Electron impact excitation-cavity ringdown absorption spectrometry of elemental mercury at 405 nm

Abstract

We report a new method of measuring elemental mercury (Hg) at the 405 nm transition line, which corresponds to the optical transition from the metastable state 5d¹⁰6s6p ³P₀ to the upper state 5d¹⁰6s7s ³S₁. Observation of the absorption spectra and measurements of the absolute number density of Hg in the metastable state are achieved by a stepwise electron impact excitation-cavity ringdown absorption technique, in which the population of Hg atoms in the metastable state is realized through electron impact excitation by the energetic electrons (< 10 eV) generated in an atmospheric argon microwave plasma torch and the detection of Hg is achieved via cavity ringdown spectroscopy (CRDS). The major difference between this method and the previously reported detection of Hg at 254 nm using the plasma-CRDS technique is that the plasma in this method not only serves as an atomization source to generate Hg atoms from the Hg-contained compounds injected in the plasma, but also functions as an electron impact excitation source. The merit in terms of analytical instrumentation is that this new method introduces an alternative way to measure Hg using a palm-size 405 nm laser source. This can potentially lead to a portable mercury ringdown spectrometer without constraints of importability of 254 nm laser sources. Compared with the 254 nm line, the 405 nm line has no plasma-associated spectroscopic interferences, such as absorption of the OH rovibrational lines. One analytical limitation of the detection of Hg at 405 nm is the low detection sensitivity achieved in this exploratory study, 50 μg ml⁻¹ in aqueous sample solutions, compared with the previously reported detection sensitivity of 9.1 ng ml⁻¹ using the plasma-CRDS technique at 254 nm. Given an improved plasma excitation source-CRDS system, theoretical detection limits at 405 nm are estimated to be 50 ng ml⁻¹ and 1.2 ppbv in aqueous samples and in gaseous samples, respectively. These sensitivities are still desirable in many applications when real-time, portable, and in-line analysis all become a major concern.