Analysis of microplastics in drinking water and other clean water samples with micro-Raman and micro-infrared spectroscopy: minimum requirements and best practice guidelines.

Microplastics are a widespread contaminant found not only in various natural habitats but also in drinking waters. With spectroscopic methods, the polymer type, number, size, and size distribution as well as the shape of microplastic particles in waters can be determined, which is of great relevance to toxicological studies. Methods used in studies so far show a huge diversity regarding experimental setups and often a lack of certain quality assurance aspects. To overcome these problems, this critical review and consensus paper of 12 European analytical laboratories and institutions, dealing with microplastic particle identification and quantification with spectroscopic methods, gives guidance toward harmonized microplastic particle analysis in clean waters. The aims of this paper are to (i) improve the reliability of microplastic analysis, (ii) facilitate and improve the planning of sample preparation and microplastic detection, and (iii) provide a better understanding regarding the evaluation of already existing studies. With these aims, we hope to make an important step toward harmonization of microplastic particle analysis in clean water samples and, thus, allow the comparability of results obtained in different studies by using similar or harmonized methods. Clean water samples, for the purpose of this paper, are considered to comprise all water samples with low matrix content, in particular drinking, tap, and bottled water, but also other water types such as clean freshwater.

Can the presence of additives result in false positive errors for microplastics in infant feeding bottles?

In recent years, it has been shown that food contact materials can be a potential source of microplastics (MP). Recently, it was reported that more than 16 million polypropylene (PP) particles L-1 may be released from infant feeding bottles (IFBs) made of PP. In the present study seven different IFBs were investigated by the same method used in the aforementioned publication. In our tests, however, only one IFB showed a level of MP above the limit of detection. More importantly, the MP detected were not of the same material as the bottle and are more likely the result of contamination. In addition, there was a notable difference in released MP particles when the water simulant was filtered for µ-Raman spectroscopy at hot temperature (70°C) instead of filtering it after cooling down to room temperature. Thermal desorption gas chromatography mass spectrometry showed that these differences may be the result of migration and precipitation of additives such as fatty acid esters, often used as release agents in bottle production. These observations, that migrating additives could result in false positive errors for MP, indicate the need for critical consideration when polymers have been subjected to heat.

Determination of particle abrasion through milling with five different salt grinders - a preliminary study by micro-Raman spectroscopy with efforts towards improved quality control of the analytical methods.

Latest findings suggest that packaging and processing of food may be a contamination source of microplastics. In this study particle abrasion from five different salt mills with grinding burrs made of plastic and ceramic were investigated using micro-Raman (µ-Raman) spectroscopy. The mills were filled with a reference salt and an amount of 0.1 g was milled into a beaker, dissolved, filtered and the residues analysed via µ-Raman spectroscopy in different size classes, beginning with ≥ 1 µm. In the unground reference salt itself 423 ± 161 microplastic particles per 0.1 g were found, mainly consisting of polyethylene terephthalate (PET). These contents were not subtracted from the samples. One of the ceramic grinders also exclusively released PET-particles (527 ± 265 per 0.1 g).The other had compartments of polystyrene (PS) in the milling system and PS particles were found in the ground salt (201 ± 37 PET; 727 ± 226 PS/). Two of the plastic grinders had burrs made of polyoxymethylene (POM) and one with burrs of poly(methyl methacrylate) (PMMA). There were large amounts of 7628 ± 2655 and 5048 + 594 POM particles in 0.1 g salt in the two POM grinders as well as 265 ± 182 and 1546 ± 884 PET particles, respectively; salt from the PMMA grinder had 240 ± 41 PMMA particles/0.1 g and 1643 ± 1174 PET particles. The majority of the PET particles were smaller than 5 µm, whereas the POM, PS and PMMA particles had average sizes greater than 10 µm. In addition to possible microplastic contamination from salt itself, salt mills with grinding burrs made of plastic can emit microplastic particles in large amounts, especially if the burrs are made of POM. Other particles may, however, also be emitted from the grinders, e.g. ceramic particles in a similar size class, leading to the question whether and which kind of particles may be of toxicological concern for humans.

Comment on "exposure to microplastics (<10 µm) associated to plastic bottles mineral water consumption: The first quantitative study by Zuccarello et al. [Water Research 157 (2019) 365-371]".

Microplastics in food is a relatively new research field with only few studies available so far. Scientists have been pointing out that some of these studies apply questionable analytical methods. Nevertheless, media often use such results to gain attention of the readers. It is therefore of particular significance, that only those scientific studies are published, clearly presenting valid data on the content of microplastics in food. Unfortunately, the study by Zuccarello et al. shows very critical aspects regarding analytical methods used and conclusions made. The applied procedure is not described and, therefore, does not allow any assessment by other groups, which is indispensable prerequisite of any scientific publication. Moreover, the analytical method used for the identification and quantification of microplastic particles - SEM-EDX - is not sound and not validated. Therefore, in our opinion the results on the contamination of bottled mineral water with microplastics published by Zuccarello et al. are more than questionable.

Analysis of microplastics in water by micro-Raman spectroscopy: Release of plastic particles from different packaging into mineral water.

Microplastics are anthropogenic contaminants which have been found in oceans, lakes and rivers. Investigations focusing on drinking water are rare and studies have mainly been using micro-Fourier Transform Infrared Spectroscopy (µ-FT-IR). A major limitation of this technique is its inability to detect particles smaller than 20 µm. However, micro-Raman spectroscopy is capable of detecting even smaller particle sizes. Therefore, we show that this technique, which was used in this study, is particularly useful in detecting microplastics in drinking water where particle sizes are in the low micrometer range. In our study, we compared the results from drinking water distributed in plastic bottles, glass bottles and beverage cartons. We tested the microplastic content of water from 22 different returnable and single-use plastic bottles, 3 beverage cartons and 9 glass bottles obtained from grocery stores in Germany. Small (-50-500 µm) and very small (1-50 µm) microplastic fragments were found in every type of water. Interestingly, almost 80% of all microplastic particles found had a particle size between 5 and 20 µm and were therefore not detectable by the analytical techniques used in previous studies. The average microplastics content was 118 ± 88 particles/l in returnable, but only 14 ± 14 particles/l in single-use plastic bottles. The microplastics content in the beverage cartons was only 11 ± 8 particles/l. Contrary to our assumptions we found high amounts of plastic particles in some of the glass bottled waters (range 0-253 particles/l, mean 50 ± 52 particles/l). A statistically significant difference from the blank value (14 ± 13) to the investigated packaging types could only be shown comparing to the returnable bottles (p < 0.05). Most of the particles in water from returnable plastic bottles were identified as consisting of polyester (primary polyethylene terephthalate PET, 84%) and polypropylene (PP; 7%). This is not surprising since the bottles are made of PET and the caps are made of PP. In water from single-use plastic bottles only a few micro-PET-particles have been found. In the water from beverage cartons and also from glass bottles, microplastic particles other than PET were found, for example polyethylene or polyolefins. This can be explained by the fact that beverage cartons are coated with polyethylene foils and caps are treated with lubricants. Therefore, these findings indicate that the packaging itself may release microparticles. The main fraction of the microplastic particles identified are of very small size with dimensions less than 20 µm, which is not detectable with the µ-FT-IR technique used in previous studies.