a review of analytical techniques for quantifying ... · pdf filea review of analytical...

A review of analytical techniques for quantifyingmicroplastics in sediments

Joanne S. Hanvey,a Phoebe J. Lewis,a Jennifer L. Lavers,b Nicholas D. Crosbie,c

Karla Pozode and Bradley O. Clarke*a

In this review the analytical techniques for measuring microplastics in sediment have been evaluated. Four

primary areas of the analytical process have been identified that include (1) sampling, (2) extraction, (3)

quantitation and (4) quality assurance/quality control (QAQC). Each of those sections have their own

subject specific challenges and require further method development and harmonisation. The most

common approach to extracting microplastics from sediments is density separation. Following

extraction, visual counting with an optical microscope is the most common technique for quantifying

microplastics; a technique that is labour intensive and prone to human error. Spectroscopy (FTIR;

Raman) are the most commonly applied techniques for identifying polymers collected through visual

sorting. Improvements and harmonisation on size fractions, sampling approaches, extraction protocols

and units for reporting plastic abundance would aid comparison of data generated by different research

teams. Further, we advocate the development of strong QAQC procedures to be adopted like other

fields of analytical chemistry. Finally, inter-laboratory proficiency testing is recommended to give an

indication of the variation and reliability in measurements reported in the scientific literature that may be

under- or overestimations of environmental burdens.

1 IntroductionSince the introduction of plastics into western society in the 1950s,deliberate and accidental environmental release has resulted inubiquitous environmental contamination with plastics debris. Asa consequence, plastic pollution has been recovered throughoutthe planet, in remote regions far from known point sources – suchas Antarctica,1 remote mountain-tops2 and the deep sea oceanoor3 – environments that should be pristine, free of humanimpact. The rst reports of marine plastic pollution in the early1970s4 were followed by similar reports throughout the worldnding widespread, ubiquitous contamination.5,6 Later analysis ofarchived samples revealed a dramatic increase of marine plasticdebris and microplastics through the late twentieth century.7

Efforts to understand the extent of widespread environmentalcontamination and subsequent ecological harm caused by plastics

debris are ongoing.8,9 To develop appropriate management andremediation strategies for this global pollution problem we musthave reliable and consistent analytical techniques for measuringplastics in environmental matrices. In this review, we will examineanalytical techniques reported in the English language peer-reviewscientic literature for quantifying microplastics in sedimentssamples.

There are a wide variety of polymers detected in the envi-ronment, with the main types being polyethylene (PE), poly-propylene (PP) and polystyrene (PS; see Table 1 for a list ofcommon plastics and their uses). Microplastic debris is broadlyclassied as either (1) primary microplastics that includeproduction pellet nurdles and microbeads (abrasive particlesincorporated into personal care products) or (2) secondarymicroplastics which are formed by mechanical and chemicalbreakdown of larger plastic debris and bres released fromsynthetic textiles (referred to as microbers).10 Environmentalplastic litter has been broadly categorised within the literatureand it was not until 2009 that ‘microplastics’ were dened asplastic particles smaller than 5mm by the National Oceanic andAtmospheric Administration (NOAA) International ResearchWorkshop on the occurrence, effects and fate of microplasticmarine debris.11–13 Initial reports of microplastics in sediments(published in 2004) dened them as being <1 mm (presumablyas it was micro (m) in size)7 and various authors since havecontinued to apply this denition.10 Variations in the collectionof size fraction makes the comparison of data between regions

aSchool of Sciences, Centre for Environmental Sustainability and Remediation, RMITUniversity, GPO Box 2476, Melbourne, Vic. 3001, Australia. E-mail: [email protected] for Marine and Antarctic Studies, University of Tasmania, 20 CastrayEsplanade, Battery Point, Tasmania, 7004, AustraliacMelbourne Water, 990 LaTrobe St, Melbourne, AustraliadFacultad de Ciencias, Universidad Catolica de la Santısima Concepcion, Alonso deRibera 2850, Concepcion, ChileeRECETOX Research Center for Toxic Compounds in the Environment, Faculty ofScience, Masaryk University, Kamenice 753/5, pavilon A29 625 00, Brno, CzechRepublic

Cite this: DOI: 10.1039/c6ay02707e

Received 29th September 2016Accepted 22nd December 2016

DOI: 10.1039/c6ay02707e

www.rsc.org/methods

This journal is © The Royal Society of Chemistry 2017 Anal. Methods

AnalyticalMethods

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and matrices problematic. The elimination of the 1–5 mm or <1mm size fraction from the study designs will underestimate theamount and type of microplastic present (e.g., production pelletnurdles are typically 2–3 mm while microbers are oen <1mm) and the overall conclusions drawn.14 Another furthercomplication is lack of consistency in reporting plasticconcentration (dened as amount per medium), units ofabundance (or mass) per mass,15 volume,10 area16 and evenlength.17 In this review, microplastics have been dened as <5mm consistent with the NOAA workshop consensus denition.However, as far as the authors are aware the lower limit remainsundened, although most recognise it as 333 mm due to thecommon use of 333 mm mesh nets used in the eld to captureplankton and oating debris.13 The smallest size fraction re-ported in environmental samples is 1 mm through the applica-tion of this size-lter, however it is very difficult to visuallydetect plastics <100 mm in size even using a microscope.18 Below1 mm scale ‘nanoplastics’ are the least known area of marinelitter, and constitute a very recent area of the environmentalscience and to date, no studies have successfully accounted fornanoplastics in either freshwater or marine environments.19

The terminology applied inconsistently through the scienticliterature would benet from harmonisation, aiding consistentstudy objectives related to size fraction and generation ofcomparable data sets. A summary of the proposed terminology

is listed in Table 2 (please note the size range categories dooverlap).

Analytical approaches applied to measuring plastic debris inenvironmental samples are not well-developed.9 Comparisonwith well-established analytical techniques for quantifying tracelevel pollution could provide valuable strategies related tomicroplastics quantitation, but also particularly for qualityassurance/quality control (QA/QC). For example, routine prac-tices in other environmental science would require appropriateeld and laboratory blanks, laboratory control samples, matrixspikes and recovery standards as part of the analytical process.One key difference between chemical quantication and plas-tics quantication is diversity of polymers with respect to type,size, colour, and morphology combined with the lack ofhomogeneity within environmental samples. This causesunique problems (compared to traditional chemical testing) atevery stage of the analytical process (sampling, extraction andquantication). Most studies evaluated include a visual count-ing procedure as part of the analytical process. Human bias hasbeen demonstrated with variable recoveries based upon poly-mer colour with blue fragments having the highest detectionprobability, while white fragments had the lowest.20,21 Further-more, other factors that impact the quantity reported are theanalyst, their experience, as well as other biological materialpresent in the sample that could be confused with plastic.21

Visual inspection and counting with a microscope is animportant technique as it is cheap and non-destructive forsample processing. Visual inspection is an unreliable analyticalprocess, particularly in the lower smaller microplastics (<1 mm)and nanoplastic (<1 mm) range, likely to be generating unreli-able and incomparable data.14

Currently there are no universally accepted methods for anyof these matrices and the methods available all have potentialbiases.9 First steps towards addressing the plastics issue shouldinclude the promulgation of standard approaches and methods

Table 1 List of common types of plastics (abbreviation) and their common usage

Type of plastic Abbreviation Application of virgin resin Density (g cm!3)

Polyethylene terephthalate PET/PETE Disposable beverage bottles, textiles(synthetic bers), tape, Mylar foodpackaging and thermal insulation

1.37–1.38

High density polyethylene HDPE Plastic bags, plastic lumber, fueltanks, bottle caps, milk crates

0.93–0.97

Polyvinyl chloride PVC Plumbing pipes, door and windowframes, garden hoses, electricalcable insulation, inatable products

1.10–1.47

Low density polyethylene LDPE Plastic bags and lms six-packrings, exible snap-on lids

0.91–0.92

Polypropylene (expanded or non-expanded)

PP Bottle caps, rope, carpet 0.89–0.92

Polystyrene (expanded or non-expanded)

PS Disposable cutlery, dinnerware, andtake-away food packaging, buildinginsulation, refrigerated bins (e.g.,sh boxes)

0.28–1.04

Other resins, such as polycarbonate,nylon, and acrylic

Other Used for engineering purposesbecause of their thermal, electricaland chemical properties. e.g.,electrical wire insulation

1.15–1.22

Table 2 Summary of proposed terminology

Size rangeProposedterminology

>20 cm Macroplastic29

5–20 cm Mesoplastic29

1–5 mm Large microplastic29

1–1000 mm Small microplastic29

<1000 nm Nanoplastic19

Anal. Methods This journal is © The Royal Society of Chemistry 2017

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for collecting, archiving, and reporting data.22 Studies tend toconcentrate on surface sediment debris (e.g. beach surveys) orburied debris (e.g. core samples), but rarely do they considerboth. Without quantifying the debris in both surface sedimentand deeper sediment, a true evaluation of the microplasticabundance cannot be achieved. Due to limits in analytical tech-niques accurately quantifying microplastics in sediments, thepercentage of plastic pollution in the microplastic scale is yet tobe accurately determined within these environments. To date,the lower size limit used in microplastic assessment studies ishighly dependent on the sensitivity of the sampling and extrac-tion technique applied and can potentially lead to reports thatunderestimate concentrations.23 Numerous problems associatedwith the characterisation and quantication of microplastics insediments can make inter-study comparisons problematic, if notimpossible. For example, method inconsistencies betweensampling techniques leading to a variety of sampling units (i.e.,mass, volume, including variations in the masses and volumes,area and length) reported and also differences in the lower andupper size limit of microplastics implemented.24 Consistentanalytical techniques are important for not only dening theextent of the plastic pollution in the environment but in deter-mining adequate risk mitigation techniques to protect overallfreshwater or marine environments.25

2 Studies reviewedIn this review, we have examined reports that have quantiedmicroplastics in sediment and collated the reported analyticaltechniques (Table 3). We have conned this review to Englishlanguage peer-reviewed scientic studies that have includedquantitative measurements of microplastics (<5 mm in diam-eter) within a certain volume of sediment (marine, freshwater)samples (dened as matter that settles to the bottom ofa liquid). Previous reviews in this eld have included papersthat broadly dened plastics in the environment using termssuch as ‘marine litter’, ‘marine debris’ or ‘plastic frag-ments’.23,24,26 We have excluded studies that have reportedmicroplastics on the beach surface sampled with the naked eyerather than within sediment. Further criteria for inclusion inthis review were that the authors (i) report microplastic particleswithin the limits dened by the NOAA workshop (<5 mm); (ii)include information on the sampling, extraction and quanti-cation techniques used to recover microplastics from withinsediment matrices and (iii) due to developments within thisnovel eld, were conducted within the past two decades. A totalof 43 studies of microplastics in sediments have been evaluatedthat include data from 18 countries, a global study,27 plus deep-sea samples, mostly from the Northern Hemisphere (Table 3).

Generally, there are three main aspects of the analyticalprocess for measuring microplastics in sediment samples; theyare: (1) sampling, (2) extraction, and (3) quantication. Theinformation provided in each of these sections has beencollated and is presented in Table 3 along with a discussionbelow using these headings. Inclusion of QA/QC practice ismissing from most studied evaluated with only 19% includingor describing QA/QC approaches. Therefore, we have included

a fourth section on this topic. This review provides an overviewand comparison of analytical methods applied within the pasttwo decades for quantifying microplastics in sediments. Whilewe do provide a summary of the data reported, it is not ourintention or aim to summarise the ndings, only the analyticalapproaches. There is a signicant amount of further workrequired by analytical chemists to harmonise the analyticalapproaches taken with respect to sampling, extraction, quanti-cation and QA/QC approaches so that reliable and comparablemeasurements of environmental burdens are reported.

3 Analytical methods3.1 Sampling

Collection of appropriate samples is the rst critical step inquantifying microplastics in the environment. Fig. 1 demon-strates the various sampling locations around the world, as wellas differing sample types. All plastic debris that is found in thesediment will have been transported by water movement (rivers,ocean currents28) or air from the emission point-source. It isestimated that there are at least 5.25 trillion particles weighing270 tons at depths between 0.5 and 2 meters below the seasurface.29 It is likely that land-based emissions that enter riversare a signicant pathway of pollution to the aquatic environ-ment.30,31 Attention has been focussed on wastewater treatmentplants (WWTPs) as a source of pollution through (1) effluentdischarges (2) sewage overow during high rain events30 (3)runoff from sewage-based fertiliser land application.32,33

However, evidence is mounting that WWTPs are also a source ofsmall microplastics34 and microbers10 with larger micro-plastics (e.g., production pellet nurdles) and macroplasticsremoved during the treatment process. The contribution ofrivers to aquatic pollution could be signicant. For example, theDanube river in Germany is estimated to release 4.2 tonnes ofplastic debris per day into the Black Sea.35 In Paris, novel dataon atmospheric deposition rates for microplastics combinedwith estimates for sewage effluent36 highlighted the signicantmass release of plastics, primarily microbers, associated withan urban environment (3–10 tons per day) into freshwaterecosystems.37 Proximity to urban centres and onshore oceancurrents is thought increase the reported concentrations ofmarine plastics debris and have an important impact on litterabundances on coastal beaches;38,39 this is indicative of thepercentage of studies that sampled beach sediment (58%) todetermine the extent of contamination (Fig. 1c). Therefore, theselection of sampling site will have a signicant impact of theabundance and type of plastic reported (see Fig. 1). For example,a South Korean north-side beach contained a 100-fold lowerabundance than two south-side beaches that faced southerlywind and currents that were prevalent through the studyseason.28 Typically, most studies have reported polyethylene asthe major polymer type detected in beach sediment since the1970s.40,41 However, this is not always the case and polymer typecan vary signicantly based upon location and point-source.28

For example, 90% of the plastics debris recovered from sedi-ments in a South Korean study were styrofoam.28 Once in theaquatic environment, it is hypothesized that while initially


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Tab

le3

Summaryofartic

lesfrom

theEn

glishlangu

agepee

rreview

edscientifi

cliterature

reportingaquan

titativemea

suremen

tofmicroplasticspartic

les(<50

00mm)in

aquatic

sedim

ent

Cou

ntry

Hab

itat

Size

rang

e

Abun

danc

e(m

ean;

rang

e)Sa

mpling

Extraction

Qua

ntication

QA/QC

Referen

ce

Japa

n,Rus

sia

Beach(n

¼26

)>3

00mm

Japa

n(261

0pieces

per

m2 ),R

ussia

(32pieces

per

m2 )

3samplelocation

s;8L

sand

;qua

drat

(40cm

"40

cm);de

pth(0–5

cm)

Fieldde

nsitysepa

ration

(seawater);ltered

(0.3

mm

meshne

t)

Visu

ally

coun

ted/sorted

(optical

microscop

e);

gravim

etric

Kus

uiet

al.,

2003

(ref.1

6)

USA

Beach(9

location

s;2sites

perlocation

)

1–15

mm

NA

Wrack

line&be

rm;

quad

rat6

1cm

"61

cm;

5.5cm

depth;

eld

sieved

(1.15mm)

Den

sity

sepa

ration

(fresh

water)s

ieved(1,

2.8,

4.75

mm);dried

Visu

ally

coun

ted/

sorted

;gravimetric

McD

ermid

etal.,20

04(ref.5

0)

UK

Estuarine/

subtidal

sedimen

t;be

ach

(n¼

17)

>1.6

mm

NA

Strand

line,

subtidal;n

¼5at

each

location

Den

sity

sepa

ration

(NaC

l1.2

kgL#

1 );

stirred;

ltered

(Wha

tman

GF/A1.6mm)

Visu

ally

coun

ted/

sorted

;polym

eriden

tication

(FTIR)

Thom

pson

etal.,20

04(ref.7

)

Sing

apore

Beach(n

¼8)

>1.6

mm

NA

1cm

surface(n

¼4);

sub-su

rface(10–11

cm,

n¼

4)

Den

sity

sepa

ration

(hyp

ersaturated

solution

,1.2

kgL#

1 );

ltered

(1.6

mm)

Polymer

iden

tication

(FTIR)

LCS(3.45mm

PE/

PPsp

iked

into

sedimen

t),n

oda

tarecovery

repo

rted

Nget

al.,

2006

(ref.5

1)

India

Marinesedimen

t(10location

s)>1

.6mm

81mg

plastics

per

kgsedimen

t

10samplinglocation

s;5(betweenhigh

tide

alowwater

mark)

"10

kgsamples

(each

location

);de

pth(0–5

cm);sieved

Den

sity

sepa

ration

(30%

NaC

l);ltered

(1.6

mm)

Visu

ally

coun

ted/sorted

(optical

microscop

e);

polymer

iden

tication

(FTIR);su

rface

characterisation

(SEM

)

Red

dyet

al.,

2006

(ref.1

5)

USA

(Haw

aii)

Beach(18

location

s)NA

NA

Strand

line;

paralle

l;15

0–19

0gsampled

;ad

dition

alsamples

(at1

beach)

perpen

dicu

larto

shoreline;

1cm

and10

cmde

pth;

6m

tran

sect

Den

sity

sepa

ration

(sod

ium

polytung

state

1.4gmL#

1 );p

articles

then

immersedin

1.2g

mL#

1sodium

polytung

state&

etha

nol:

water

mixturesof

variou

sde

nsities,

includ

ing1g

mL#

1 ,0.95

49gmL#

1 ,0.94

08gmL#

1 ,an

d0.91

1gmL#

1

Visu

ally

coun

ted/sorted

(stereom

icroscop

e);

polymer

iden

tication

(FES

EM;F

TIR;m

icro-

ATR)

Corcoran

etal.,20

09(ref.7

9)

Brazil

Beache

s(11

location

s)1–20

mm

NA

Strand

line;

quad

rats

(900

cm"

900cm

);2

cmde

pth

Ovendried(100

$ C);

sieved

(1mm);sorted

2–5,

<10,

<15,

<20mm

(smallitems)

<25,

<30,

<50,

<100

mm

(med

ium

item

s);d

ensity

sepa

ration

;seawater;

debris

<1mm

not

cons

idered

Visu

ally

coun

ted/sorted

Ivar

doSu

let

al.,20

09(ref.8

0)



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Tab

le3

(Contd.)

Cou

ntry

Hab

itat

Size

rang

e

Abun

danc

e(m

ean;

rang

e)Sa

mpling

Extraction

Qua

ntication

QA/QC

Referen

ce

Englan

dEs

tuary(3

location

s,2sites)

<100

0mm,1

–10

mm

NA

Strand

line;

rand

omly

placed

quad

rates(5

replicates

!50

cm!

50cm

);de

pth(0–3

cm)

Den

sity

sepa

ration

(saturated

NaC

l)Po

lymer

iden

tication

(FTIR)

Browne

etal.,

2010

(ref.8

1)

USA

(Haw

aii)

Beach(n

¼5)

NA

NA

Tran

sect

line40

mpa

ralle

ltosh

oreline;

swaths

at10

mintervals

perpen

dicu

larto

shoreline;visibleplastic

+5gsu

rrou

nding

sedimen

t

NA

Polymer

iden

tication

(FTIRmicroscop

y);

polymer

characterisation

(SEM

)

Coo

peret

al.,

2010

(ref.6

9)

Brazil

Beach(1

location

)>5

00–100

0mm

NA

Strand

line;

100m

tran

sect;d

epth

(0–2

cm);9replicates;

quad

rat(988

cm!

988

cm);wirecloths

eld

sieving(0.5,1

mm)

Den

sity

sepa

ration

(ltered

seaw

ater)

Visu

ally

coun

ted/sorted

Costa

etal.,

2010

(ref.8

2)

Globa

lBe

achsedimen

t(18location

s)<1

000mm

8–12

4pieces

perL

Strand

line;

depth(0–1

cm)

Den

sity

sepa

ration

,50

mLsedimen

t,saturated

NaC

l,3extraction

s

Polymer

iden

tication

(FTIR)

Browne

etal.,

2011

(ref.1

0)

USA

(Haw

aii)

Beachsedimen

t(5

location

s;3

sites)

<250

–500

0mm

1.34

%w/w

3samplingpo

ints

per

location

(stran

dline,

1m

seaw

ard,

2m

past

strand

line);c

ore

sampling(5

cmdiam

eter,2

5de

pth)

Den

sity

sepa

ration

(NaC

l1.2

gcm

#3 );

sieved

(0.25,

0.5,

1,2,

4mm)

Visu

ally

coun

ted/

sorted

;polym

eriden

tication

(FTIR)

Carsonet

al.,

2011

(ref.4

0)

Belgium

Marinesedimen

t(three

harbou

rs,

3–4sitesat

each

)

>63mm

170(49–39

0)pieces

perkg

3samplingpo

ints

(stran

dline,

intertidal,

subtidal

zone

;parallel);

sedimen

tcores

(2–7

cm)

Den

sity

sepa

ration

,1kg

sedimen

t,3Lconc

salin

esolution

;stirred

;settle

1h;

sieved

(38

mm)

Visu

ally

coun

ted/sorted

(binocular

microscop

e);

polymer

iden

tication

(FTIR)

LCS;

size

rang

e‘sim

ilarto

wha

twas

foun

din

eld’;6

8.8–

97.5%

recovery

Claessens

etal.,20

11(ref.4

5)

Portug

alBe

ach(5

location

s)>1

mm

190(29–39

3)pieces

perm

2Strand

line;

two

quad

rats

(50cm

!50

cm;2

m!

2m)in

duplicate;

depth(0–2

cm);samples

sieved

insitu

(2.5–3.5

mm)

Den

sity

sepa

ration

,conc

entrated

NaC

l140

gL#

1 ;stirred

vigo

rous

ly;

ltered

(Wha

tman

GF/C1mm)

Polymer

iden

tication

(m-FTIR)

Martins

etal.,

2011

(ref.8

3)

Can

ada

Freshw

ater

sedimen

t<5

mm

NA

Paralle

ltosh

oreline;

60m

tran

sect;v

isible

debris

sampled

(within

1m);tw

oreplicates

(2m

!2m)

Sepa

ration

into

3catego

ries

(<5mm,>

5mm

andPS

);ultrason

icated

inDI

water

for4min

Polymer

iden

tication

(FTIR);su

rface

characterisation

(SEM

)

Zbyszewski

etal.,20

11(ref.6

7)



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Tab

le3

(Contd.)

Cou

ntry

Hab

itat

Size

rang

e

Abun

danc

e(m

ean;

rang

e)Sa

mpling

Extraction

Qua

ntication

QA/QC

Referen

ce

USA

Beache

s(n

¼8)

2–5mm

NA

Visu

alsample

colle

ction(m

acroplastic

<50

mm);in-eldsied

(2mm);18

replicates;

quad

rats

(1m

"1m);

surface(1

inch

)

Den

sity

sepa

ration

,conc

entrated

NaC

l140

gL#

1 ;stirred

vigo

rous

ly;

ltered

(Wha

tman

GF/C1mm)

Visu

ally

coun

ted/sorted

Vanet

al.,

2012

(ref.8

4)

Portug

alBe

ach(n

¼10

)>3

mm

2400

/m#2

Strand

linelin

e;2"

2m;3

–5replicates;2

cmde

pth;

eldsieved

(3mm)

Den

sity

sepa

ration

,conc

entrated

NaC

l140

gL#

1 ;stirred

vigo

rous

ly;

ltered

(Wha

tman

GF/C1mm)

Visu

alcoun

ting

sorting

Antune

set

al.,20

13(ref.8

5)

German

yBe

ach(n

¼2)

>0.45mm

2rand

omsamples;

depth(0–2

cm)

Den

sity

sepa

ration

,NaC

l(1.2gcm

#3 );

man

uals

haking

;ltered

(0.45mm

nitrocellulose)

Stereo

microscop

e;SE

M;P

yr-GC

combina

tion

withmass

spectrom

etry

(MS)

Labo

ratory

controls

ample

(black

PEpe

llets,

1"

0.2"

0.2m;

10pieces

adde

dto

175g

sedimen

t;recovery

80–

100%

)

Frieset

al.,

2013

(ref.5

2)

South

Korea

Beach(n

¼10

)>2

mm

980pieces

perm

2Strand

linelin

e;50

mtran

sect;1

0replicates;

quad

rat(50cm

"50

cm);de

pth(0–5

cm)

Fieldsieved

(2mm)

Visu

aliden

tied

and

sorting

Heo

etal.,

2013

(ref.5

6)

Italy

Suba

lpinelake

sedimen

t(n

¼2)

NA

NA

Ran

dom

grid

sampling

Den

sity

sepa

ration

,no

furthe

rde

tails

provided

Visu

alcoun

ting

(microscop

e);p

olym

eriden

tication

(Ram

an)

Imho

fet

al.,

2013

(ref.8

6)

India

Beach(n

¼4)

1–5mm

69(12–96

0)pieces

perm

2Strand

line;qu

adrats(50

"50

cm);de

pth(0–2

cm);sieved

(1mm)

Den

sity

sepa

ration

,NaC

l,14

0gL#

1 ;oa

ting

plasticrecovered;

washe

d;dried

Visu

alcoun

ting

and

sorting

Jayasiriet

al.,

2013

(ref.8

7)

Italy

Marinesedimen

t(10location

s)>0

.7mm

670–22

00pieces

perkg

Supe

rcialsedimen

t(0–

5cm

depth)

Den

sity

sepa

ration

,conc

NaC

l,12

0gL#

1 ;sh

aken

;sieved32

mm

steelw

ire;

resu

spen

ded

ltered

(GF/F0.7mm)

Polymer

iden

tication

(m-FTIR)

Metho

dblan

k;am

bien

tmicroplastics

contam

ination

from

laban

deq

uipm

ent

Vian

ello

etal.,20

13(ref.4

1)

German

yBe

ach(3

location

s;6

samples

ateach

)

100–10

00mm

NA

Upp

eran

dlower

dri

line;

rand

omof

uppe

r,lower

perpen

dicu

larto

those;

6accu

mulations

and6‘bare’

sedimen

tssampled

;0.25"

0.25

m

Sieved

(1mm

mesh);

dens

itysepa

ration

(two

step

;saturated

NaC

l;NaI)

Visu

alcoun

ted(>1mm

fraction

byna

kedeye;

<1mm

microscop

e);

polymer

iden

tication

(TD-Pyr-GC/M

S)

Dek

iffet

al.,

2014

(ref.2

0)



Publ

ishe

d on

23

Dec

embe

r 201

6. D

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oade

d by

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on

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View Article Online

Tab

le3

(Contd.)

Cou

ntry

Hab

itat

Size

rang

e

Abun

danc

e(m

ean;

rang

e)Sa

mpling

Extraction

Qua

ntication

QA/QC

Referen

ce

Belgium

Beach,

river

>250

mm

1.5pieces

per

m2

Paralle

ltosh

oreline;

50m

tran

sect;ran

dom;3

replicates;q

uadrats(25

cm!

25cm

)

Den

sity

sepa

ration

,NaC

l,(360

gL"

1 );

shak

ing2minutes

!2;

sieved

(250

mm)

Visu

aliden

tication

andcoun

ting

(ligh

tmicroscop

e)

Laglba

uer

etal.,20

14(ref.8

8)

Sing

apore

Intertidal

man

groves

(n¼

7)

<20to

>500

0mm

37(12–63

)pieces

perkg

‘Und

isturbed

areas’;

quad

rats

(1.5

m!

1.5

m);2–3m

apart;de

pth

(3–4

cm)

Den

sity

sepa

ration

,conc

salin

esolution

,(1.18gL"

1 );s

haking

(mecha

nicals

hake

r2

min

200rpm);settle

6h;

sieved

0.1cm

(rem

ovede

bris);ltered

(1.7

mm

GF/A)

Visu

alsortingan

dcoun

ting

(optical

microscop

e);p

olym

eriden

tication

(FTIR)

LCS50

0–60

0mm

PEsp

heresinto

sedimen

t(n

¼2;

recovery

55–

72%)

Moh

amed

Nor

etal.,

2014

(ref.8

9)

German

yBe

ach(n

¼3)

>1mm

NA

‘Recreationzone

’;rand

omsamples

14km

beach;

max

80m

apart;

depth(0–3

cm)

Sieved

1mm

mesh;

uidisation

(saturated

NaC

l);de

nsity

sepa

ration

(NaI

1.8g

cm"3 );s

hake

n10

s;ltered

(nitrocellu

lose

0.45

mm);matrix

removal

(30%

H2O

2

treatm

ent)

Visu

alcoun

ting

/sorting

(microscop

e);p

olym

eriden

tication

(Pyr-GC/

MS)

Labo

ratory

controls

ample

(1mm

size

PE,

PP,P

ET,P

S,PV

C,

EPS,

PUR;

recovery

68–

99%);proced

ural

blan

k

Nue

lleet

al.,

2014

(ref.5

3)

Brazil

Beach(n

¼15

)>1

mm

5400

pieces

perm

3

(surface

layer

repo

rted

)

100!

1m

2tren

ches;1

0cm

depthsampled

;10

samples

at0.1m

depth

intervalsun

tild

epth

(1.0

m)

Den

sity

sepa

ration

(seawater);sieve1mm

mesh

Polymer

iden

tication

(Ram

an)

Turraet

al.,

2014

(ref.4

6)

NA(deep-

sea)

Deep-sea

sedimen

t(n

¼12

)

>32mm

1.4–40

pieces

per50

mL

Cores;1

000–35

00km

depth;

megacorersor

boxcorer;5

.7,7

.4or

10cm

diam

eter

Metho

d1-centrifuge;

dens

itysepa

ration

,NaC

lorLu

dox-TM

40;

sieve(32mm)

Visu

alcoun

ting

/sorting

(binocular

microscop

e);

polymer

iden

tication

(FTIR)

Woo

dall

etal.,20

14(ref.4

3)

Metho

d2-de

nsity

sepa

ration

;con

cNaC

lsolution

;ltration

(Wha

tman

GF/A1.6mm)

Can

ada

Freshw

ater

lake

(3lake

s;26

location

s)

<10cm

NA

Paralle

ltosh

oreline;

60m

tran

sect;1

0m

interval;v

isible

<10cm

particlescolle

cted

Clean

ed(ultrasonic

bath,D

Iwater);

sepa

ratedby

hand

into

4catego

ries

Visu

alcoun

ting

/sorting;

polymer

iden

tication

(FTIR);

surfacech

aracterisation

(SEM

)

Zbyszewski

etal.,20

14(ref.9

0)



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ishe

d on

23

Dec

embe

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6. D

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Tab

le3

(Contd.)

Cou

ntry

Hab

itat

Size

rang

e

Abun

danc

e(m

ean;

rang

e)Sa

mpling

Extraction

Qua

ntication

QA/QC

Referen

ce

Can

ada

Freshw

ater

lake

sedimen

t(2

cores)

500–50

00mm

Bottom

sedimen

t;2

boxcoresamples

(7cm

diam

eter);15

!2cm

samples

(dep

th30

cm,

150samples);sieved

(0.5,0

.71,

0.85

,1mm)

Den

sity

sepa

ration

(two

step

;distille

dwater

oa

ting

particles

removed

);sedimen

tdried(60

" C),de

nsity

sepa

rated(sod

ium

polytung

state,

1.5g

cm#3 );

oating

particles

removed

Visu

ally

coun

tedan

dsorted

(microscop

e);

polymer

iden

tication

(FTIR)

Corcoran,

2015

(ref.7

3)

Switzerlan

dFreshw

ater

lake

sedimen

t(3

beache

s,n¼

67)

300–50

00mm

1300

(20–

7200

)pieces

perm

2

Strand

line;

4location

s;qu

adrats

(30cm

!30

cm)

Den

sity

sepa

ration

;250

mLsedimen

twith7L

salin

esolution

,320

gNaC

lper

L;airow

;sieve(300

,100

0,50

00mm);over

driedat

60" C

;matrixremoval

(35%

H2O

2with0.05

MFe

catalyst)

Visu

ally

coun

ted

(stereom

icroscop

e);

polymer

iden

tication

(FTIR)

Faureet

al.,

2015

(ref.9

1)

NA(Pacic

ocean)

Deep-sea

sedimen

t(n

¼12

)

0.3–20

0mm

60–200

0pieces

perm

2Deepsea;bo

xcorer;0

.25

m2 ;su

bsam

ples

0–2cm

,2–20

cm

Sieve(300

,500

,100

0mm)

Visu

ally

coun

tedwith

someaidof

stereo

microscop

e

Fische

ret

al.,

2015

(ref.4

7)

Hon

gKon

gBe

ach(n

¼25

)31

5–50

00mm

5595

(106

–15

554)

pieces

perm

2

Tide

line;

paralle

l;30

mtran

sect;d

epth

(0–4

cm);qu

adrat50

cm!

50cm

Ineldsieve(315

mm);

dens

itysepa

ration

(tap

water,s

onication);w

etsieved

(315

mm)

Visu

ally

coun

tedan

dsorted

(microscop

e);

gravim

etrically

Foket

al.,

2015

(ref.9

2)

South

Korea

Beachsedimen

t(3

location

s;n¼

21)

50–500

0mm

4600

0(56–

29000

0)pa

rticlespe

rm

2

Tide

line;

divide

dinto

districts(every

30m);

quad

rat(50cm

!50

cm);de

pth(0–2

cm);on

-site

sieved

(100

0,50

00mm)

Den

sity

sepa

ration

,NaC

l,2.16

gcm

#3 ;

sieved

4000

,280

0,20

00,

and10

00mm;

supe

rnatan

tof1

000mm

sieved

(300

,500

mm)

andltered

(0.75mm)

Visu

ally

coun

ted

(microscop

e);p

olym

eriden

tication

(FTIR;

particles>3

00mm)

Kim

etal.,

2015

(ref.2

8)

German

yRiver

sedimen

t(n

¼8)

63–500

0mm

4000

particlespe

rkg

Betw

eenwrack

linean

dwater

line;

3–4kg

compo

site

sample

comprising35

–40

rand

omsamples

over

10–15m

rang

e;qu

adrat

(30cm

!30

cm);de

pth

(2–3

cm)

Ovendried;

sieved

(63,

200,

630mm);63

0fraction

visu

ally

sepa

rated;

dens

ity

sepa

ration

,NaC

l365

gL#

1 ;settle

overnigh

t;ltered

(glass

bre);

matrixremoval

(H2O

2/H

2SO41:3

ratio)

Visu

ally

coun

ted

(binocular

microscop

e);

gravim

etrically

;classied

bysh

ape;

polymer

iden

tication

(FTIR)

Klein

etal.,

2015

(ref.6

3)

South

Africa

Beachsedimen

t(21sites)

65–500

0mm

670pa

rticles

perm

2Strand

line;

triplic

ate

compo

site

samples;

depth(0–5

cm);12

00mLsu

bsam

pletake

n

Den

sity

sepa

ration

;saturatedsalin

esolution

;rep

eatedve

times

Visu

ally

coun

ted

(dissecting

microscop

e);v

isua

l

Nel

etal.,

2015

(ref.7

7)



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ishe

d on

23

Dec

embe

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View Article Online

Tab

le3

(Contd.)

Cou

ntry

Hab

itat

Size

rang

e

Abun

danc

e(m

ean;

rang

e)Sa

mpling

Extraction

Qua

ntication

QA/QC

Referen

ce

sorting(fragm

ents

&bre,

colour)

Korea

Beachsedimen

t(6

beache

s;10

sites)

>1mm

Strand

line;

6location

s;qu

adrat(50!

50cm

);de

pth(0–5

cm)

In-eldsieved

(1mm);

dens

itysepa

ration

,50

mLsand

with33

mL

saturatedNaC

l;ltered

(1.2

mm);oven

dried(60

" C)

Visu

alcoun

ting

(nak

edeye>1mm;m

icroscop

efor<1

mm);po

lymer

iden

tication

(FTIR)

Blan

kan

alysis

(distille

dwater,

n¼

3)

Song

etal.,

2015

(ref.7

2)

German

yBe

achsedimen

t(5

location

s;14

sites)

>55.5mm

Strand

line;

500mL;

depth(1–2

cm)

Den

sity

sepa

ration

;CaC

l 2(1.30–

1.35

gmL$

1 );a

irventing;

settle

overnigh

t;55

mm

zoop

lank

tonlter;

H2O

2digestionfor

matrixremoval

Visu

ally

coun

ted

(stereom

icroscop

ewith

camera)

LCS(200

PE10

0–10

00mm

particlessp

iked

incleane

dsedimen

t;recovery

rang

edvaried

basedon

colour;%

55%

recovery)

Stolte

etal.,

2015

(ref.6

4)

UK

Atlantic

ocean

marinesedimen

t(n

¼2)

>32mm

NA

Deepsea;

corers;0

–2cm

and>5

cmsu

bsam

ples

Cen

trifug

e;1.16

spec

gravity,

8sp

incycle

(400

0rpm,5

min);32

mm

sieve

Visu

ally

coun

ted

(stereom

icroscop

e)Ba

ckgrou

ndmicrobe

rlevels

tested

Woo

dall

etal.,20

15(ref.9

3)

Spain

Marinesedimen

t(n

¼3)

63to

>210

0mm

0.90

pieces

perg

Coretube

s30

cm!

3.5

cm;sitereplicates

1.5m

apart;8–

10m

depth

Man

uals

ieve

($0.06

3,0.12

5,0.25

,0.5,1

,$2

mm);de

nsity

sepa

ration

;distille

dwater;m

ixed

15m;

lter

details

notprovided

Visu

ally

coun

ted

(stereom

icroscop

ewith

camera)

Petrid

ishe

sle

inworksp

ace

andch

ecke

dfor

particle

contam

ination

Alom

aret

al.,

2016

(ref.7

8)

Taiwan

Beachintertidal

(n¼

4)$38

to$40

00mm

32–4256

0pieces

perm

3Middleof

inter-tida

lzone

;qua

drat

(50cm

!50

cm);de

pth(0–5

cm,

5–10

cm),dried60

" Cfor2da

ys

Man

uals

ieve

($0.03

8,0.12

5,1,

2,$4mm);

dens

itysepa

ration

,(1.17gNaC

lper

cm3 );

vigo

rous

shak

ing30

s

Visu

ally

coun

ted

(stereom

icroscop

e);

polymer

iden

tication

(ATR

-FTIR;S

R-FTIR)

Kun

zet

al.,

2016

(ref.5

8)

India

Beachsu

rface

sedimen

ts(n

¼9)

2–5mm

24–145

pieces

perm

2Tide

linepa

ralle

l;qu

adrats

(1m

!1m)

Den

sity

sepa

ration

;1.2

kgNaC

lper

L;stirred;

ltered

(Wha

tman

GF/A

1.6mm)

Visu

ally

coun

ted

(stereom

icroscop

ewith

digitalc

amera);

polymer

iden

tication

(FTIR)

Veerasinga

met

al.,20

16(ref.9

4)

USA

Estuary,

intertidal

sand

ysedimen

t(n

¼7)

200–50

00mm

51(5–117

)pieces

perm

2Tide

linepa

ralle

l;12

replicates;q

uadrats(25

cm!

25cm

);de

pth

(top

3–6cm

)

On-site

man

uals

ieve

(0.5–5

mm

sieve);

dens

itysepa

ration

(elutriation

;aeration

200mm)

Visu

ally

coun

ted;

iden

tication

(FTIR)

LCSrecovery

97.25%

(unk

nown

polymer

orsize)

Wessele

tal.,

2016

(ref.5

9)



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buoyant, in time plastics will biofoul42 or following ingesting,will sink to the aquatic sediment where it will persist fordecades. Due to long-range environmental movement, plasticmicrobers have been detected in deep-sea sediments (1000–3500 m depth) ranging from 1.4 to 40 pieces per 50 mL (mean!s.e.: 13.4 ! 3.5).43 The proximity to city, ocean currents, type ofsediment sampled must be considered when choosing anappropriate sampling site.

Once the study location is decided upon, the samplingapproach on-site is applied. Each study considered in our reviewhas applied unique methodology for the sampling approachand in many instances the methodology is incomplete to allowthe reader to fully understand the technique applied which hasimplications for reproducibility. Transects are a commonapproach when conducting a beach survey (14%), with quadratsof various sizes utilised (51%), potentially the top surface layerof variable depth and then either collected or sieved on site. Thevariation in results from each of these decisions will stronglyinuence the results and the comparability of the ndings. It is

well known that the strandline (tide-line) will contain anabundance of marine debris, including microplastics, and sothis area is oen targeted to be part of the transect (56% ofstudies). This has the strong potential to bias the results, as theresearchers are aiming to nd debris rather than conductinga systematic measurement of the environment.

The sediment depth is another aspect of sampling that islikely to inuence the reported concentrations and apparentenvironmental burden of plastics. The majority of studies willunderestimate the levels of plastics by only focusing on thesurface layer and not accounting for buried litter.44 Furtherthere are contradictory reports drawing conclusions about theabundance of plastic with depth. Studying the stratication ofsediment cores to a depth of 25 cm in Hawaii indicates that 50%of microplastic fragments were contained in the topmost 5 cmof each core, and that the top 15 cm hosted 95% of all detectedplastic particles.40 Similar results were reported at the high-water line at a Belgian beach, where the top 16 cm layer con-tained 65% of all microplastics, with 40% detected in the top 8

Fig. 1 (a) A world view of the locations and year of publications where microplastics have been recorded within sediment samples, (b) the totalnumber (n) of publications per year, and (c) percentage of studies based upon type of sediment and location.



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cm layer alone.45 While another study demonstrated that thesurface layer accounted for <10% of the total abundance in thesediment column in their sample location.46

General sampling strategies are dened as bulk or volume-reduced, depending on the type of microplastic accumulationbeing investigated. For beach sediments and shorelines, bulksampling – taking a dened volume of sediment,24 usually froma dened quadrat area and depth –was themost commonmethodfor sampling beach sediments (n ¼ 22). Marine sediments (n¼ 6)were collected by bulk sampling, with a box corer or sedimentgrab.47,48 Volume-reduced sampling, used by 30% of the studiesreviewed, involved processing sand in situ with 1–5 mm sieves.

Further method inconsistencies between these samplingtechniques have led to a variety of sampling units reported.23

Reporting units are generally in microplastic abundance persurface area (m2),49 however some quadrats are also sampled todepth, reporting volume in cubic metres (m3),46 mL or L (ref. 7and 50) or weight (g to kg).51 Conversion between measure-ments requires density estimates, leading to assumptions andfurther inconsistencies comparing microplastic abundancebetween littoral zones.23 The top 15 cm of sediment contains95% of microplastic particles, with 50% found in the topmost 5cm.40 Depths sampled across the studies reviewed range from 1to 5 cm, collected using metal spoons or spatulas, whilea rotating drum sampler has been used for sediment subsurfacebulk sampling (10–11 cm depth).51 Analysis of pellet distribu-tion below the subsurface involved a number of 1 m2 trenchesdug using a shovel, however how sediment was sampledremained unspecied.46 Across all bulk and volume-reducedsampling methods, sediment samples are either stored in PETdrinking bottles52 or glass bottles.20,53

The current lack of standardisation between samplingmethods hinders inter-study comparison on the reported abun-dances of microplastics in sediments worldwide.23,26 Differencesbetween sampling design could be overcome by rigorous reportingmethods on the complete set of sampling details including depth,weight or volume, density and water content details of sedimentssampled.24 Simple improvements for standardisation couldinclude reporting using SI Units for sediment mass (g) rather thanvolume and reporting dry weight values.

Differences in sampling design hinder spatiotemporalcomparisons between studies, as microplastic abundance re-ported within a number of studies is due to deposition over anunknown time scale.54 Shoreline and beach studies havedened quadrat sizes at random or regular intervals alonga transect, usually running parallel to the ‘wrack line’ or‘strandline’, referring to the most recent otsam deposited atthe high tide line.10 It is clear that microplastics are nothomogeneously distributed across the shoreline and results canvary widely even within a close geographic distance, howevermost studies assume small-scale variations within beaches areinsignicant.55 To date, no clear relationship has been foundbetween the amount of microplastic litter and general surveymethod, except methods that included the total length of thevegetation line (or ‘berm’) as well as ‘random’ sampling tends tohave a higher mean.54 A single study detailed using a randomnumber generator for quadrat sampling.56

3.2 Extraction

3.2.1 Physical separation. Once the samples have beencollected the microplastics of varying polymer, size, colour,morphology must be removed and isolated from what can bea complicated matrix. The most commonly applied techniquefor isolating microplastics is with on-site sieving which hasbeen successfully applied for the large microplastics (>1 mm).Depending upon the nature of the study, the plastics are simplyremoved and placed in a collection bag for visual processing ata later stage. However, this approach is not adequate for smallmicroplastics (<1 mm) and further laboratory extraction tech-niques must be applied.

3.2.2 Density separation. To extract the small microplastics(<1 mm) from a sediment sample, the most commonly reportedtechnique is density separation combined with ltration. Thismethod was applied in 84% of studies examined (36/43; Table3). The density separation extraction technique generallyinvolves four main steps; (1) introduction of aqueous solvent ofspecic density, (2) mixing for dened periods of time, (3)a settling equilibration time and (4) ltering to specic sizefractions.

The density separation approach to isolate microplasticsfrom sediment is possible by exploiting the difference in densityof polymers. For example, PE/PP have a density lower than waterand would be expected to oat on the water surface withoutdensity modication (Table 1; LDPE 0.91–0.92 g mL"1; HDPE0.93–0.97 g mL"1; PP 0.89–0.92 g mL"1). By increasing thedensity of the solvent, it is possible to create a solution wherehigher density polymers oat (Table 1; PS 0.28–1.04 g mL"1;PVC 1.10–1.47 g mL"1). Common environmental samples (i.e.soil, sand) have higher densities (approximately 2.55 g mL"1

and >1.44 g mL"1 respectively) than these polymers makingseparation a practical technique for separation.24 The sepa-rating solution is most commonly a concentrated salt solutionsuch as NaCl of varying densities (n¼ 19/43), the most commonbeing 1.2 g mL"1 (50%). Other solutions used include zincchloride (ZnCl2) (n ¼ 1), calcium chloride (CaCl2) (n ¼ 1),sodium iodide (NaI) (n ¼ 1), sodium polytungstate (n ¼ 2),ltered seawater (n ¼ 4) and distilled water (n ¼ 2). This leavesa total of 17 out of the 40 density separation techniques that donot specify a separation uid and 15 that also lack a corre-sponding density. The variation in separating uids used inaddition to these being unknown in 43% of the density sepa-ration studies highlights the need for a standardised methodand appropriate reporting of these methods.

Shaking or stirring following the introduction of a separatinguid is important to ensure polymers can adequately detachfrom the matrix. Shaking of the sample is either donemechanically with a centrifuge57 or mechanical shaker,51 or byvigorous manual shaking.58 Stirring was the other preferredmethod of mixing, done either manually or mechanically(magnetic). Time frames were rarely indicated for stirring,which introduces a subjective element in terms of decidingwhen the polymer has sufficiently separated from the matrix.Aeration,59–61 and inversion48 were other techniques used forthis step.



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Settling times aer mixing varied between each study, withsome le to settle for as little as ve minutes,62 and as long as‘overnight’ (assumed to be upwards of 12 hours).63,64 The timingof settling was only specied in seven studies, highlighting thenecessity for better reporting of methods.

3.2.3 Filtration. The most common ltration techniquesamong studies are sieving and vacuum ltration (generally drysorting and wet sorting, respectively). Dry sorting by sieving canbe done before the density separation method to reduce thebulk of a sample or separate them into size fractions, or insteadof the density separation technique. Wet sorting is generallyconducted aer the density separation method to isolate theoating plastic particles, and is carried out by vacuum ltration.These methods are consistent throughout all studies, witheither one of these methods occurring to isolate polymers. Thediffering factor between these is the lter type and size.

Seventeen studies reported ltering their samples during theextraction process. Filter types used for wet sorting includemost commonly glass bre lters (Whatman® GF/A, GF/C orGF/F; n ¼ 11), but also include nitrocellulose lters (n ¼ 1),isopore lters (n ¼ 1), and zooplankton lters (n ¼ 1). Twentystudies reported using sieving as their method of isolatingpolymers during their extraction step, 14 of which used it astheir only sorting step within their extraction process (sansdensity separation). This leaves the remaining 6 studiescombining sieving and ltration during their extractionprocess. Regardless of the method or technique used, the sizeand type of the lter paper is a large inconsistency between allstudies, with no particular size fractions being used to deter-mine the quantity of ‘microplastics’. It is this that emphasisesthe need to clearly dene the size ranges of microplastics in theenvironment, with our recommendations listed in Table 1.

The typical size categorisation of microplastics diameter usehere is <5 mm in diameter here, we endorse the use of large-microplastic, small-microplastic, and nanoplastic as beingthree separate categories within this range. With the imple-mentation of these three size fractions, more structuredresearch can be achieved that can be compared betweenresearchers, sites, and nations. Another inconsistent analyticalapproach is the pore size of the lter paper used. These rangefrom 0.3 mm (ref. 65) to 5 mm,66 but do not necessarily corre-spond to the size ranges reported in their ndings. For example,one study report using the Whatman® GF/F lter with pore size0.7 mm, and that their smallest plastic particle detected was 15mm.41 This infers that anything between 0.7 mm and 15 mm isstill being captured. This becomes problematic when weighingis introduced as a quantication method, but also begs thequestion as to whether all plastic particles were counted andmeasured correctly.32

3.2.4 Matrix removal. Matrix removal is a process that canbe applied to the destructive removal of interfering organicmatter present in the sample. The difficult task of separatingpolymers from their matrix is aided using solutions to digestorganic matter; making ltration and quantication moreaccurate and less time consuming later. Studies have includedchemical matrix removal as well as physical matrix removalsteps and reported higher ltration rates and a need to remove

material (organic matter, calcium carbonate, etc.) from theplastic particles.53,63,64

Hydrogen peroxide (30% H2O2) was found to leave polymersize, shape and resulting spectra unchanged,53 and was deter-mined to be the best solution to digest samples with higherorganic matter, compared to 20% H2O2, 37% hydrochloric acid(HCl), and various concentrations of sodium hydroxide (NaOH).53

Mixtures of H2O2 and sulphuric acid (H2SO4) (1 : 3) have beenused to aid in the digestion of organic matter,63with bleach beingutilised to destroy natural debris in another study.34 Mechanicalmatrix removal is also seen in studies where samples wereultrasonicated to remove excess matrix from the samples.67–69

The matrix removal step is only present in 12% (5/43) ofstudies, despite it being an important step in the quanticationof microplastics from sediment samples. Its importance isbased on the content of organic matter that is likely to beubiquitous throughout all sediment samples, making it neces-sary to ensure all matter is digested before analysis of theplastics. The inconsistencies between studies indicate that thematrix removal step needs to be validated and included in allsediment extraction processes going forward. Gravimetrictechniques are likely to overestimate concentration withoutmatrix removal. A complimentary approach for measuring thetotal mass of plastics is through gravimetric analysis. Theapproaches vary and can include measuring the mass of indi-vidual microplastics particles or measuring the lter paper asa whole.16,50

3.3 Quantication and identication

3.3.1 Manual counting – optical microscope. The mostcommon quantication technique for microplastics is visualcounting and sorting into various categories, typically basedupon polymer type, colour, size and morphology. Visual sortingand identication of microplastics under a dissection micro-scope has been a common method reported in terms of quan-tication (n ¼ 27), but has many limitations in terms ofaccuracy. Visual counting can lead to either extreme over18 orunderestimations of plastic content,70 based on the extent of thesize ranges of plastics in the environment, as well as the risk ofcounting non-plastic particles as plastic. For example, approx-imately 20% particles initially identied as microplastics byvisual observation, were later identied as aluminium silicatefrom coal ash using scanning electron microscope (SEM).32

The application of spectroscopy in the conrmation of thepresence of plastic is extremely important, and can increase theaccuracy of visual counting. Studies have used a combination ofmicroscopy and spectroscopy (i.e. microscope and Fouriertransform infrared spectroscopy (FTIR)) to rst count thepotential microplastic particles, followed by either conrmingor denying that they are plastics.65

Recovery of visual counting methods is highly unreliable andquantitation using quality control samples demonstrate thatthe colour of microplastics with 70–100% recovery for blue,violet, green hues and 0–40% recover for yellow, orange, pinkand orange microplastics with transparent microplasticsrecovered about 45–63%.64



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3.3.2 Polymer identication (FTIR; Raman; scanning elec-tron microscope). Spectroscopy is required to conrm theidentication of plastics, and their synthesis polymer forparticles <1 mm in size.9 Identication methods used to classifymicroplastics from environmental samples include Ramanspectroscopy, Fourier transform infrared spectroscopy(including micro-FTIR and ATR-FTIR), and scanning electronmicroscopy (SEM). These techniques assist in identifyingcolour, shape, morphology, chemical composition and struc-ture. Each of these identifying features is important in deter-mining the types of plastic pollution that is found in theenvironment.

FTIR is used to obtain infrared spectrum of emission orabsorption spectra as well as to collect high spectral resolutiondata, which facilitates the determination of the structure ofmolecules.71 FTIR is the most common method in the identi-cation of microplastics from sediment and water samples re-ported (23/43; 53%; Table 3). A combination of FTIR and opticalmicroscopy has also been utilised (n ¼ 4) (micro-FTIR) in theidentication of polymers from sediment samples.72,73 The FTIRanalytical method has many advantages in terms of plasticidentication. Based on the high concentration of studiesrelying on FTIR spectroscopy we are condent that this is anappropriate measure of polymer type within sediment samples.One advantage of FTIR reectance spectroscopy is its non-invasive nature, as it allows for samples to be analysed withoutdestroying the sample.

Raman spectroscopy is a spectroscopic technique based onthe interaction of a sample and the consequent changes ofphotons in monochromatic light and has the ability to providestructural information about plastics which can then be inves-tigated to identify their polymer type.74 Raman spectroscopy wasonly used in two studies, where it was combined to both identifyand quantify plastics in sediment using Raman micro-spec-troscopy. This allows for a non-destructive identication tool,where particles in the mm range can be quantied.

SEM gives detailed information about the size and shape ofa particle. SEM can assist in identifying inorganic plastic addi-tives, and gives a clear image of the size and topography of theparticle. Seven studies employed SEM to obtain identicationinformation about plastic particles from sediment samples.Combining SEM with other identication techniques such asFTIR can be advantageous, where certain plastic types could beparticular shapes. A study showed that most of their polyethylenesamples were rounded, and most of their polypropylene sampleswere fragments.67 This information can help to give an indicationof the source of the plastics sampled, i.e. broken down fragmentsfrom large plastic, or primary nurdles.15,67

3.3.3 Emerging techniques. Three studies used pyrolysisgas chromatography/mass spectrometry (GC/MS) to identify thepolymer found in environmental samples. Pyrolysis is a tech-nique where a sample is burnt in the absence of oxygen and ittypically applied for thermogravimetric analysis. The decom-position of the polymer related to temperature provide a specicsignature relative to specic polymers. If combustion is fed intoa gas chromatography for separation of chemical constituents

being determining the chemical identity through mass spec-trometry (MS). This is combined with Pry-GC obtains structuralinformation about macromolecules by GC/MS analysis and isa powerful tools for identication of polymer.53 The majordisadvantages are that you determine the mass of polymer persample and do not get any information on the number, type andmorphology of the plastics present in the sample.

A new technique for quantifying the amount of plastics insolid samples has been reported using pressurised uidextraction (PFE).75 Whole polymers (melting, destructive) areappropriate for measuring the total amount of plastics presentin a sample but they will fail to capture the morphologicalcharacteristics, that currently can only be captured through thelabour intensive visual counting.

3.4 QA/QC

Analytical approaches applied to measuring plastic debris inenvironmental samples are not well-developed.9 The quanti-cation of microplastics from environmental samples has notbeen optimised, and involves many hurdles such as contami-nation, overestimation and underestimation.53 These hurdlesbecome apparent at the extraction process, where validationstudies and blanks should be included as part of the analyticalprocess.

Only seven studies of the 43-analysed conducted some sort oflaboratory control sample (LCS) or validation trials. Validationstudies retrieved recovery rates of up to 100% (ref. 61) and aslittle as 54.9%.76 The size range of spiked plastics varied greatlyand higher recoveries were reported for the larger spiked plas-tics compared to the smaller size fraction.

Laboratory blanks were used in three of the studies evalu-ated, which determined whether contamination from thelaboratory or clothing of scientists was effecting results.57,77,78

Determining whether contamination is occurring in the labo-ratory environment is imperative to ensure plastic content is notoverestimated by counting or weighing said contamination.77

The combination of validation studies and blanks are essentialin reporting ndings of a study, without them the it cannot beconcluded that the method used was reliable or valid.

4 ConclusionIn this review, we collated the analytical approach for measuringmicroplastics in sediment. The four primary areas of the analyt-ical process that have been summarised; (1) sampling, (2)extraction, (3) quantitation and (4) QA/QC. Each of those sectionshave their own subject specic challenges and require furthermethod development and harmonisation. Specically, improve-ments and harmonisation on size fractions, samplingapproaches, extraction protocols and units for reporting plasticabundance would be improve the comparability of data in thisresearch area. The issues facing scientists measuring micro-plastics in sediment samples are like other matrices and bio-logical samples. Further, we would advocate for the developmentof strong QA/QC procedures to be adopted like other elds ofanalytical chemistry. Finally, inter-laboratory prociency testing



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is recommended to give an indication of the variation inmeasurements reported in the scientic literature to this point.While visual counting has been an important technique forbeginning our understanding of microplastics in environmentalsamples, it is labour intensive and prone to human error, andanalytical techniques must move forward.

AcknowledgementsThe authors thank projects Fondecyt 1161673 (K. Pozo), theCzech Ministry of Education, Youth and Sports projects(LO1214 and LM2015051) and the anonymous reviewers forproviding comments on earlier dras of this manuscript.

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