Determination of Mercury by Cold-Vapor Atomic Absorption Spectrometry with Preconcentration on a Gold-Trap

Abstract

Cold-vapor atomic absorption spectrometry (CVAAS) is one of the most popular techniques for determining mercury in a wide variety of samples. [1][2][3][4][5][6] Generally, this technique is applied to determine the total mercury content. 7 Either tin chloride 1-3 or sodium tetrahydroborate(III) 4,5 is used to reduce mercury to its elemental form. Mercury vapor is purged from solution by a carrier gas, such as air, nitrogen or argon. After passing through a gas-liquid separator, it is introduced into the optical path of an atomic absorption spectrometer.Several modifications have been introduced to this technique, aiming to improve the sensitivity, precision, lower interferences and increased practicality of the methodology. Some of the proposals for preconcentrating mercury include extraction 8 and the amalgamation on silver or gold, 1,2,9,10 as well as the application of solid sorbents. 3,8,11 Especially an enrichment procedure based on the amalgamation of generated mercury vapor on a gold trap is recommended for the quantification of a very low mercury content in biological and environmental samples. Beside the preconcentration effect, separation from other compounds in the gas phase could be achieved. The gold trap is a simple on-line preconcentration system that does not require the use of reagent and could be easily adopted to the flow-injection technique.Recently, the use of gold amalgamation has been reported in combination with the pyrolysis of solid samples in a combusion tube at 750˚C under an oxygen atmosphere, as reported for mercury reduction. 12 Mercury and its compounds, even at low concentration, are known to be a dangerous neurotoxins for living organisms. Therefore, it is essential to use an accurate and precise analytical method for the determination of trace amounts of mercury. Because the average concentration of Hg in natural water samples was found to be a few ng l -1 and the typical detection limit for CVAAS is 0.05 µg l -1 level, 6,13,14 the preconcentration step seems to be essential in this case. Low detection limits are also necessary in studies where speciation is involved, because some species may account for only a small fraction of the total mercury present in the sample.The aim of this work was to investigate and optimize the chemical condition for mercury reduction, as well as the parameters of the flow system, which was coupled on-line to the preconcentration and separation unit. Various parameters affecting the mercury preconcentration and its final determination by the CVAAS method were evaluated. Studies were carried out for inorganic mercury (Hg(II)) and phenylmercury (PhHg). The data were compared to those obtained without a preconcentration step. Experimental InstrumentationA FIMS flow-injection atomic absorption spectrometer (Perkin-Elmer, Germany) was used for all experiments. A sample loop of 1 ml volume was used. The pump action, injection and measurement (Table 1) as well as data processing were controlled through Perkin-Elmer WinLab software. Measurements were perfo...

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