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Abstract The measurement of mercury in aqueous solutions by ICP-AES is adversely affected by the memory effect wherein mercury accumulates within the sample introduction system and is slowly released over time to give increasing response signals at the same initial mercury concentration. The memory effect is obviated by the addition of Hg(II) complexants: thiourea and gold(III) chloride are both effective in preventing mercury sorption and vapor buildup with the latter being preferred because the memory effect vanishes more rapidly. Conditions are described wherein it is possible to quantify low levels of mercury(II) in aqueous solutions by ICP-AES under routine operating conditions that can be applied to other metal ions by adding 1 mg of gold(III) chloride per 3 mg of mercury(II) to those solutions.1. IntroductionThe toxicity of mercury at very low levels has led to its stringent control with a maximum contaminant level of 2 g/L being set by the US Environmental Protection Agency 1. This has led to the development of techniques for accurate monitoring of mercury levels 2 via cold vapor atomic absorption spectrometry 3,4, plasma atomic emission spectrometry 5, atomic fluorescence spectrometry 6,7, inductively coupled plasma (ICP) optical emission spectrometry 8 and ICP-mass spectrometry 9-11. Despite methods with good sensitivity and selectivity, few can directly measure Hg(II) in aqueous solutions under routine operating conditions as can be done with other ions 12. Mercury accumulates in instruments irrespective of its initial there is a gradual increase of the mercury signal strength with time, a non-linear calibration, and a long wash-out time 13. Mercury contamination of the instruments may be attributed to its adsorption on the transfer tubing, spray chamber and nebulizer 14.Mercury in solution exists as an equilibrium of Hg(0), Hg(I), and Hg(II). Since only Hg(0) is volatile, one method of eliminating the memory effect entails complete reduction of the mercury to Hg(0) with the reducing agents stannous chloride and sodium borohydride 15. The mercury vapor is then purged from solution into an absorption cell. This cold vapor technique 16 is frequently used for mercury determination in solution when coupled with atomic absorption or atomic fluorescence spectrometry. The standard EPA mercury determination method is based on this technique 17. The generation of mercury vapor does, however, require additional equipment for its generation and measurement. An alternative technique involves the stabilization of mercury in solution as Hg(II). Gold(III) chloride and potassium dichromate are typical stabilizing agents that give complete conversion to Hg(II) thus obviating volatilization 18. The technique has been applied to the measurement of mercury via ICP-mass spectrometry (MS) 19. The addition of chelants that bind Hg(II) also stabilizes it within aqueous solutions and limits loss through volatilization. Thus the addition of Triton X-100, ammonia and EDTA allows for the accurate measurement of mercury in aqueous solutions by ICP-MS 20. The use of 2-mercaptoethanol reduces the memory effect to the point that ICP-MS gives a detection limit for mercury of 5.1 g/L 21. Techniques have been developed to allow the use of cold vapor coupled atomic absorption spectrometry (CV AAS) 22 and ICP-MS for mercury determination. The detection limits are 10 g/L for the former and 3 g/L for the latter 23. While ICP is thus an excellent technique for mercury determination, its extension to the more commonly available ICP-atomic emission spectroscopy (AES) remains problematic due to the memory effect 24. It is reasonable to expect that mercury-stabilizing agents added to solutions would eliminate the memory effect in ICP-AES as they have with ICP-MS. The use of a 2% L-cysteine solution containing Hg(II), but only at a very low level (1 g/L), obviates the memory effect in ICP-AES 25. We now report a rapid and accurate method for the determination of total mercury in aqueous solutions at the mg/L level using ICP-AES through a comparative study of gold(III) chloride, triethylamine, ethylenediaminetetraacetic acid (EDTA), disodium ethylenediaminetetraacetate (Na2EDTA) and thiourea.2. Experimental section2.1. Reagents Mercury solutions are prepared by diluting a 1000 mg/L mercury(II) nitrate reference solution containing 1.8% nitric acid. Au(III) solutions are prepared from a 1000 mg/L gold atomic absorption standard solution in 0.5 M HCl. Triethylamine, ethylenediamine tetraacetic acid (EDTA), disodium EDTA (Na2EDTA), and thiourea (analytical grade) were purchased from Acros and used directly without purification. Nanopure water with resistivity N 18 M was used from a Barnstead ultrapure water system.2.2. InstrumentationA Spectroflame M120E (Spectroanalytical Instruments) inductively coupled atomic emission spectrometer (ICP-AES) controlled by Spectro Smart Analyzer Vision software was used for analysis of the mercury in solution. Sample solutions were introduced by peristaltic pump. Ultra-pure argon from Airgas was used. The wavelength of analysis was set at 184.956 nm. The operating conditions (also appropriate for the analysis of other transition metal ions) are detailed in Table 1. Measurement was initiated immediately after the preflush.2.3 Analytical procedure Mercury(II) solutions with 2.0 mg Hg(II)/L were prepared in50 mL volumetric flasks also containing one of the following: Au(III) at 1.0 mg/L, 0.1% triethylamine, EDTA (saturated solution), 0.1% Na2EDTA, or 0.1% thiourea (following reports in the literature with ICP-MS, 0.1% Triton X-100 was added to the amine and thiourea solutions as a wetting agent). One set of solutions was allowed to stand for 1 h before analysis and a second set was allowed to stand for 17 h. Au(III) solution at1.0 mg/L was made by successive dilutions of a 1000 mg/L standard reference solution. Triethylamine, Na2EDTA and thiourea solutions were prepared by dilution of their 1% stock solutions. Due to its low solubility (0.5 g/L), the EDTA solution was prepared by adding 0.05 g directly to the Hg(II) solution in a 50 mL volumetric flask and then using only the supernatant for the ICP measurement. Mercury standard solutions at levels up to 10.0 mg/L containing up to 10.0 mg Au(III)/L were prepared by successive dilution of 1000 mg/L Hg(II) and Au(III) reference solutions.After the plasma was ignited, nanopure water acidified with 1% nitric acid was used to wash the sample introduction system and the instrument allowed to stabilize for 15-20 min. The optics were then reprofiled, the sample solution aspirated into the ICP-AES, and measurement initiated immediately after a 20 s preflush. Three measurements were taken as the solution was continuously aspirated and averaged. Nitric acid solutions (1 M) containing 1.0 mg Au(III)/L were used to rinse the sample introduction system between samples for 1-2 min.3. Results The memory effect increases the mercury count and leads to erroneous results for the concentration of mercury in successive samples. Fig. 1 shows the changes in mercury signal intensity measured at different times when Hg(II) standard solution at 0.2 mg/L is aspirated into the ICP-AES. The same result is seen with a Hg(II) solution at 0.005 mg/L, the lowest level in this study. Continuously introducing the same mercury solution results in a gradual increase of signal response, with equilibrium not being achieved. The intensity of the mercury signal in Fig. 1 at the fourth measurement has increased about twice compared to that of the first measurement. The mercury signal does not correlate with the sample concentration and those concentrations are not reproducible. Mercury in the sample solution must thus be adsorbed on the walls of the transfer tubing, spray chamber and/or nebulizer during its transport before atomization and released over time. No maximum mercury sorption within the sample introduction system is observed. Further-more, signal response only slowly returns to the baseline level after aspiration is changed from the mercury solution to distilled water. Washing the system with water for 10 min does not remove all of the adsorbed mercury: when 2.0 mg/L Hg(II) solution is used, a mercury signal with intensity of 166410 cps is still observed (Fig. 2). This significant memory effect obviates the routine analysis of Hg(II) by ICP-AES without the addition of mercury-stabilizing agents.TEA, EDTA, Na2EDTA, thiourea and gold(III) chloride are compared to determine which can most effectively stabilize Hg (II) thus eliminating the memory effect in the ICP-AES. Since the mercury standard concentrations are low (2.0 mg/L) and high background concentrations can affect plasma temperature and nebulization, the matrix composition is kept as dilute as possible. The concentrations of TEA, Na2EDTA and thiourea in the mercury sample solutions were thus set at 0.1% and the Au(III) set at 1.0 mg/L; the saturated EDTA solution had the same Au (III) level.Table 2 shows the change in mercury signal intensities over three measurements when TEA, EDTA, Na2EDTA, thiourea, or gold(III) chloride is added to the mercury solutions and the solutions allowed to stand for 1 h before measurement. Within each set of three measurements, the second and third measurements were taken immediately after the one that preceded it. A significant memory effect remains with all but the gold-stabilized solution. With TEA, the third measurement gives a mercury signal that is 2.5 times greater than the first measurement while, with EDTA and Na2EDTA, it is nearly twice as great. Thiourea shows a variation of less than 20% in the maximum mercury signal intensity though the initial mercury response is still much greater than that of the Au(III)-stabilized solution. The latter shows no memory effect.While complexants with nitrogen-and sulfur-based ligands are known to form stable Hg complexes (e.g., immobilized tertiary amines display a high affinity for Hg(II) from dilute solutions after a contact time of less than 30 min 26 and soluble thiourea can complex Hg2+ and CH3Hg+ from aqueous solutions 27), it is possible that attaining equilibrium requires a time longer than 1 h. The respective solutions were thus allowed to stand for 17 h after which point the measurements were made (Table 2). Though the situation improves compared to the results with a 1 h contact time, the increase in mercury signal response remains evident with TEA, EDTA, and Na2EDTA. (The memory effect is evident with Na2EDTA even after a 5-day contact time.) The lengthy contact time, however, clearly diminishes, and may eliminate, the memory effect when thiourea is used as the complexant.Table 3 shows the effect of adding varying amounts of Au (III) to solutions of mercury at 2.0 mg/L. The addition of a level as low as 1.0 mg/L eliminates the memory effect: reproducible signal intensities are observed over three measurements after a 1 h contact time. A change in Au(III) level from 1.0 to 10.0 mg/L does not affect the signal intensity at a Hg(II) level of 2.0 mg/L. Each signal intensity varies from the average by b 1.5%. Au(III) at the low level of 1.0 mg/L is sufficient to keep Hg(II) at low concentrations stable in aqueous solutions. Given that the contact time required to achieve this reproducibility is less than that required by thiourea, gold(III) chloride is the agent of choice for stabilizing the mercury signal.A linear calibration curve was obtained (brN = 0.999-1.000) with solutions of 1.0 mg Au(III)/L and increasing levels of Hg (II) (0.1, 0.2, 0.4, 0.6, 1.0, and 2.0 mg/L). No matrix effect was found upon the addition of 0.04 N sodium nitrate, chloride, or sulfate. Mercury at the g/L level can be measured with the gold technique. For example, contacting a solution containing 10 mg Hg(II) /L with crosslinked polystyrene beads bearing immobilized dimethylamine ligands for 17 h gave ICP-AES spectra that were nearly identical to the baseline of a blank solution, indicating a Hg(II) level of less than 10 g/L.In order to determine the lower limit of the Au:Hg ratio, six solutions of 10 mg Hg(II)/L were prepared with 0.5, 1.0, 2.0, 3.0, 6.0, and 10.0 mg Au(III)/L (Table 4). Three measurements were recorded for each solution; 1 M HNO3 containing 1.0 mg Au (III)/L was used to wash the sample introduction system between Hg(II)-containing solutions. The solution with 10 mg of gold was read first in the ICP, followed by the solutions with lower gold levels, as given in Table 4. The steady Hg signals from the last three solutions were as expected from the earlier runs. However, the first solution with 0.5 mg Au showed a decrease in signal intensity as each measurement was taken whereas an increase in intensity would have been expected due to the memory effect when the level of Au got too low. The solutions with 1.0 and 2.0 mg Au also suggested a decrease but one much less pronounced than the first solution. The first four solutions (i.e., 10.0 mg Hg/L with 0.5, 1.0, 2.0, and 3.0 mg Au/L) were then prepared again for re-investigation and to study the effect of Au(III) in the wash solution by changing that solution to 2 M HCl since the chloride ion is known to bind both mercury and gold ions. The order of reading in the ICP was as given in Table 5, from left to right; the solutions with 0.5, 1.0, and 2.0 mg Au show a decrease in the Hg signal over the four measurements taken (though the decrease is not pronounced with the 2.0 mg solution) and a steady Hg(II) signal is obtained with the 3.0 mg Au solution. These results suggest that there is an effect on the Hg(II) signal if the intensities are read in a sequence from more concentrated to less concentrated Au solutions (Table 4) or less concentrated to more concentrated solutions (Table 5); this is discussed in the following section.4. DiscussionThe results clearly indicate that the addition of a low level of gold(III) chloride to mercury-containing solutions yields a reproducibly stable signal. The mercury may be stabilized by binding to gold(III), though the nature of that binding is unclear. In optimizing the gold level, it became evident from the lowest Au level in Table 4 that there is another effect operative since the Hg signal intensity decreases with time (rather