Desenvolvimento e aplicação de um sistema com arco elétrico para SS-FF AAS para a determinação de impurezas elementares

Abstract

An alternative system for solid sampling and element determination by heated tube coupled to flame atomic absorption spectrometry was developed for the determination of Cd, Co, Mo, Ni and Pb in high purity graphite and carbon nanotubes (CNTs). Three quartz systems were developed and evaluated: without atomizer tube (system A) and with atomizer tube, being one of them of 80 mm of length without slit (system B) and other with the same length and with a 80 mm slit (system C) that allow the flame inlet. Two electrodes were positioned in the system and were connected to an external device, to generate an electric discharge which was responsible to ignite the samples. Samples were introduced in the system, with ethanol as auxiliary reagent to start the combustion before the generation of arch discharge between electrodes, in the presence of a constant O2 flow, that was responsible to react with organic matrix producing CO2 as main product and driving the gaseous products of combustion with the analytes to the atomizer tube. The system B was the most suitable having a quartz atomizer tube connected to a perpendicular tube (“T” shape). This atomizer tube was positioned over the spectrometer flame and acted as an atom trap to increase the atomic density in front of the optical beam, reducing the limits of detections (LODs). The following parameters were investigated to optimize the system: flame stoichiometry of air/acetylene (620/100, 565/100, 430/100 and 400/120 L h-1), distance between atomizer tube and burner (3, 6, 9, 12 e 15 mm), O2 flow-rate (0.3, 0.5, 1.0 and 2.0 L min-1), linear calibration range and the sample mass influence in the determination. The calibration was performed by matrix matching, being added the analyte to a high purity graphite, used as a standard, and the results were statistically compared with reference values obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) and by inductively coupled plasma mass spectrometry (ICP-MS) before decomposition of samples by microwave-induced combustion (MIC). The proposed method allows the introduction of up 50 mg of solid sample, without a sample decomposition for the determination of Cd, Ni and Pb in graphite and CNTs. Low LODs were achieved for the SS-FF AAS system (between 0.107 and 4.20 μg g-1), allowing the determination of analytes at low concentrations. Moreover, the proposed method agreed with several recommendations of green analytical chemistry as reduced sample pre-treatment, low sample consumption, no use of toxic reagents for sample preparation, low residues generation (basically CO2), low electric energy consumption and relatively safety for the analyst.