WESTPAC Training Workshop on
“Distribution, Source, Fate and Impacts of Marine
Microplastics in Asia and the Pacific”
20–22 September 2017, Phuket, Thailand
Methodological Limitations for Microplastic
Quantification in the Ocean: Recommendations for
Overcoming the Defects
Director, ProfessorPLASTIC MARINE DEBRIS RESEARCH CENTER
EAST CHINA NORMAL UNIVERSITYInvestigate Principal
IOC-WESTPAC Marine MP Program
Daoji Li
Plastic Marine Debris Research Center East China Normal University
1. Microplastic Collection
2. Microplastic Separation
3. Microplastic Identification
4. QA/QC
CONTENTS
1 MICROPLASTIC COLLECTION
1.1 Collection of microplastics from the environment
Influenced by many factors
Water, sediment, soil, air or biota
Determine the abundance, size and shape
No universally accepted methods
The methods available all have potential bias
1.2 Standardization of collection
No standardization for collecting microplastics
The abundance or the concentrations with
differing units of measurement, incomparable
Unit conversion , both numerical and mass
concentrations
Method standardization in microplastic analysis
by the EU Joint Programme Initiative (JPI)
Ocean
Laboratory methods by the National Oceanic
and Atmospheric Administration (NOAA) in the
United States
Selective sampling
Items visible to the naked eye are directly extracted from
the environment
1.3 Sampling method
Volume reduced sampling
The volume of the bulk sample is reduced until only
the specific items of interest for further analysis remains
1.3 Sampling method
Bulk sampling
The entire sample is taken without reducing its volume
Barrows et al. 2017
1.3 Sampling method
Environmental parameters
To consider and record the prevailing weather conditions
wind direction
the time of Sampling
tidal height
rainfall
1.3 Sampling method
wind velocity instrument
Contamination mitigation
Microfibers
From ambient air, but also via the use of sampling or laboratory
equipment, improper storage of samples or even from the
clothing of the researchers themselves
downwind sampling
using non-plastic tools or containers
avoid synthetic clothing and natural fibres
avoid fleece
1.3 Sampling method
Monofilament nylon mesh plankton nets of
various designs
1.4 Surface water sampling
Neuston net
Manta trawl
Catamaran
the typical mesh sizes : 53 μm to 3 mm, most frequent
size being 333 μm (used for plankton studies)
fine mesh will clog quickly, it will need to be towed
slowly.
commonly 350–400 cm long
a flow meter should be fitted to the net
Flow meters is strongly recommended
1.4 Surface water sampling
A neuston catamaran (a) and a manta trawl (b)
Neuston net tow 333 or 335 μm
1.4 Surface water sampling
Vertical sampling of the water column
1.5 Water column sampling
Bongo net can be used for horizontal or vertical sampling of the water column
direct in situ filtration
bulk sampling with subsequent filtration
continuous Plankton Recorder (CPR)
water pumps
Other water column sampling
1.5 Water column sampling
Advantage:
large volume
easy comparison between data sets
Drawback:
size of plastic captured is limited
samples are prone to be contaminated
Advantage and Drawback of net sampling
Advantage and Drawback of bulk sampling
Advantage:
no size limitation
highly efficient
easy to control contamination
Drawback:
small sampling volume
Larger volume of samples by bulk sampling
is highly recommended to assessing MPs in
the waters
Recommendations
Sediments and sands
sediments are considered to be ideal mediums for
environmental pollutants and tend to experience
long-term contamination
1.6 Sediment Sampling
Microplastics (<5–1 mm) \mini-microplastics (<1 mm to 1 μm) in size on coastal sediments
Using visual identification and sorting by hand
Exclusion of mini-microplastics can result in highly underestimated concentrations of microplastics
Mini-microplastics are considered to represent 35–90% of all the microplastics present in the marine environment
As the size of microplastics decrease, they become progressively
more difficult to collect from the environment
The lower size limit of microplastics reported
is highly dependent on the sampling and
separation methods used.
For small irregularly shaped microplastics,
bulk sampling is also the favored method to
obtain a representative sample
1.6 Sediment Sampling
For sediment samples, may also be reported the
units of weight (g or kg) in both wet and dry
weight, as well as volume (mL or L)
Recommended !
1.6 Sediment Sampling
sampling strategy of sedimental samples
TransectsQuadrats
Dekiff et al. 2014
Recommendation:
from the wrack line
to the vegetation line
1.6 Sediment Sampling
Sampling depth
Variant sampling depths reported: from 1 to 200 cm
Recommendation:
The top 5 or 2.5 cm of sand and selectively
sampling variable depth layers
1.6 Sediment Sampling
2 MICROPLASTIC SEPARATION
2.1 Separation of microplastics from samples
Microplastics (<5–1 mm) ,visual detection or binocular
microscopy
Mini-microplastics (<1 mm to 1 μm) , high
magnification fluorescence microscope
Suspected microplastics, based upon their physical
characteristics, such as their size, shape, texture, color
and lack of biological structures
Further identification , using spectroscopic techniques,
such as Raman spectroscopy, to positively confirm the
type of plastic
2.2 Water samples
Filtration
The pore size of the filter paper
used can vary between studies, it
is generally 1–2 μm in size
Sieving
The size in most studies
ranged from 38 μm to 4.75 mm,
mesh size must not exceed 5 mm
A vacuum filtration system
Crawford 2017
Multi-tier sieving
2.2 Water samples
Crawford 2017
2.3 Sediment samples
Density separation
density separation : using a saturated sodium
chloride (NaCl) solution, density of 1.202 g/cm3
The atmosphere should be avoided
The experimental setup of the three-
necked round-bottomed flask
A publication by the
National Oceanic and
Atmospheric Administration
(NOAA) in the United States
recommends the use of 5.4
M lithium metatungstate (1.6
g/cm3) for sediment density
separations
the extraction ,three times to
achieve high efficiencies
2.3 Sediment samples
Crawford 2017
Heavy liquids that have successfully been used to create high
density solutions for the separation of microplastics from
sediment
2.3 Sediment samples
Crawford 2017
A novel technique based on density separation
has been developed, the Munich Plastic
Sediment Separator
Utilizing a solution of zinc chloride, the MPSS
is reported to reliably separate mini-
microplastics from sediments with an
improvement in the recovery rate from 40% to
95.5% , the rate of recovery for microplastics
greater than 1 mm in size is even higher at 100%
2.3 Sediment samples
Elutriation
Separation with a sodium iodide (NaI)
solution and demonstrated an excellent
recovery
Efficiency for PVC particles at 100% and
fibres at 98%
2.3 Sediment samples
2.4 Biological samples
Visual detection and separation from biological
material
Chemical and enzymatic digestion
fish stomachs and shellfish
Other methods :use of nitric acid, hydrogen peroxide and sodium
hydroxide and have demonstrated effective rates of tissue digestion
(particularly in mussels), with high recovery yields of polystyrene
microbeads at 94–98%, but highly variable results for nylon fibers at
0–98% recovery
The acid destruction method : a mixture of 65% nitric acid (HNO3)
and 68% perchloric acid (HClO4) in a 4:1 ratio (HNO3:HClO4 4:1 v:v)
3 MICROPLASTIC IDENTIFICATION
3.1 Identification of microplastics
Sample purification (mechanical, chemical and
enzymatic methods)
Any surface exposed to the marine or freshwater environment will
be subject to some form of fouling, particularly biofouling where a
layer of microorganisms (bacteria, algae, fungi or plankton)
accumulates on the surface, resulting in misidentification
FTIR or Raman spectroscopy
may also be negatively impacted
Mechanical removal methods
ultrasonic bath-might artificially introduce secondary
MPs into samples by breaking weathered plastics
Chemical removal methods
H2O2, NaOH, KOH, HNO3, HCl, HClO4, NaClO, HNO3
and HClO4, Sodium hypochlorite and HNO3, KOH and
NaClO
Sample purification
some chemicals alter the physical properties (color, shape
and size, etc.)
Enzymes
MP samples from river water (Nuelle et al. 2014),
mussels tissue (Catarino et al.2016), and digestive
tracts of turtles (Duncan et al. 2017)
Advantages and Recommendation
1. enzymatic purification ensures MPs integrity
2. less harmful to human and environmental health
Enzymatic purification should be considered
Visual identification
Selection criteria to identify microplastics
Microscope-aided visual inspection:
size range 0.5–5 mm
Naked eyes inspection:
size range >1 mm
High-magnification microscopic examination, as well as
fluorescence microscopy, to preclude the possibility of
biological origin
Drawback--misidentification rate:
from 20 % to 70 % (Hidalgo-Ruz et al. 2012;
Eriksen et al. 2013; Song et al. 2015)
Drawback of Visual identification
Complementary techniques are needed (e.g.
FTIR, Raman, GC/MS).
Scanning electron microscope
Pyrolysis–gas chromatography–mass
spectrometry(Pyr-GC–MS)
Nuclear magnetic resonance (NMR) spectroscopy
Fourier-transform infrared (FTIR) spectroscopy
Raman spectroscopy
3.2 Analysis
SEM can offer a clear image of the size and surface
texture of the plastic-like particles
Scanning Electron Movroscopy (SEM) /
Energy-Dispersive X-Ray Spectroscopy (EDX)
X-ray spectroscopy (EDX) can determine the mainatomic composition of the putative plastic particles,which is useful for identifying carbon-dominant plasticsfrom inorganic particles
Drawback: 1. colors cannot be provided by SEM
2. expensive
Methods: Pyro-GC/MS, TDS-GC/MS, TGA-SPE-
TDS-GC-MS
Fries et al. 2013; Fischer M. and Scholz-Böttcher B.M. 2017
Thermo-chemical analysisl
1. destructive to the analyzed particles
2. not routinely applicable in common labs
3. size limitation
Drawback of thermal-chemical analysis:
Advantage: nondestructive andstraight-forward
characterization techniques
Higher resloution
Spectroscopic Methods
FTIR and Raman spectroscopy
BRUKER
Lumos
Fourier-transform infrared
(FTIR) spectroscopy
FTIR - DRAWBACK
Micro-ATR-FTIR are that: 1) aged and brittle MPs can
be destroyed by the pressure of the ATR probe; 2) ATR crystal
is prone to be damaged by hard inorganic particle remnants
The reflectance mode bears the disadvantage that
measurements of irregularly-shaped microplastics may result
in non-interpretable spectra due to refractive error
The transmittance mode needs IR transparent filters (e.g.
aluminium oxide) and is, owing to total absorption patterns,
limited by a certain thickness of the microplastics sample
Raman spectroscopy
RAMAN - DRAWBACK
Undesired fluorescence
Coming from additives (e.g. filler, colorants) or
environmental organic matter (e.g. biofilm, algae)
superposes the Raman spectra of polymer matrix
and hampers identificationfluorescent spectra
FTIR AND RAMAN IMAGING
The analysis of small microplastics without any visual
presorting.
Käppler et al. 2016
4 QA/QC
Minimizing Contamination in labs
1. Filtering all the liquid solvents
2. Glassware
3. Combustion
4. Airclean Hood with HEPA
field sampling
laboratory environments
atmospheric depositions
samples filtration
negative control samples during the digestion and separation
Procedural Blank
At all stages:
Thanks for Your Attention !
Plastic Marine Debris Research Center East China Normal University