Determination of Subnanogram Amounts of Fluoride with the Fluoride Electrode

An extremely sensitive method of determining fluoride with the Orion electrode is described having an absolute detection limit of 10 pg (10⁻¹¹ g) of F⁻. One-microliter volumes of sample solution are confined as thin layers between the fluoride- ion sensing element of a standard Orion electrode and the flat sleeve-junction of a calomel reference electrode mounted immediately below it. One hundred picograms (10⁻¹⁰ g) of F⁻ have thus been determined with a precision (relative standard deviation) of 5%: the precision Improving to 2.8% at the 1-ng level. Sample preparation procedures and a fluoride microdiffusion technique of great sensitivity and range (<1 ng to 100 ng F⁻) are also described. These techniques, while developed primarily for the mineralized tissues, can be used to analyze many other fluoride-containing materials. Similar electrode assemblies, used to determine ultramicroamounts of chloride ion (1 ng) and hydrogen ion are discussed.