Overview of methods and challenges for microplastic analysis · Overview of methods and challenges for microplastic analysis Jes Vollertsen, Professor of Environmental Engineering,

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

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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

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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