EVALUATION OF ASHING PROCEDURES FOR THE GAS-CHROMATOGRAPHIC DETERMINATION OF FLUORIDE IN BIOLOGICAL-MATERIAL

Fluoride in acidic aqueous solution was extracted with chlorodimethylp henylsilane in cyclohexane and converted to fluorodimethylphenylsilane , which was quantified by gas-liquid chromatography (GC) with flame io nization detection. The potential of GC, combined with different ashin g procedures, including high pressure closed vessel acid digestion, to determine trace amounts of fluoride was evaluated with different biol ogical standard reference materials. Major constituent elements such a s Ca, Mg, Al and Si were also measured, but no correlation was apparen t as to their effect on fluoride yields. Cluster analysis of fluoride yields with the various ashing methods revealed a difference between p lant- and non-plant materials. Using alkaline fusion (KOH) for plant m aterials and a retaining agent (CaO) for non-plant materials good agre ement with either certified or published values was obtained. The comb ination of both ashing aids gave satisfactory results regardless of th e sample type. Application of this ashing method to Swiss total diet s amples from the year 1983 and comparison with an ion-selective procedu re in another laboratory yielded consistent results and identical mean values of 1.9 mu g F g(-1) (dry matter), corresponding to a daily die tary intake of 0.9 mg F per adult.