A New Microwave Acid Digestion Bomb Method for the Determination of Total Fluorine

Abstract

A new microwave acid digestion method for total fluorine analysis was compared to the reliable reverse-extraction technique. The commercially available Parr bombs which are compatible with microwave heating were modified for this purpose. The Mann-Whitney statistical test did not show any significant differences (p > 0.05) in the determinations of total fluorine in various samples between the two above-mentioned methods. The microwave method also gave high fluorine recoveries (> 97%) when fluoride was added to different samples. The great advantage of the microwave acid digestion bomb method is that the digestion under pressure is so aggressive that only a few minutes is needed for complete digestion (also of covalently bonded fluorine), which reduces the time for fluorine analysis dramatically, while no loss of fluorine or contamination from extraneous sources could take place during the ashing procedure. The digestion solution was made up of 300 μl of concentrated nitric acid plus 537 μl of water. After digestion 675 μl of ∼ 8.5 M sodium hydroxide plus 643 μl of citrate/TISAB buffer was added resulting in an alkaline solution (pH ∼ 12) which was finally adjusted to a pH of ∼ 5.3 for fluoride determination.

Quantification of Fluoride in Food by Microwave Acid Digestion and Fluoride Ion-Selective Electrode

“…The use of closed systems in microwaves for the analysis of fluoride was not described until 1998, by Grobler and Louw, who used Parr bombs to obtain excellent recoveries in various samples, including food. 22 This use may be considered a late arrival in comparison with the extensive application of microwave digestion since the 1980s for the quantification of other trace elements and macronutrients. 26 At present, laboratories are still optimizing methods employing dry ashing or isolation by facilitated diffusion for the determination of fluoride in food.…”

“…Those authors attributed the improvement to the complete solubilization of the fluoride in the sample and the elimination of losses through volatilization. The use of closed systems in microwaves for the analysis of fluoride was not described until 1998, by Grobler and Louw, who used Parr bombs to obtain excellent recoveries in various samples, including food . This use may be considered a late arrival in comparison with the extensive application of microwave digestion since the 1980s for the quantification of other trace elements and macronutrients .…”

“…In the literature there are descriptions of the use of nitric acid for digestion in open systems with heating to 150 °C12 and in closed vessels with heating by microwave oven. 10,22 There are many methods for quantifying fluoride after solubilization, 23 although direct potentiometry with fluoride selective electrode is the method of choice for the determination of fluoride in foods. [9][10][11]14 It has advantages because of its low cost, satisfactory sensitivity and selectivity.…”

“…The alternative of using wet digestion to mineralize the organic matter has scarcely been applied for the analysis of fluoride in food. In the literature there are descriptions of the use of nitric acid for digestion in open systems with heating to 150 °C and in closed vessels with heating by microwave oven. , …”

Fluoride in Human Health and Nutrition

“…[129][130][131] Commonly used procedures nowadays for total sample decomposition involve oxygen bomb combustion, 132,133 open ashing 134,135 fusion with alkali metal hydroxides or carbonates 136,137 and microwave acid digestion. 138,139 Reactions proceeding during decomposition of metal fluorides by pyrohydrolysis, oxygen bomb combustion and alkaline carbonate fusion and their related thermochemistry were discussed in our recent papers. 140,141 Calcium and magnesium fluoride, which can be in variable amounts present also in human, animal, vegetation and other environmental samples, were suggested as the most resistant ones to decompose among all the metal fluorides studied.…”