Coupling of cold vapor generation with an atmospheric pressure glow microdischarge sustained between a miniature flow helium jet and a flowing liquid cathode for the determination of mercury by optical emission spectrometry

Abstract

A direct current atmospheric pressure glow microdischarge (dc-μAPGD), generated between a miniature flow He jet nozzle anode and a small-sized flowing liquid cathode, was combined with a continuous flow cold vapor generation (CVG) system to improve the sensitivity of the determination of Hg by optical emission spectrometry (OES). In this arrangement, Hg(II) ions were converted to cold vapor in the reaction with NaBH₄ and subsequently delivered in a stream of He carrier/jet-supporting gas to the microdischarge through the nozzle anode. Additional He shielding gas was used to prevent discharge zones from the access of ambient air. A vertical distribution of emission from the Hg I 253.7 nm line between both electrodes was acquired, and the highest response for Hg was established in the near-anode region of the microdischarge. Several operating parameters that affect the CVG reaction and discharge were optimized. Under compromised conditions, the intensity of the Hg I line was improved over 4000 times compared to that obtained in a μAPGD-OES system without the CVG system. The efficiency of CVG of Hg and its transport to the microdischarge was evaluated to be 98 ± 1%. For comparison, in the μAPGD system without CVG, the efficiency of sputtering was merely lower by about 20%, i.e., 77 ± 4%. A likely explanation of the enhancement of Hg response observed for CVG-μAPGD was discussed. The detection limit (DL) of Hg assessed for CVG-μAPGD-OES was 0.14 μg L⁻¹ (3σ criterion). To assess the accuracy of the new method, Hg was quantified in a certified reference material (CRM) of human hair (NCS ZC 81002). Excellent agreement between certified and measured concentrations of Hg was obtained. In addition, recoveries of Hg added to samples of different waters were evaluated. They were in the range of 96–103% proving the good accuracy of CVG-μAPGD-OES. The repeatability of the signal over the linearity concentration range of 5–500 μg L⁻¹ of Hg was within 2.1–4.1% (as relative standard deviation, RSD).