On-line photoassisted vapour generation implemented in an automated flow-injection/stopped-flow manifold coupled to an atomic detector for determination of selenium

Abstract

A flow-through UV reactor has been implemented in an automated flow-injection/stopped-flow manifold, and used for on-line generation of Se volatile derivatives prior to detection by quartz furnace atomic absorption spectrometry. The presence of a low molecular weight organic acid (formic or acetic) in the carrier was a necessary requirement so that efficient vapour generation was achieved. Stopped-flow conditions were employed with the aim of increasing the time for which the sample zone was subjected to UV irradiation while it was transported to the detector. The simultaneous formation of H₂ during the photochemical decomposition of formic acid facilitated the atomization of the Se derivatives in the quartz furnace, and therefore the addition of H₂ to the carrier gas, which is necessary for efficient atomisation, could be eliminated. Nevertheless, the presence of H₂ in the carrier gas (Ar) was required for efficient atomisation when using acetic acid as carrier. This manifold allowed a sample throughput of 26 h⁻¹ with a detection limit of 0.5 ng mL⁻¹ Se and a repeatability, expressed as relative standard deviation, of 4%. Effect of concomitants in the online generation of Se vapours with the new manifold was investigated. Chloride salts of alkaline and alkaline-earth elements did not interfere at concentrations up to 10 000 mg L⁻¹, but nitrate salts caused a positive effect. Tolerance of typical hydride-forming elements such as As and Sn was much higher with UV vapour generation as compared to chemical hydride generation. The presence of Cu(II) was critical since interference effects are observed from 0.05 mg L⁻¹, but tolerance of Ni(II) was greater with UV vapour generation.