Evaluation and application of argon and helium microstrip plasma for the determination of mercury by the cold vapor technique and optical emission spectrometry

Abstract

The suitability of a 2.45-GHz atmospheric pressure, low-power microwave microstrip plasma (MSP) operated with Ar and He for the determination of Hg by continuous-flow cold vapor (CV) generation, using SnCl2/HCl as the reducing agent, and optical emission spectrometry (OES) using a small CCD spectrometer was studied. The areas of stability for a discharge in the Ar and in the He MSP enclosed in a cylindrical channel in a quartz wafer were investigated. The excitation temperatures as measured for discharge gas atoms (Ar I, He I), and the electron number densities at 35-40 W and 15-400 mL min-1 were found to be at the order of 3,200-5,500 K and 0.8∈×∈1014-1.6∈×∈1014 cm-3, respectively. The relative intensity of the Hg I 253.6-nm line and the signal-to-background ratio as a function of the forward power (35-40 W) as well as of the flow rate of the working gas (15-400 mL min-1) were evaluated and discussed. For the selected measurement conditions, the Ar MSP was established to have the lower detection limit for Hg (0.6 ng mL -1) compared with the He MSP. The linearity range is up to 300 ng mL-1 and the precision is on the order of 1-3%. With the optimized CV Ar MSP-OES method a determination of Hg in spiked domestic and natural waters at concentration levels of 20-100 μg L-1 and an accuracy of 1-4% could be performed. In an NIST domestic sludge standard reference material, Hg (3.64 μg g-1) could be determined with a relative standard deviation of 4% and an agreement better than 4%. © 2007 Springer-Verlag.