###### Abstract  
Fluoride ions are the major constituents of dental products because they prevent cavities through bacterial growth inhibition. However, excessive consumption of fluoride ions results in fluorosis, thereby causing tooth staining and roughness. Therefore, there is a crucial need to develop rapid and effective methods for monitoring fluoride levels in dental products. The current study describes a greener water-based microwave-assisted digestion (WB-MAD) prior to fluoride-ion-selective electrode (F-ISE) measurement for the determination of fluoride ions in various toothpaste products. The optimum conditions of the developed WB-MAD method were 180 \({}^{\circ}\)C digestion temperature, 60 min digestion time, 0.05 g toothpaste amount and 10 mL distilled water. Under the optimum conditions, the method detection limit (MDL) of 0.00302 \(\mu\)g/kg and the method quantification limit (MQL) of 0.01007 \(\mu\)g/kg obtained were favorably comparable with the literature reports. The proposed WB-MAD method was both accurate (99.2 to 101%) and precise (\(\leq\)0.75%) for the quantitative determination of F\({}^{-}\) in toothpaste samples using F-ISE. Furthermore, the newly developed WB-MAD method showed better accuracy (97-100%) than the traditional microwave-assisted acid digestion methods (71-92%). It is worth indicating that since water was used as the only digestion reagent, it was possible to validate the F-ISE results with ion chromatography (IC). The percentage recoveries obtained from IC (91-104%) and F-ISE (93-100%) were statistically insignificant. In view of the validation data, the proposed WB-MAD method can be considered as an alternative to the conventional microwave-assisted acid digestion (MAAD) methods for the determination of F\({}^{-}\) in toothpaste samples containing sodium monofluorphosphate. water-based microwave-assisted digestion; fluoride-ion-selective electrode; method detection limit; method quantification limit; ion chromatography; toothpaste samples +
Footnote †: journal: Applied Sciences  
Various analytical techniques such as laser-induced breakdown spectroscopy (LIBS) [8; 9], X-ray fluorescence spectrometry (XRF) [10], electrothermal vaporization coupled to inductively coupled plasma optical emission spectrometry (ETV-ICP-OES) [11] and high-resolution graphite furnace molecular absorption spectrometry (HR-GF-MAS) [12] have been used for the direct determination of F\({}^{-}\) in different solid sample matrices. Even though direct analytical techniques are non-destructive, sensitive and rapid, they have limitations such as the need for sophisticated calibration curves, especially in the absence of matrixed-matched certified reference materials (CRMs), and evenly homogenous samples to obtain acceptably accurate and reproducible results, and they are also costly, which is a serious drawback for routine analysis in developing countries like South Africa [13]. To resolve some of the disadvantages of the direct analytical methods, researchers have made use of indirect analytical techniques such as fluoride-ion-selective electrode (F-ISE) measurement [14], gas chromatography (GC-MS) [15] and ion chromatography [16; 17; 18]. However, the indirect analytical techniques are only compatible with aqueous solutions; therefore, there is the requirement of a sample preparation step to convert solid matrices into solutions prior to analysis [8]. The most documented sample preparation methods for halogen determination in solid matrices are acid digestion [19] pyrohydrolysis [20], combustion in bombs [21] and microwave-induced combustion [22]. Classical combustion and wet digestion methods use different acidic reagents for digestion, which result in loss of F\({}^{-}\) as volatile HF gas, and corrosive acidic matrices can cause interferences and damage nebulization systems and chromatographic columns. The nitrous oxides normally produced during acid digestion when using HNO\({}_{3}\) as a reagent are carcinogenic [8; 22].