Emprego da piroidrólise como método de preparo de amostras para a determinação de tungstênio em carbeto de silício por técnicas baseadas em plasma indutivamente acoplado

Abstract

In this work, pyrohydrolysis was proposed as a sample preparation method for the tungsten volatilization of the silicon carbide (SiC) and subsequent determination by induced coupled plasma optical emission spectroscopy (ICP OES) and mass spectrometry with inductively coupled plasma (ICP-MS). The following parameters were evaluated: the type of pyrohydrolysis reagent, H2O and H2O2 (10 to 50%, v/v), flow rate of the pyrohydrolysis reagent (0.2 a 1.4 mL min-1), type of carrier gas (oxygen and compressed air) and flow of this gas (0.1 a 1.0 L min-1), adding of auxiliary reagent (V2O5, PTFE, NH4Cl e NaCl), reaction time (30 to 180 min), oven temperature (1000 to 1200 °C), sample mass (2.5 to 20 mg) and type of absorbing solution (50% H2O2 or H2O). The use of H2O2 resulted in recoveries of 55% of tungsten, which is greater than when compared to recoveries only with the use of water as a reagent for pyrohydrolysis (30%). In addition, it was necessary to adapt a inlet at the end of the tube using 50% H2O2, where condensation of volatile tungsten species occurs. Without this inlet with 50% H2O2, tungsten was not fully collected, causing lower recoveries, in addition to causing memory effects. The condition that resulted in the best recovery was using 50% H2O2 at a flow rate of 1.2 ml min-1, oxygen flow rate of 0.1 L min-1, without using an auxiliary reagent, at temperature of 1000 ºC, for 60 min of reaction and using H2O as absorbing solution. The results were compared with the reference results of the alkaline fusion method and with the instrumental neutron activation analysis technique and they do not differ significantly (Student's t-test, confidence level of 95%). The proposed method with optimized conditions was applied to samples with different tungsten concentrations. The detection limit was 23 μg g-1 for the ICP OES technique and 2.6 μg g-1 for ICP-MS (estimated for 200 mg of mass). The proposed pyrohydrolysis method proved to be adequate for the determination of tungsten in SiC with minimal use of reagents and generation of waste.