An in situ pre-concentration method for fluorine determination based on successive digestions by microwave-induced combustion.

A new strategy based on successive digestions by microwave-induced combustion (MIC) in the same reaction vessel with a single absorbing solution was proposed. As a proof of concept, F was determined by ion-selective electrode (ISE) in seafood digests. Samples were pressed as pellets (up to 0.7 g) and combusted in closed quartz vessels pressurized with oxygen. Sequential digestions were each performed (up to 4 combustion cycles) in the same vessel and using the same absorbing solution. In each cycle, a new filter paper, igniter and sample pellet (0.7 g of sample) were used. Ammonium hydroxide solutions (10-100 mmol L-1) were evaluated for F absorption. Accuracy of the proposed method was evaluated using certified reference material of oyster tissue (NIST 1566a) and also by comparison of results after pyrohydrolysis method. Up to 3 digestion cycles (total mass of 2.1 g) could be used with 50 mmol L-1 NH4OH as absorbing solution. Results were in agreement with those obtained using pyrohydrolysis and also with certified reference value; the coefficient of variation after 3 cycles was below 5%, which was considered as suitable for F determination even at low concentration. The residual carbon in digests was lower than 25 mg L-1, allowing F determination by ISE virtually free of interferences due to dissolved organic matter. The limit of quantification (LOQ) for F was 1.3 µg g-1 (using 2.1 g of seafood), which is almost 4 times lower than the LOQ obtained using the reference method (pyrohydrolysis). Contrary to the reference method, this relatively low LOQ allowed the determination of F in all the seafood samples analysed. Taking into account that only 6 mL of diluted NH4OH solution (50 mmol L-1) were used and the suitable LOQ, the proposed sequential digestion MIC method can be recommended for further F determination in trace levels in seafood, even using a low-cost technique such as ISE, instead of other, more powerful techniques, such as ion chromatography.