## 2 Materials and Methods  
Samples were collected from different bottled drinking waters between April and June 2022. The selection criteria included: plain spring/mineral water, packaged in plastic or glass bottles up to 2 L in size, and purchased before the expiration date indicated on the label. We also made sure to include samples from the same brand but sourced from different springs of origin. The bottled waters were obtained from major supermarkets and grocery stores in Romania.  
In order to perform F content readings, the combination ISE was used (HI-4110 F Combination ISE Electrode, Hanna Instruments, Woonsocket, RI, USA). The manufacturer's instructions were followed, and direct calibration and measurement technique was utilised [22]. The HI-4110 is a potentiometric device, specifically designed for the rapid determination of free F ions in various samples, including water, soft drinks, wine, emulsified foods, and plating and pickling acids. The electrode functions as a sensor or ionic conductor. Furthermore, the HI-4110 has a reference electrode incorporated and requires the use of an electrolyte fill solution of 1.7 M KNO\({}_{3}\) + 0.7 M KCl (HI-7075 Electrolyte Solution, Hanna Instruments, Woonsocket, RI, USA). This F ISE HI-4110 was connected to a research-grade benchtop ISE meter (HI 5222-02 pH/mV/ISE/Temperature Bench Meter, Hanna Instruments, Woonsocket, RI, USA), which allowed us to measure F directly in mg/L. The ISE HI 4110 meter determines the concentration of the sample by a direct reading after calibrating the meter with standards [23]. Usually, the meter is calibrated with two or more freshly made standards if the measurement range of the samples is linear. If the measurement range is non-linear, additional calibration standards are required. After calibration, the samples are read directly. Thus, we performed a five-point calibration of the HI 5222-02 meter. F calibration standards ranging in concentration from 0 to 1.0 mg/L F were prepared using 10 mg/L F standard with incorporated total ionic strength adjustment buffer (TISAB) II (HI 4010-10 10 mg/L F Standard with TISAB II, Hanna Instruments, Woonsocket, RI, USA) and double distilled water (Chemical Company, Iasi, Romania). The F concentration range of the calibration standards was chosen based on the estimative F concentration of the samples. TISAB II is formulated for water sample analysis to provide samples and standards with a constant ionic strength and pH background, which stabilises the solutions' activity coefficient and allows the concentration to be measured directly. It also enables the measurement of the total real F concentration by de-complexing metal (i.e., Al+3, Fe+3)-F complexes that could be found in the sample.  
Plastic beakers were used for both the calibration standards and the sample solutions. The solutions were stirred at the same rate using a magnetic stirrer (HI 180, Hanna Instruments, Woonsocket, RI, USA) and identical-sized TFE-coated stir bars. The purchased samples were kept in their original sealed containers at room temperature until examination. Prior to the direct measurement technique of the F concentration, the samples were prepared by adding TISAB II solution (HI-4010-00, Hanna Instruments, Woonsocket, RI, USA) in a 1:1 ratio.  
The samples were analysed twice, and the intraclass correlation coefficient (ICC) was used to measure the reliability of the obtained values [24]. A paired samples \(t\)-test was used to evaluate the differences between the obtained and labelled F values. A \(p\)-valueof \(<\)0.05 was considered statistically significant. The data were analysed using SPSS (IBM SPSS Statistics for Windows, Version 26.0. Armonk, NY, USA: IBM Corp; 2019). The design of the study was approved by the Research Ethics Committee of the \"[PERSON]\" University of Medicine and Pharmacy Iasi, Romania.