Effect of the Surface Treatment of a Graphite Furnace with a Refractory Element (Hafnium, Titanium, Tungsten and Zirconium) by a One-Drop Coating Method on the Atomization Mechanism of Indium in Electrothermal Atomic Absorption Spectrometry

Abstract

In the present work we investigated the atomization mechanisms of indium in electrothermal atomic absorption spectrometry with pyrolytically coated graphite (PG) and non-pyrolytically coated graphite (NPG) furnaces treated with a refractory element, such as Hf, Ti, W or Zr, by a one-drop coating method. A single-peak signal was observed in Hf-, Ti- and Zr-treated PG and NPG furnaces, and an unresolved double peak signal was observed in W-treated PG and NPG furnaces. The effects of the surface treatment on the charring curve (charring temperature-absorbance curve), kinetic data and Raman spectroscopic data were investigated. The charring curve was shifted to a high-temperature side with a great sensitivity enhancement after the surface was treated, corresponding to suppression of the sensitivity loss reaction of In (In₂O₃(s)+2C(s)→In₂O(g)+2CO(g)). The Arrhenius activation energy (E_a) was estimated to be 158±10, 146±10 and 145±10 kJ mol^-1 and 170±15, 194±15 and 165±10 kJ mol^-1 with Hf-, Ti-and Zr-treated PG and NPG furnaces, 180±15 kJ mol^-1 for the first peak with W-treated NPG one, 115±10 and 111±10 kJ mol^-1 for the second peak with W-treated PG and NPG ones, respectively. The Raman spectra of the treated graphite surface were measured at various sites in the sample compartment. The ratio of the intensity of the D band (disordered mode) to that of the G band (E_2g mode), I_D/I_G, was increased by a surface treatment. This suggests an increase in the number of disordered sites, such as edge carbon atoms and the boundaries of graphite crystallites, which exist in uncoated regions of graphite within the treated area. Arrhenius activation energy, except for the second peak with the W-treatment, decreased along with an increase in I_D/I_G when using a treated furnace. From this relationship, it was proposed that the atomization of indium in treated furnaces takes place due to collisions between In₂O(g) and the uncoated region of graphite. When using a W-treated furnace, dissociation of the indium dimer was attributed to a rate-determining step for the second peak.