Transition metal ion assisted photochemical vapor generation of niobium and tantalum

Abstract

Photochemical vapor generation (PVG) uses radicals and aqueous electrons generated from a sample solution exposed to ultraviolet radiation for the transformation of elements into volatile species. It has been used as a sample introduction method in atomic spectrometry to replace traditional pneumatic injection for element analysis since 2003. Niobium (Nb) and tantalum (Ta) are regarded as high field strength elements in the field of geochemistry. In this work, the PVG of Nb and Ta is reported for the first time. In formic acid (FA) solution, Nb and Ta can be converted into volatile species via photochemical reduction. The addition of transition metal ions, including Cu²⁺, Co²⁺ and Cd²⁺, into the PVG system can promote the PVG yields of Nb and Ta. Among them, Cu²⁺ was found to be the best “sensitizer” with an enhancement of 76- and 91-fold for Nb and Ta, respectively, compared to the system without transition ions. Inductively coupled plasma mass spectrometry (ICP MS) was used to investigate main parameters affecting the photochemical reduction of analytes, including the concentration of low molecular weight organic acids (LMWOAs), the concentration of transition ions, the wavelength and duration of UV radiation, and the influence of coexisting ions. The mechanism of Cu²⁺-assisted PVG for Nb and Ta was studied using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Nanoparticles of zerovalent Nb and Ta were found in collected gaseous and liquid phases after photochemical reduction. It was speculated that the generated volatile species of those elements were carbonyl compounds. The new findings in this study will help in further understanding the photochemical properties of Nb and Ta and formation of nanoparticles under UV irradiation. Moreover, it provides the possibility of further enhancing the PVG efficiency of Nb/Ta and developing a sensitive method for those two elements.