A Quantitative Comparison of Methods Measuring Fluoride in Solutions or in Enamel

Fluoride in solution is most usually measured with the ion-specific electrode, although a few other methods are available. The electrode can be used in a straightforward manner, its lower concentration limit may be extended by special methods such as the standard addition of fluoride, or microdiffusion methods may be used to extract fluoride from the sample and concentrate it in base. Specifications of all methods are tabulated. A choice of method depends on the requirements in terms of specifications but also on the personal experience of the investigator and the required robustness. Microdiffusion of some kind is preferable for concentrations below 5 mumol/L or when electrode-interfering molecules such as proteins are present in rather high concentrations. Fluoride in enamel may be determined by removal of layers by etching, abrasion, or microdrilling, followed by dissolution of the layer removed. Specifications of depth and area resolution are tabulated. Alternatively, probing methods of a physical nature can be used. These methods require sophisticated instruments that are not widely available. Some of them are destructive, others are not. All but one require exposure to vacuum. However, most of them combine a very good sensitivity with a very good resolution in depth and/or measured area. Therefore, their use in research (requiring cooperation with instrument specialists/owners) may well be worth the effort.