it cannot measure particle sizes.)
Size ranges and analytical methods Microplastic range: 1 5000 μm 1 μm 10 μm 100 μm 1000 μm 10000 μm Increasing uncertainty Increasing uncertainty Optical microscopy Mikro-ATR-FTIR (single point analysis of particles on a filter) ATR-FTIR (particles handpicked, analyzed on bench) Possibly down to a few μm (not proven) Imaging μft-ir using filters, windows, or slides Imaging μraman possible methods, not well proven Macro Raman (particles handpicked, analyzed on bench) NIR (pre sorting) + Hy-Spec. Imaging NIR (not well proven) TDU-Pyr-GC/MS; TED-GC-MS
Infrared is the most suitable wavelengths for spectroscopic methods ENERGY INCREASE Wavenumber (cm -1 ) WAVELENGHT DECREASE 10 10 10 9 10 8 10 7 10 6 10 5 10 4 10 3 10 2 10 1 10-1 10-2 10-3 Electromagnetic spectrum GAMMA RAYS X-RAYS ULTRAVIOLET (UV) VISIBLE INFRARED (IR) MICROWAVES RADAR, RADIO, TELEVISION WAVES 10-6 10-5 10-4 10-3 10-2 10-1 1 10 10 2 10 3 10 4 10 5 10 6 10 7 Wavelenght (µm) ENERGY DECREASE WAVELENGHT INCREASE 14000 4000 500 Wavenumbers (cm -1 ) 20 IR region NEAR- IR MID-IR FAR-IR 0,7µm 2,5µm 20µm Micrometers 500µm 0.16 0.15 0.14 0.13 0.12 0.11 NIR 0,95 0,90 0,85 0,80 0,75 0,70 0,65 FT-IR 0,80 0,75 0,70 0,65 0,60 0,55 Raman Absorbance 0.10 0.09 0.08 0.07 0.06 Absorbance 0,60 0,55 0,50 0,45 0,40 0,35 Intensity 0,50 0,45 0,40 0,35 0,30 0.05 0,30 0,25 0.04 0.03 0.02 0.01 0,25 0,20 0,15 0,10 0,05 0,20 0,15 0,10 0,05 4700 4600 4500 4400 4300 Wavenumbers (cm -1 ) 4200 4100 4000 3500 3000 2500 2000 Wavenumbers (cm -1 ) 1500 1000 500 3500 3000 2500 2000 Raman shift (cm -1 ) 1500 1000 500
Typical work flow for single-particle analysis Methods ranked according to certainty (in my opinion.) Sampling Extraction Cleanup Single particle analysis - Compound or stereo microscopy - Fluorescence microscopy Concentration Visual sorting - Single point Raman - TDU-Pyr / TGA-GC-MS Main issues: Sorting is operator dependent Very difficult and time consuming for small particles - Single point FT-IR - ATR-FT-IR
The work flow for imaging analysis Sampling Extraction Cleanup Concentration Depositing on filter or window Scanning of deposited surface analysis Interpreting spectral maps μft-ir - Well proven and tested. Most used approach μraman - Possible method but not well proven and tested
Analytical flow I believe the solution is not one method, but a suit of methods Vis. Sorting A potentially suitable analytic scheme MP sample LMPPs SMPPs FT IR Raman NIR HySpec µft IR imaging µraman imaging TED GC/MS Pyr GC/MS GC/MS ICP/MS ICP/OES HySpec imaging Info obtained Polymer s ID N particles Size Morphology Color Polymer s ID Polymer s mass Additives POPs Metals
The Aalborg University microplastic research group Foundation in urban polluted waters Wastewater, stormwater, sludge, Receiving environment impacted here by Soil, water, air Focus on developing better, faster, and more valid methods for microplastic quantification Focus on quantifying microplastic in the environment