RESUMO
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 0.8x10(14)-1.6x10(14) 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 microg L(-1) and an accuracy of 1-4% could be performed. In an NIST domestic sludge standard reference material, Hg (3.64 microg g(-1)) could be determined with a relative standard deviation of 4% and an agreement better than 4%.