Mechanistic Studies of Chromium Speciation with Thermospray

Abstract

Thermospray (TSP) coupled with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) or inductively coupled plasma-mass spectrometry (ICP-MS) has been developed as a non-chromatographic method for chromium speciation to quantitatively separate and determine two chromium oxidation states: Cr(III) and Cr(VI). The limits of detection can reach 0.5 ng/mL with ICP-AES detection and 50 pg/mL with ICP-MS detection. The basis for this speciation method is that Cr(III) can selectively and nearly quantitatively deposit inside a thermospray system as Cr₂O₃, while Cr(VI) does not. To fully understand the mechanism of this deposition process, four questions were investigated: is aerosol formation necessary for the reaction to occur? Does the deposition occur in the aerosol or liquid regime? Does the deposit tend to be retained on the surface of the fused silica capillary? Can the reaction be predicted from thermodynamic calculations? These studies show that this reaction happens before solvent evaporates (i.e., the liquid regime). The high temperature inside the thermospray system is the major factor triggering this reaction. At the same time, the high pressure is important for its influence on the solvent boiling point, which affects the residence time (the time that the analyte spends in the solution before the solvent evaporates) and the kinetics of the reaction. The effects of the other parameters (vaporizer length, heating length, drawn tip, etc.) on the efficiency of the deposition reaction, represented as background residual signal (BRS), were also studied.