Comparison of Raman and Fourier Transform Infrared Spectroscopy

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Environmental Measurements Methods

Comparison of Raman and Fourier Transform Infrared Spectroscopy for the Quantification of Microplastics in the Aquatic Environment Livia Cabernard, Lisa Roscher, Claudia Lorenz, Gunnar Gerdts, and Sebastian Primpke Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03438 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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Environmental Science & Technology

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TITLE: Comparison of Raman and Fourier

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Transform

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Quantification of Microplastics in the Aquatic

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Environment

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AUTHOR NAMES: Livia Cabernard,*, ‡, † Lisa Roscher, ‡ Claudia Lorenz, ‡ Gunnar Gerdts, ‡

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Sebastian Primpke ‡

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AUTHOR ADDRESS:

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Helmholtz Center for Polar and Marine Research, Biologische Anstalt Helgoland,

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Kurpromenade 201, 27498 Helgoland, Germany

Infrared

‡

Spectroscopy

for

the

Department of Microbial Ecology, Alfred Wegener Institute,

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†

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Engineering and Institute of Science, Technology and Policy, Swiss Federal Institute of

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Technology, ETH Zurich, Universitätsstrasse 41, 8092 Zurich, Switzerland

Department of Civil, Environmental and Geomatic Engineering, Institute of Environmental

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ABSTRACT: Microplastics (MPs, 500 µm were visually sorted and manually analyzed by µ-Raman and

19

attenuated

20

concentrated on gold-coated filters and analyzed by automated single-particle exploration

by

total

analyzing

reflection

MPs

extracted

(ATR)-FTIR

from

spectroscopy.

ACS Paragon Plus Environment

North

Sea

surface

Microplastics ≤500 µm

waters.

were

1


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coupled to µ-Raman (ASPEx-µ-Raman) and FTIR imaging (reflection mode). The number of

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identified MPs >500 µm was slightly higher for µ-Raman (+23%) than ATR-FTIR analysis.

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Concerning MPs ≤500 µm, SPE-µ-Raman quantified two-times higher MP numbers but

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required a four-times higher analysis time compared to FTIR imaging. Since SPE-µ-Raman

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revealed far higher MP concentrations (38–2621 particles m–3) compared to the results of

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previous water studies (0–559 particles m–3), the environmental concentration of MPs ≤500 µm

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may have been underestimated until now. This may be attributed to the exceptional increase in

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concentration with decreasing MP size found in this work. Our results demonstrate the need for

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further research to enable time-efficient routine application of SPE-µ-Raman for reliable MP

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counting down to 1 µm.

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INTRODUCTION

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With their unique properties, plastics have become both indispensable in our daily life and an

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emerging global environmental problem.1-3 Global plastics production has demonstrated an

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exponential growth predicted to continue in the future.4 Unfortunately, the same holds for

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plastic litter ending up in the environment. Depending on the stability of the polymer type and

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the environmental conditions, plastics persist in the environment for years, decades, centuries

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or even millennia with many negative impacts.5-8 In the past decade, environmental plastics

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pollution became even more popular due to the ubiquitous evidence of so-called microplastics

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(MPs, 500 µm were visually sorted and sequentially analyzed by µ-Raman and FTIR spectroscopy.

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The latter was mainly performed by attenuated total reflection (ATR)-FTIR and, in some cases,

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by FTIR imaging. Microplastics ≤500 µm were concentrated on a gold-coated polycarbonate

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filter to analyze identical samples by automated single-particle exploration coupled to µ-Raman

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spectroscopy42 (ASPEx-µ-Raman) and FTIR imaging18, 20, 36, 38, 43, 44 in reflection mode.19, 45, 46

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The gold-coating of the filters allowed the measurement in reflection–absorption resulting in

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spectra similar to those obtained by the use of the transmission mode. Based on these analyses,

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the MP number, polymer composition, and size distribution were assessed and compared

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between the two methods. Finally, the presence of MPs of 1–10 µm was exemplarily confirmed

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in one sample by ASPEx-µ-Raman.

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MATERIALS AND METHODS

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Sampling. Eight surface water samples were collected during two sampling campaigns in the

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southern North Sea in autumn 2013 for Raman evaluation (n = 1, R) and in summer 2014 for

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Raman and FTIR comparison (n = 7, Station S1–S7) (Supporting Information (SI) Table S1

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and Figure S1). All samples were collected with a neuston net (HYDRO-BIOS Apparatebau

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GmbH) fixed to a catamaran, which ensured constant net positioning towards the water surface

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and sideward trawling to prevent contamination by the ship. The net had a mesh size of 100 µm

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and a net opening of 0.15 m x 0.3 m. The sampled volumes were determined by a flow meter

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(HYDRO-BIOS Apparatebau GmbH) attached to the net opening (SI Table S1). The material

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in the cod end of the net was rinsed into a 1 L Kautex bottle and stored in a freezer at –20 °C.

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Laboratory Preparation. Each sample was fractionated at a size of 500 µm with a stainless-

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steel filter. The sample material >500 µm from Stations S1–S7 was placed into a Bogorov

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chamber and visually sorted under a stereomicroscope (Olympus SZX16) at 16-fold

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magnification (Figure S1). A total of 235 putative MP particles including 44 fibers were ACS Paragon Plus Environment

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identified and photographed for size determination (cellSens, Olympus). The size (d) of each

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particle was determined, with the length (l) as the longest dimension and the width (w) as the

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longest distance rectangular to the length:24

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𝑑 = √𝑙 × 𝑤

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The size fraction ≤500 µm of each sample underwent extensive enzymatic and oxidative

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purification according to the procedure of Löder et al. (2017)47 (SI Figure S1). This purification

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step was followed by density separation with a zinc chloride solution (1.7 g cm–3) to remove

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inorganic residues. A blank control sample of Milli-Q water (ultrapure water filtered with a

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pore size of 0.22 µm, Merck Millipore) was prepared simultaneously to account for possible

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contamination during sample processing in the laboratory.

(1)

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Raman and FTIR Comparison for MPs >500 µm. Out of the 235 visually sorted items, a

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total of 200 particles were analyzed first by µ-Raman and then by ATR-FTIR spectroscopy

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(SI Figure S1), since the latter led to partial physical destruction of the material.

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For µ-Raman analysis, each particle was placed on a gold-coated mirror and manually

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measured by the Single Particle Explorer for Life & Science (rap.ID Particle Systems GmbH)

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using a laser exposure time of 30 s, a laser intensity of 50% and a laser wavelength of 785 nm

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(SI Paragraph S1 and S2). In some cases, these parameters were adapted to higher exposure

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times and lower intensities to improve the spectral quality.

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For ATR-FTIR analysis, each particle was fixed onto a diamond crystal, and three IR spectra

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were recorded (SI Paragraph S3). A total of 20 particles had sizes slightly 500 µm. In total, 235 particles were isolated from

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the investigated environmental samples. At Station S4, 45 highly similar particles (size, shape,

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color) could be observed. From these, a subsample of 10 particles was further analyzed by

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Raman and ATR-FTIR analysis. In both approaches, all particles were identified as

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polyethylene. Thus, all 45 particles were considered MPs for calculation of the concentration

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and size distribution of MPs.

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Out of the 200 individually investigated particles, 140 particles (including 6 plastic fibers)

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were confirmed as MPs by µ-Raman, compared to 114 MP particles (including 2 plastic fibers)

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identified by ATR-FTIR (n = 112) and FTIR imaging (n = 2, Figure 1 and SI Table S5 for

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individual results of Station S1–S7). In both approaches, the majority was assigned to

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polyethylene (n = 70 for µ-Raman and n = 75 for ATR-FTIR) and polypropylene (n = 29), and

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a lower number was attributed to varnish (n = 20 for µ-Raman and n = 2 for FTIR imaging),

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polystyrene (n = 5), polymethylmethacrylate (n = 2), and cellulose acetate (n = 1). In contrast

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to FTIR analysis, µ-Raman enabled identification of another four polymer types, including


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polyvinyl alcohol (n = 6), polyester (n = 5), polyacrylonitrile (n = 1), and rubber (n = 1,

200

Figure 1).

201 140

140 (70%) polyethylene

120

114 (57%)

polypropylene polypropylene fiber

Identified MPs [n]

100

varnish polyvinyl alcohol

80

polystyrene polyester

60

polyester fiber 40

polymethylmethacrylate rubber

20

polyacrylonitrile fiber cellulose acetate fiber

0

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µ-Raman

ATR-FTIR & FTIR imaging

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Figure 1. Number and chemical compositions of MPs >500 µm investigated by µ-Raman and

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FTIR analysis. Plastic fibers are marked by the striped areas. Total number of identified MPs

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and the share of identified MPs in total/ all investigated particles (n = 200) is indicated by the

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numbers on top of the bars, respectively.

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The combined results enabled identification of 148 MPs (including 6 fibers), increasing the

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individual µ-Raman and FTIR analyses by eight MP particles (6%) and 34 MP particles (30%),

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respectively. Of these, 103 particles were assigned to the same polymer type, 34 were only

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identified by µ-Raman, eight were solely detected by ATR-FTIR, and three were assigned to

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different plastic types (SI Table S6). Out of the 34 particles only identified by µ-Raman,

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18 particles were too small for ATR-FTIR analysis and were not identified by FTIR imaging.

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The same held for six plastic fibers which were too thin for ATR-FTIR and whose threeACS Paragon Plus Environment

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dimensional orientation impeded analysis by FTIR imaging. The remaining 10 MP particles

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only confirmed by µ-Raman included six varnish particles with the same bright blue color (SI

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Figure S6a–d). All 8 particles only identified by ATR-FTIR showed either blackish, brownish

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or greenish color (SI Figure S6e–h). Regarding the three particles identified as different

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polymers, two showed a bright blue and green color, respectively, which was in accordance

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with the color of the associated substance in the Raman library (SI Figure S6i–j).

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In total, 52 particles were not confirmed as MPs by µ-Raman or ATR-FTIR spectroscopy (SI

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Table S7). Of these, eight particles were assigned to cellulose (n = 1), cotton (n = 1), graphite

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(n = 2), hematite (n = 3), and blue pigment (n = 1) by µ-Raman. The other 44 particles were not

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identified by µ-Raman or ATR-FTIR spectroscopy, of which the majority were fibers (n = 30)

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and showed black color (n = 26). In terms of µ-Raman spectroscopy, 8 particles were not

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identified due to fluorescence (SI Table S7).

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In terms of ASPEx-µ-Raman analysis of 100 selected MPs >500 µm, at least 95 of the 100

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particles were identified in the fully automated procedure, and the remaining 5 particles were

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identified by repeated analysis in the verification mode (SI Table S8). The chemical

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composition, as well as the size distribution, was in agreement in each of the four automated

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runs and, thus, independent of the orientation of the sample holder (SI Figure S7). Additionally,

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these results were in complete accordance with the manual µ-Raman analysis. The work load

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and analysis time could be substantially reduced by ASPEx-µ-Raman analysis (SI Table S9).

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Raman Evaluation for MPs of 10–500 µm. To compare Raman and FTIR results, it was

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first necessary to optimize the parameters for ASPEx-µ-Raman analysis. While particles >500

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µm could be targeted with a laser intensity of 50%, this was not possible for MPs of 10–500

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µm concentrated on gold-coated filters, where laser intensities >18% led to destruction of the

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filter surface. Different Raman settings were investigated for particles of 10–500 µm with focus

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on the hit quality, the signal to noise ratio and the percentage of identified MPs as well as the

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percentage of fluorescent particles out of all analyzed particles of the filter. The best results ACS Paragon Plus Environment

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were achieved using a laser exposure time of 30 s and an intensity of 18% (Filter R4, SI Table

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S10 and Figure S8). In this context, the mean hit quality was 861 ± 81 with a signal to noise

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ratio of 44 ± 7. Additionally, MPs shared 1% of all counted particles on Filter R4 and the

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percentage of fluorescent particles was lowest, with only 0.25% (Filter R4, SI Table S10).

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Statistical evaluation revealed a significantly higher hit quality (p-value 500 µm can be automated by using ASPEx-µ-Raman with no loss in the MP

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identification rate but a substantial decrease in the labor input and analysis time. This and the

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25% higher identification rate found for µ-Raman spectroscopy in this work confirms

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ASPEx-µ-Raman as the optimal technique for reliable and time-efficient quantification of MPs

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>500 µm.

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Raman Evaluation for MPs of 10–500 µm. Compared to MPs >500 µm, which required

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previous visual sorting and careful arrangement of the particles on the sample holder, MPs

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≤500 µm were analyzed directly on the filter by ASPEx-µ-Raman. This was performed with

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low laser intensities to prevent damage to the filter material. While many studies reported

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fluorescence as a major issue regarding the identification of environmental MPs ≤500 µm by

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µ-Raman spectroscopy25, 38, 40, 52, we found a way to eradicate fluorescence almost entirely

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(500 µm

402

(Figure S10) indicates that the particle identification mechanism might count smaller particles

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in close proximity to each other as one larger one, overestimating the particle size and

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underestimating the particle number.

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Unlike the studies by Käppler et al. (2016)38 and Elert et al (2017)41, who revealed a similar

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or even identical number of different polymer types by µ-Raman and FTIR imaging,

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respectively, the number of detected polymer types was nearly two times higher for

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ASPEx-µ-Raman than for FTIR imaging in the present work. This may be ascribed to the

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clustering process of the spectral library that is inevitable for FTIR imaging53 but redundant for

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ASPEx-µ-Raman, wherein each spectrum is compared to the reference library separately in a

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fully automated procedure. Thus, ASPEx-µ-Raman demonstrates further benefits in its higher

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resolution regarding the allocation of different polymer types and in its complete automation.

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In terms of the time effort, Käppler et al. (2016)38 and Elert et al. (2017)41 revealed a more

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than one 100-times higher measurement time for µ-Raman imaging than for FTIR imaging

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followed by manual spectra evaluation, in which the work input was comparable for both

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techniques. The fact that the analysis time (including automated spectra evaluation and data

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compilation) was only six times higher for ASPEx-µ-Raman (43 h per aliquot) than FTIR

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imaging (8 h per aliquot) in this study may indicate that ASPEx-µ-Raman is more time-efficient

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compared to µ-Raman imaging.

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Concentration and Size Distribution of MPs of 10 µm – 5 mm. The concentrations of

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MPs >500 µm found by µ-Raman (0.15–2.68 particles m–3) and ATR-FTIR analysis (0–

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2.63 particles m–3) in North Sea surface waters in this work are in the range of those found by

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previous studies offshore from Ireland (0–22.5 particles m–3),54 in the Rhine River (0.3– ACS Paragon Plus Environment

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21.8 particles m–3),55 in the Mediterranean Sea (3.13 particles m–3),56 in the North Atlantic

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Ocean (0–8.5 particles m–3),57 in the western English Channel (0.27 particles m–3),58 and in

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Portuguese coastal waters (0–0.4 particles m–3).59 These studies have in common that they all

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used either µ-Raman,54 ATR-FTIR,55, 56 or µ-FTIR spectroscopy57-59 for the identification of

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visually sorted MPs (Figure 4).

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In contrast, the concentrations of MPs of 10–500 µm in North Sea surface waters found by

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ASPEx-µ-Raman in this work (38–2621 particles m–3) are far higher compared to those

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detected by FTIR imaging (22–228 particles m–3). In addition, they are far higher compared to

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those found by previous studies in the North Atlantic Ocean (13–501 particles m–3)24 and in

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Singapore’s coastal waters (100–400 particles m–3),60 which used µ-Raman24 and µ-FTIR

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spectroscopy60 to analyze MPs >10 µm directly on the filter (Figure 4). Thus, the concentration

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of MPs of 10–500 µm may have been underestimated until now. This underestimation may be

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ascribed to false-negative errors due to previous lack of automation, meaning that only particles

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visually resembling MPs underwent spectroscopic investigation. In contrast, ASPEx-µ-Raman

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involves automated spectroscopic investigation of each particle, excluding both false-positive

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and false-negative errors.

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Another reason for the previous underestimation of the concentration of MPs of 10–500 µm

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may be ascribed to the huge increase in MP concentration with decreasing particle size. This

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increase, probably ascribed to the fragmentation process of (micro)plastics in the environment,

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was first detected by Enders et al. (2015)24 and confirmed in this study by both Raman and

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FTIR analysis. Fitting the size distribution of the found MPs into a power law regression,

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Enders et al. (2015)24 achieved an empiric correlation of R2 = 0.77 and a scaling exponent of

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l = 2, with the latter corresponding to the dimensionality of the fragmentation process of

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plastics in the environment. A lower empirical correlation of R2 = 0.67 but a similar scaling

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exponent of l = 2.1 was demonstrated in this study for FTIR analysis. The lower empiric

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correlation was ascribed to the gap in the size class of 200–500 µm since particles with this size ACS Paragon Plus Environment

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were too small for ATR-FTIR analysis but were influenced by the total absorption in FTIR

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imaging. For Raman analysis, an empiric correlation close to one (R2 = 0.94) and a scaling

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exponent of l = 2.8 was found. These results may confirm the ongoing fragmentation process

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of plastics into three dimensions, as well as the reliability of ASPEx-µ-Raman for the

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quantification of MPs in all size classes.

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The increase in the MPs concentration as well as the comparably high concentration of MPs

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with sizes of 1–10 µm found in the sample for Raman evaluation is surprising, considering that

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all samples were taken with a mesh size of 100 µm. On the one hand, this could be explained

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by early clogging of the net by the sample material, holding back also particles 10 µm (lower figure) found in this study and in previous studies using comparable

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techniques. Studies that did not mention the range but indicated the mean MP concentration are

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illustrated by dashed bars.

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Future Recommendations. In this study, we demonstrate the importance of ASPEx-µ-

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Raman for future MP analysis down to 1 µm. However, further research is strongly needed to

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further improve the methodology and to validate its reliability by testing the recovery. In this

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context, environmental samples should be spiked with known MP numbers to compare the

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achieved results with the known values in terms of MP number, polymer composition and size

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distribution. Here, a special focus should be devoted on the risk of particle agglomeration,


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meaning that particles lying to close on the filter are counted as once. In this study, we have

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tried to mitigate this issue by concentrating only small aliquots on the gold-coated filter (SI

477

Paragraph 7). Nevertheless, the underestimation in the number of MPs due to particle

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agglomeration is an open question and must be investigated in further studies.

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In addition, with an analysis time of 147 h per 100 L investigated water volume for particles

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of 1–10 µm (SI Table S12), there is a compelling need to improve the time efficiency of

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ASPEx-µ-Raman. Since the analysis time was the product of the particle number and the

482

exposure time, and since a decrease in the exposure time was shown to impair MP

483

identification, future research is needed to further enhance the target/nontarget ratio. Despite

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the extensive enzymatic and oxidative purification performed in this work, most particles on

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the filter either showed uninterpretable Raman spectra (>66%), suggesting a high percentage

486

of biological material or an even higher number of MPs, or were assigned to salt precipitates

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originating from the laboratory preparation itself (>16%, SI Table S19). Thus, further research

488

is needed to improve the decomposition of the biological matrix and to reduce the formation of

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salt precipitates on the filter. The former may be achieved by an additional enzymatic treatment

490

with Proteinase-K58 and prolonged oxidative purification with hydrogen peroxide (SI Figure

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S11).

492

To prevent underestimation in the MP number13, we further recommend sampling with a pore

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size equal to the lower detection limit of 1 µm for ASPEx-µ-Raman. A highly promising

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sampling technique in this context is direct fractionated pressure filtering, which divides MPs

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into size fractions down to the lower micrometer range and enables sampled water volumes in

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the lower cubic meter range

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pressure filtering with ASPEx-µ-Raman analysis to reliably quantify MPs down to 1 µm (SI

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Figure S11). This may enable an environmental risk assessment based on data that do not

499

underestimate the concentration of the MP fraction that is most significant concerning

500

ecotoxicity.

24, 40

. Thus, future research may combine direct fractionated


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ASSOCIATED CONTENT

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Excel file “ESM_Ramandata.xlsx” with four spreadsheets:

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Spectra+ID. Raman spectra of all polymers detected by SPE-µ-Raman in Station S1–S7 with

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a hit quality >700 and a size of 10–500 µm as well as quality factors derived from comparison

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of these polymer hits to the reference spectra.

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Evaluation. Validation results of the manual reanalysis of these polymer hits (from the

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spreadsheet Spectra+ID) for different minimum hit qualities, involving the threshold shift of

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Raman spectra from a hit quality of >700 to >800.

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Final Result. Raman spectra of all MPs of 10–500 µm detected by SPE-µ-Raman in Station

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S1–S7 with a hit quality >800 which were considered in this study.

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Raman Database. Raman spectra of the self-recorded Raman library used in this study.

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World file “Supporting Information”

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Table S1: Information on the date of sampling, duration of trawl, and geographic coordinates

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and the sampled distance, area, and volume of the sample for Raman evaluation (R) and the

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samples of Stations S1–S7 collected in the southern North Sea.

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Figure S1. Scheme of sampling, laboratory preparation, and spectroscopic analysis of the

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samples for Raman evaluation (R) and Stations S1–S7.

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Paragraph S1 on Raman Analysis.

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Paragraph S2 on Spectral Libraries for Raman Analysis.

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Paragraph S3 on ATR-FTIR Analysis.

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Paragraph S4 on FTIR Imaging Analysis.

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Paragraph S5 on ASPEx-µ-Raman Analysis of MPs >500 µm.

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Figure S2. Arrangement of 100 visually sorted MP particles >500 µm on a gold-coated mirror

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and particle imaging by ASPEx-µ-Raman in the bright field mode.


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Paragraph S6 on FlowCam measurements to determine the fraction of samples collected

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on gold-coated filters.

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Table S2. Total water volume of collected plankton samples, volume after laboratory

529

preparation, surface area of all particles per prepared volume, volume of the prepared sample

530

filtered on gold-coated filters, percentage of filtered sample in total sample, corresponding

531

sampled water volume per filter, and corresponding sampled water volume that was

532

spectroscopically investigated

533

Figure S3: Geometry and percentage of filtered area and five sectors analyzed at the border

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and in the center of each gold-coated Filter R1–R4 by ASPEx-µ-Raman to investigate if

535

particles are distributed equally over the filtered area and to explore the optimal Raman

536

parameter setting.

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Table S3. Raman setting used for identification of particles of 10–500 µm on Filters R1–R4

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and particles of 1–10 µm on Filter R2 and the corresponding investigated water volumes.

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Paragraph S7 on Statistical Analysis for ASPEx-µ-Raman Evaluation.

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Table S4. One statistical model to investigate if particles are equally distributed over the

541

filtered area, and four statistical models performed to investigate the influence of the laser

542

exposure time (10 s and 30 s) and the laser intensity (11% and 18%) on MP identification and

543

fluorescence.

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Figure S4. Overlapping areas with a joint midpoint measured first by FTIR imaging in

545

reflection mode and then by SPE-µ-Raman on a gold-coated filter.

546

Equation S1 for blank correction.

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Figure S5. Overview of the different quality factors for the assignment of polystyrene reference

548

spectra to measured spectra.

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Table S5. Number of MP particles identified by µ-Raman and ATR-FTIR per station.

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Table S6. Compliances and differences between µ-Raman and FTIR analysis concerning the

551

identification of MP >500 µm. ACS Paragon Plus Environment

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Table S7. Summary of 200 visually sorted MPs investigated by µ-Raman, ATR-FTIR and

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FTIR imaging as well as combined results.

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Figure S6. Four of six blue varnish particles only identified by µ-Raman (a–d), three

555

polyethylene particles (e–g) and one polypropylene particle (h) only identified by ATR-FTIR,

556

one blue (i) and one green particle (j) identified as polyethylene by ATR-FTIR but assigned to

557

blue polypropylene reference fibers (ref. i) and light blue polyvinyl alcohol reference fibers

558

(ref. j) by µ-Raman spectroscopy.

559

Table S8. Number of MPs >500 µm identified by ASPEx-µ-Raman, with a 90° shift of the

560

sample holder after each of the four automated runs.

561

Figure S7. Chemical composition and size distribution of 100 MPs >500 µm measured by

562

ASPEx-µ-Raman in each of the four automated runs.

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Table S9. Expenditure of time and labor for manual µ-Raman and ASPEx-µ-Raman analysis

564

of 100 visually sorted MPs >500 µm.

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Table S10. Summary of totally counted particles, identified MPs, and fluorescent particles as

566

well as the command variables of the statistical evaluation for particles of 10–500 µm, analyzed

567

on Filter R1–R4.

568

Table S11. Statistical evaluation of the influence of the laser exposure time and intensity on

569

the identification of MPs and on fluorescence for particles of 10–500 µm.

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Figure S8. Raman spectra of four polyethylene particles analyzed on filters R1–R4 with

571

varying laser exposure times and intensities, respectively.

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Table S12. Number and concentration of MPs of 10–500 µm on Filter R4 and MPs of 1–10 µm

573

on Filter R2 and the derivation of the respective analysis time per 100 L investigated water

574

volume.

575

Equation S2 to derive the analysis time for ASPEx–µ-Raman.

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Figure S9. Chemical composition and size distribution of MPs of 10–500 µm and MPs of 1–

577

10 µm detected by ASPEx-µ-Raman on Filters R4 and R2, respectively. ACS Paragon Plus Environment

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Table S13. Validation results of a manual reanalysis of all polymer hits identified by ASPEx-

579

µ-Raman in Stations S1–S7.

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Table S14. Number of all counted particles and MP particles detected by ASPEx-µ-Raman and

581

µ-FTIR imaging per aliquot of Station S1–S7 and in the blank control sample.

582

Table S15. Compliances and differences in ASPEx-µ-Raman and FTIR imaging analysis for

583

the identification of MP of 10–500 µm in aliquots of Station S1–S7.

584

Figure S10. Size distribution of MPs of 10–500 µm detected by ASPEx-µ-Raman and FTIR

585

imaging in aliquots of Stations S1–S7.

586

Table S16. Expenditure of time and labor for ASPEx-µ-Raman and FTIR imaging to analyze

587

one aliquot of Stations S1–S7.

588

Table S17. Conclusions concerning the analysis of MP >500 µm from Stations S1–S7 by

589

µ-Raman and ATR-FTIR.

590

Table S18. Conclusions regarding the analysis of MP of 10–500 µm from Stations S1–S7 by

591

ASPEx-µ-Raman and FTIR imaging.

592

Table S19. Composition of all particles of 10–500 µm and 1–10 µm analyzed on Filters R4 and

593

R2 by ASPEx-µ-Raman, respectively.

594

Figure S11. Suggested scheme for the reliable quantification of MPs down to 1 µm by ASPEx-

595

µ-Raman with adjusted sampling and enhanced purification.

596 597

AUTHOR INFORMATION

598

Corresponding Author

599

*E-mail: [email protected], Phone: +41 44 632 03 38

600

Present Addresses

601

†Department of Civil, Environmental and Geomatic Engineering, Institute of Environmental

602

Engineering and Institute of Science, Technology and Policy, Swiss Federal Institute of

603

Technology, ETH Zurich, Universitätsstrasse 41, 8092 Zurich, Switzerland ACS Paragon Plus Environment

26

Page 27 of 32


604

Author Contributions

605

The manuscript was written by Livia Cabernard and completed by Sebastian Primpke, Lisa

606

Roscher, Claudia Lorenz, and Gunnar Gerdts. Laboratory preparation and visual sorting were

607

conducted by Lisa Roscher with the support of Claudia Lorenz. Raman and FTIR analysis were

608

performed by Livia Cabernard and Lisa Roscher with the support of Sebastian Primpke. Data

609

analysis for FTIR imaging and the manual reanalysis of SPE-µ-Raman Data of MPs of 10–

610

500 µm with threshold shift was performed by Sebastian Primpke. All authors have given

611

approval to the final version of the manuscript.

612

Funding Sources

613

Claudia Lorenz was financed by a Ph.D. scholarship of the Deutsche Bundesstiftung Umwelt

614

(DBU). The Single Particle Explorer for Life & Science was funded by the German Federal

615

Ministry of Education and Research (FKZ Project of JPI O BASEMAN - Defining the baselines

616

and standards for microplastics analyses in European waters, BMBF grant 03F0734A).

617

Notes

618

The authors declare no competing financial interest.

619 620


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Page 28 of 32

621

ACKNOWLEDGMENT

622

We thank the team of the Heincke cruise HE430 for collection of the surface water sample in

623

the southern North Sea in 2014 and the German Federal Ministry of Education and Research

624

for the funding (FKZ Project of JPI O BASEMAN - Defining the baselines and standards for

625

microplastics analyses in European waters, BMBF grant 03F0734A). Further thanks to Prof.

626

Dr. Gerhard Furrer from ETH Zurich for supervision of the master thesis of Livia Cabernard.

627 628 629

ABBREVIATIONS

630

MPs, microplastics; R, sample for Raman evaluation; FTIR, Fourier transform infrared

631

spectroscopy; ASPEx-µ-Raman, single-particle exploration coupled to µ-Raman spectroscopy;

632

ATR-FTIR, attenuated total reflectance Fourier transform infrared spectroscopy; FPA, focal

633

plane array;

634 635 636

REFERENCES

637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653

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SYNOPSIS

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Comparison of Raman and Fourier Transform Infrared Spectroscopy

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