Post on 20-Apr-2019
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.
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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?
• What dimension is it we report when we say “size”?
1 particle of1000 x 1000 x 1000 μm
1,000,000 particles of10 x 10 x 10 μm
Because 1003 = 1,000,000
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 methods1 μm 10 μm 100 μm 1000 μm 10000 μm
Microplastic range: 1 – 5000 μm
Optical microscopy
Imaging μFT-IR using filters, windows, or slides
Possibly down to a few μm (not proven)
Mikro-ATR-FTIR (single point analysis of particles on a filter)
Increasing uncertainty
ATR-FTIR (particles hand-picked, analyzed on bench)
Imaging μRaman – possible methods, not well proven Macro Raman (particles hand-picked, analyzed on bench)
TDU-Pyr-GC/MS; TED-GC-MS
Increasing uncertainty
NIR (pre sorting) + Hy-Spec. Imaging NIR (not well proven)
Infrared is the most suitable wavelengths for spectroscopic methods
ENERGY DECREASEWAVELENGHT INCREASE
ENERGY INCREASEWAVELENGHT DECREASE
Wavelenght (µm)
Wavenumber (cm-1)
10310-6 10-5 10-4 10-3 10-2 10-1 1 10 102 105 106
RADAR, RADIO,
TELEVISION WAVES
MICROWAVESINFRARED (IR)
VIS
IBLEULTRAVIOLET
(UV)X-RAYSGAMMA RAYS
1010 109 108 107 106 105 104 103 102 10 1 10-1 10-2 10-3
107104
Electromagnetic spectrum
IR region NEA
R-
IR MID-IR FAR-IR
0,7µm 2,5µm 20µm 500µmMicrometers
Wavenumbers (cm-1)14000 4000 500 20
0,05
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Inte
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500 1000 1500 2000 2500 3000 3500 Raman shift (cm-1)
Raman
0,050,100,150,200,250,300,350,400,450,500,550,600,650,700,750,800,850,900,95
Abso
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500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)
FT-IR
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4100 4200 4300 4400 4500 4600 4700 Wavenumbers (cm-1)
NIR
Typical work flow for single-particle analysis
Sampling
Extraction
Cleanup
Concentration
Single particle analysis
Visual sorting
Main issues: Sorting is operator dependent Very difficult and time
consuming for small particles
Methods ranked according to certainty (in my opinion….)
- Compound or stereo microscopy
- Fluorescence microscopy
- Single point Raman
- TDU-Pyr / TGA-GC-MS
- Single point FT-IR
- ATR-FT-IR
The work flow for imaging analysis
Sampling
Extraction
Cleanup
Concentration
Scanning of deposited surface analysisDepositing on
filter or window
μFT-IR- Well proven and tested.
Most used approach
μRaman- Possible method but not
well proven and tested
Interpreting spectral maps
I believe the solution is not one method, but a suit of methods
A potentially suitable analytic scheme…
FT‐IRRaman
NIR‐HySpec TED‐GC/MSPyr‐GC/MS
• Polymer’s ID• N particles• Size• Morphology• Color
• Polymer’s ID• Polymer’s
mass• Additives
µFT‐IR‐imagingµRaman‐imagingHySpec‐imaging
SMPPs
LMPPs
Vis. Sorting
MP sample GC/MS
POPsInfo obtained
ICP/MSICP/OES
Metals
Analytical flow
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