For weighing, either a Radwag balance AS 220.R2 (readability limit: 0.1 mg) or a Kern balance ABT 220-5DM (readability limit: 0.1 mg/0.01mg) was used. FT-IR spectra of the toothpastes’ solid contents and their ash were measured as KBr pellets with ThermoNicolet Nexus 670 and Perkin Elmer Spectrum Two FT-IR spectrometers.

To 7 - 10 g of toothpaste was added 140 - 200 mL water (at exactly 1.0 g toothpaste/10.0 mL H₂O) and the resulting suspension was stirred at 50˚C (WiseStir, Wisd Laboratory Instruments) until it was homogeneous. Then, it was filtered through a paper filter (Filter-Lab^®, 1300/80, pore size 43 - 48 μm). The filtrates were refiltered until they looked transparent. The filter cake was washed with water (50 mL), dried at 37˚C for 14 h (Ecocell MMM, Medcenter Einrichtungen GmbH) and weighed. The experiments were carried out in triplicate. An FT-IR spectrum was taken of the respective solid. Should the solid content contain microbeads, these were separated manually from the remainder of the solid with the help of either a needle or a set of tweezers. The microbeads thus collected were analyzed and photographed under a stereoscope (model SZ2-ILST). ImageJ software was used to evaluate the microbeads for size and shape [34] [35], including the Feret’s diameter. The Feret’s diameter was used to represent the size of plastic particles. The Feret’s diameter corresponds to the longest distance between any two points along the particle boundary [36].

The colorants except for Fe₂O₃, which could be isolated, and phthalocyanine green, the presence of which can be identified by the persistence of color under thermolytic conditions, were directly obtained from the product labels.

500 - 900 mg samples of toothpaste solids were heated in a crucible (79C-00, Waldenwanger, Berlin) at 600˚C for 2 h (Carbolite electric oven ELE 11/6), during which all adhering organic components were combusted. The thermolysis experiments were carried out in triplicate for the solid of each toothpaste. After cooling, the ash content was weighed and subjected to FT-IR spectroscopic analysis. Thereafter, the ash contents of selected toothpastes were examined with energy-dispersive-X-ray spectroscopy coupled with scanning electron microscopy (SEM-EDS) as well as with X-ray fluorescence spectroscopy (XRF). The alkalinity content of the ash was measured (as w% CaCO₃) by reacting a precisely determined amount of the ash (50 - 100 mg) with aq. HCl (0.1 M, 20 mL) at 60˚C for 15 min. and subsequently back-titrating the reaction mixture with aq. NaOH (0.1 M) using phenolphthalein as indicator [37]. The titrations were carried out in triplicate.

Sample preparation—The ash sample obtained after thermolysis was ground to a fine powder. The resulting fine powder was pressed in a 13 mm bore steel die in a manually operated hydraulic press (Specac). The pressure was applied until the reading was stable at 10 tons and left for the 40s. This produced mechanically stable round pellets of 13 mm diameter. The pellets were generally analyzed within an hour and great care was taken that the two flat surfaces intended for XRF analysis were not touched. The weight and exact diameter of the pellet were measured and used for the semi-quantitative X-Ray analysis.

XRF Analysis—The XRF analysis was done on a wavelength dispersive (WD) XRF spectrometer (Rigaku ZSX Primus IV) equipped with an Rh X-ray tube. The instrument is controlled by ZSX Guidance software intended for the analysis of approximately 70 elements from F to U. The resulting pellet was placed in a sample holder cup with the aid of 10 µm polypropylene film which had a high X-ray transmission rate and low level of impurities. All samples were arranged on a sequential basis controlled by an automated autosampler system. The spectra were processed with a semi-quantitative SQX software package, capable of automatically correcting all matrix effects, including line overlaps. SQX also corrected for secondary excitation effect by photoelectrons (light and ultra-light elements), varying atmospheres, impurities, and different sample sizes. Finally, the spectra of each sample were matched with a library and Perfect Scan Analysis Programs [38].

The microstructural features of the toothpaste samples were obtained using a JEOL Analytical Scanning Electron Microscope (Model: JSM-6010PLUS/LA, Tokyo, Japan) equipped with a secondary electron imaging mode. The samples were mounted on brass stubs using double-sided adhesive carbon tape as a conductor path and were sputter-coated with gold up to 15 nm thickness using a Cressington 108 auto sputter coater and thickness controller MTM-20. Fields of the sample were inspected under a high-vacuum (ULVAC KIKO lnc, Model: G-100DB, Miyazaki, Japan) and micrographs of the sample were recorded using InTouch Scope JSM software using a power of 20 kV. The elemental composition was examined by JEOL-SEM equipped with an energy dispersive X-ray detector (EDS).

Most microplastics, especially those consisting of polythene, have a lower density than water, i.e., less than 1.0 g/mL. Following the method of Ustabasi and Baysal [30], the filtered and dried solids of the respective toothpastes were stirred rapidly in water (1.0 g solid/100 mL H₂O) at rt for 5 min. Then, the solids were allowed to settle for 2 h. Thereafter, the surface of the water was inspected visually for any floating solids. Any floating solids were skimmed off with a spatula spoon and refloated in deionized water (150 mL) in a separate beaker to remove any water-soluble material and any adhering particles. The refloated particles were transferred to a weighing paper and dried at 37˚C for 10 h (Ecocell MMM, Medcenter Einrichtungen GmbH). Thereafter, the solids were weighed [Kern balance ABT 220-5DM (readability limit: 0.1 mg/0.01mg)], studied under a stereoscope (model SZ2-ILST) in regard to their apparent morphology and their size, and submitted to FT-IR spectroscopy (Perkin Elmer Spectrum Two). In case of an observed presence of microplastic, the material was subjected to DSC (differential scanning calorimetry) analysis (see below).

The thermal response of the isolated microplastic from toothpaste (H-1) was measured using a differential scanning calorimeter (Shimadzu DSC-60 Plus, Japan). About 7 mg of powdered sample was precisely weighed into a DSC sample pan, which was sealed with a top lid utilizing a pellet press. The sample was measured in the temperature range of 25˚C - 300˚C at a heating rate of 10˚C/min under a constant flow of N₂ (50 mL/min). Data analysis was performed using the LabSolutions TA software [39].

The examined toothpastes exhibited a solid content of 11.18 ± 0.12 w% to 48.55 ± 0.28 w%, as shown in Table 1. Previously collected data shows this to be in the normal range of abrasive content (8 w% - 50 w%) in toothpastes [40] [41]. As

major abrasive components, the toothpastes for the most part had either silica or calcium carbonate, whereas toothpaste brands typically offered at least one toothpaste of each version. Those toothpastes with silica as the major ingredient have little alkalinity. Table 1 shows the alkalinity values obtained through titration expressed in w% CaCO₃. Numbers of CaCO₃ exceeding 100 w% of the ash, as found in T3, T10, T13-T14, and T18 indicate that these toothpastes have in addition to CaCO₃ other basic salts.

Also, the ash content of selected toothpastes was used the determination of the elemental composition of the solids using the WD XRF analysis method. All the results are expressed in mass% and calculated based on the weight of ash obtained after pyrolysis. The multi-element analysis report for each sample is shown in Table 2.

*The numbers of selected ash content of toothpaste.

As mentioned above, most of the samples contain silica and calcium carbonate/calcium hydrogen phosphate as major components, even though samples T16 and T27 carry alumina/aluminum hydroxide as the major component. All of these compounds mainly act as abrasives and are usually used as major components in formulations of toothpastes [42]. The presence of calcium, silica and alumina is identified and quantified by characteristic elemental spectra of each element (calcium, silicon, and aluminum) as presented in Figure 1. The spectrum for each element is the same for each toothpaste. Solely, the intensity varies (expressed in Kcps as shown in Table 2) based on the concentration of the element. A small peak of the Kα satellite is characteristic of silica and alumina.

The other minor elements in the ash observed were titanium, strontium, iron, zinc, magnesium, potassium, phosphorus, and sulfur. The X-fluorescence spectra for magnesium, sulfur, phosphorous, potassium and sodium are presented in Figure 2 (taken from sample T26), the spectra for strontium and iron are presented in Figure 3 (for sample T26) and the spectra for zinc and titanium are presented in Figure 4 (for sample T28). Titanium dioxide (TiO₂) is used as an opacifying agent in toothpastes and was found in T10, T27 and T28 in quantities of <5.0 w%. The European Chemicals Agency (Echa) considers that TiO₂ may cause cancer if inhaled. In 2022, the EU classified the substance as a suspected carcinogen by inhalation in certain powder forms. Thus, titanium dioxide (as E171) is no longer considered safe, when used as a food additive [43].

Also, ash samples of selected toothpastes were analyzed with energy-dispersive- X-ray spectroscopy coupled with scanning electron microscopy (SEM-EDS) (Figure 5 and Figure 6). As expected silicon and calcium were found to be the most abundant elements, attributable to SiO₂ and CaCO₃. Mg is attributed to CaMg(CO₃)₂ or MgCO₃, Ti to TiO₂, Fe mostly to Fe₂O₃. P we believe to be attributed to Ca₃(PO₄)₂ as sodium monofluorophosphate (Na₂PO₃F) is water-soluble.

Floating experiments were conducted with all 34 toothpastes (Table 1). Only

four toothpastes (T15, T18, T22, and H1) exhibited solid content when floated on the water. Two others (T7 and T13) had floating solid content that settled after several hours. The floating material of T15, T18, and T22 was filtered off and analyzed by IR spectroscopy to be inorganic components such as mica and small crystallites of CaCO₃. Only three toothpastes (T5, T8, and T11) bought in UAE markets and the toothpaste (H1) purchased online were found to have microbeads. They could be isolated by direct filtration of the aqueous mixture of the toothpaste and subsequent manual separation (see 2.2., above). Of these, T5 was found to have red and blue beads, composed of silica and microcrystalline cellulose, respectively, T8 possessed blue-green beads made of silica and T11 blue beads, again made of silica. These microbeads were studied under the stereoscope. The micrographs were evaluated to determine the size distribution of the beads. Typically, for T8 the sizes of the microbeads were in the range of 20 - 70 μm (Figure 7).

Only H1 was found to have plastic beads, namely green and white microparticles made from polythene. Polythene was unequivocally identified by IR spectroscopy—2919 (CH₂ asymmetric stretching vibration), 2851 (CH₂ symmetric stretching), 1473 and 1463 (bending deformation), 1366 and 1351 (wagging deformation), 1306 (twisting deformation), 1176 (wagging deformation), and 731 - 720 cm⁻¹ (rocking deformation) [44]. Also, differential scanning calorimetry (DSC, Figure 8) with a peak at 125˚C showed the typical thermal behavior of polythene [45]. The polythene microbeads made up 1.31 ± 0.39 w% of the filterable solid content of H1.

The indication that microplastic containing toothpastes are no longer available directly from UAE markets contrasts with recent studies from Turkey [30] and India [33] that seem to suggest that microplastic containing toothpastes can still be bought in these countries. Also in China microplastics have still been found in toothpastes [46]. Nevertheless, the current study is an indicator that the ban on microplastic content in rinse-off cosmetics, including toothpastes, in an ever increasing number of countries has a beneficial effect also on products that are sold in regions where bans have not yet taken hold.

21 out of the 31 toothpastes (67.7%) bought in UAE markets were significantly colored (Figure 9). 10 out of the 31 toothpastes (32.3%) contained titanium dioxide (TiO₂) as a white pigment. As colors green (11 toothpastes, 35.5%) and blue (7 toothpastes, 22.6%) optically featured the most often. While clearly not all toothpastes list dye or pigment contents, phthalocyanine green (CI 72460, 4 toothpastes, 12.9%), chlorophylline (CI 75810, 1 toothpaste, 3.2%), and fast green FCF (CI 42053, 1 toothpaste, 3.2%) are green colorants used, brilliant blue FCF (E 133, CI 42090, also known as blue lake 1, 5 toothpastes, 16.1%), phthalocyanine blue (pigment blue, CI 74160, 4 toothpastes, 12.9%), and acid blue (CI 42051, 1 toothpaste, 3.2%) are the blue colorants used. Brilliant blue FCF (E 133) has been banned as a food colorant in several European countries, including France and Germany. Iron oxide (CI 77491, 4 toothpastes, 12.9%) and CI 12490 (pigment red 5, N-(5-chloro-2,4-dimethoxyphenyl)-4-[[5-[(diethylamino)sulphonyl]-2-methoxyphenyl]azo]-3-hydroxynaphthalene-2-carboxamide, 2 toothpastes, 6.5%) are used as red ochre and bright-red colorants, respectively. Finally, tartarzine (E 102, CI 19140, 3 toothpastes, 9.7%), quinoline yellow (quinoline WS, CI 47005, 1 toothpaste, 3.2%), and yellow iron oxide (CI 77492, 1 toothpaste, 3.2%) represent the yellow colorants used. Tartarzine (E 102) is banned as a food colorant in Norway and Austria.

The two Syrian bought toothpastes carried TiO₂ (for S1) and quinoline yellow WS (E104, CI 47005) and brilliant blue CFC (for S2, E133, CI 42090). Finally, also the toothpaste that was bought online, having an expiry date of 2016, used brilliant blue FCF (E133, CI 42090) and quinoline yellow WS (E104, CI 47005). Quinoline is deemed not to present any health risk, and is even permitted as a colorant in beverages in the European Community and in Australia. The toothpaste also included iron oxide.