RESUMO
An on-line automated flow injection system with microwave-assisted sample digestion was used to perform silicate rock dissolution in acid medium for iron determination. For this purpose, a continuous flow system was built up by using an automatic flow injection analysis (FIA) system coupled to a flame atomic absorption spectrometer (FAAS), including a focused microwave oven unit. Inside the microwave cavity was inserted a polytetrafluoroethylene (PTFE) reactor coil (300cm length and 0.8mm i.d.) where the dissolution takes place. Chemical and flow variables as well as iron determination parameters were studied. In the flow system, a slurry of the rock sample (50mg in 200ml of acid mixture HF+HCl+HNO(3)) is pumped through the reaction coil and the microwaves are turned on. After elapsed the time required to complete the sample dissolution, the mixture is pumped again in order to fill the sampling loop (500mul). Then, by changing the valve position, a water carrier stream pushes the sample solution through the flame atomic absorption spectrometer nebulizer. To achieve an accurate determination of the rock certified materials, the slurry sample was irradiated during 210s at 90W power. Working in that condition, a detection limit of 0.80mugml(-1) (which corresponds to an Fe(2)O(3) content of 0.46%) and an analytical throughput of 10h(-1) were achieved. The relative standard deviation (R.S.D.) of the method varied between 1 and 11% when applied to the rock certified materials.
