Susana Río Segade

Determination of inorganic mercury and total mercury in biological and environmental samples by flow injection-cold vapor-atomic absorption spectrometry using sodium borohydride as the sole reducing agent

Abstract A simple, fast, precise and accurate method to determine inorganic mercury and total mercury in biological and environmental samples was developed. The optimized flow-injection mercury system permitted the separate determination of inorganic mercury and total mercury using sodium borohydride as reducing agent. Inorganic mercury was selectively determined after reduction with 10−4% w/v sodium borohydride, while total mercury was determined after reduction with 0.75% w/v sodium borohydride. The calibration graphs were linear up to 30 ng ml−1. The detection limits of the method based on three times the standard deviation of the blank were 24 and 3.9 ng l−1 for total mercury and inorganic mercury determination, respectively. The relative standard deviation was less than 1.5% for a 10 ng ml−1 mercury standard. As a means of checking method performance, deionized water and pond water samples were spiked with methylmercury and inorganic mercury; quantitative recovery for total mercury and inorganic mercury was obtained. The accuracy of the method was verified by analyzing alkaline and acid extracts of five biological and sediment reference materials. Microwave-assisted extraction procedures resulted in higher concentrations of recovered mercury species, lower matrix interference with mercury determination and less time involved in sample treatment than conventional extraction procedures. The standard addition method was only needed for calibration when biological samples were analyzed. The detection limits were in the range of 1.2–19 and 6.6–18 ng g−1 in biological and sediment samples for inorganic mercury and total mercury determination, respectively.

DETERMINATION OF METHYLMERCURY AND INORGANIC MERCURY IN WATER SAMPLES BY SLURRY SAMPLING COLD VAPOR ATOMIC ABSORPTION SPECTROMETRY IN A FLOW INJECTION SYSTEM AFTER PRECONCENTRATION ON SILICA C18 MODIFIED

A novel method for preconcentration of methylmercury and inorganic mercury from water samples was developed involving the determination of ngl(-1) levels of analytes retained on the silica C(18) solid sorbent, previous complexation with ammonium pyrrolidine dithiocarbamate (APDC), by slurry sampling cold vapor atomic absorption spectrometry (SS-CVAAS) in a flow injection (FI) system. Several variables were optimized affecting either the retention of both mercury species, such as APDC concentration, silica C(18) amount, agitation times, or their determination, including hydrochloric acid concentration in the suspension medium, peristaltic pump speed and argon flow-rate. A Plackett-Burman saturated factorial design permitted to differentiate the influential parameters on the preconcentration efficiency, which were after optimized by the sequential simplex method. The contact time between mercury containing solution and APDC, required to reach an efficient sorption, was decreased from 26 to 3min by the use of sonication stirring instead of magnetic stirring. The use of 1moldm(-3) hydrochloric acid suspension medium and 0.75% (m/v) sodium borohydride reducing agent permitted the selective determination of methylmercury. The combination of 5moldm(-3) hydrochloric acid and 10(-4)% (m/v) sodium borohydride was used for the selective determination of inorganic mercury. The detection limits achieved for methylmercury and inorganic mercury determination under optimum conditions were 0.96 and 0.25ngl(-1), respectively. The reliability of the proposed method for the determination of both mercury species in waters was checked by the analysis of samples spiked with known concentrations of methylmercury and inorganic mercury; quantitative recoveries were obtained.

Evaluation of two flow injection systems for mercury speciation analysis in fish tissue samples by slurry sampling cold vapor atomic absorption spectrometry

Two simple flow injection (FI) systems were used for mercury speciation analysis in slurried fish tissue samples by cold vapor-atomic absorption spectrometry (CV-AAS). The solid samples were first suspended in hydrochloric acid containing Triton X-100 as dispersing agent, and then the slurries were injected in an acid carrier stream, which was either sequentially mixed with sulfuric acid, potassium persulfate oxidizing agent and stannous chloride reducing agent streams or merged with sodium borohydride reducing agent stream. Mercury speciation analysis was carried out as function of the oxidation coil temperature or sodium borohydride concentration, respectively. The detection limit of the method was 57 and 67 ng g−1, referred to dry weight, using the FI system with on-line oxidation for inorganic mercury and total mercury determination, respectively. Nevertheless, the FI system without on-line oxidation resulted in being simpler, faster and more sensitive for inorganic mercury and total mercury determination. Furthermore, the last system permitted the separate determination of methylmercury and inorganic mercury by selecting an adequate hydrochloric acid concentration in the suspension medium and the suitable sodium borohydride reducing agent. The detection limit of the method was 4.04, 0.977 and 11.0 ng g−1, referred to dry weight, for total mercury, inorganic mercury and methylmercury determination, respectively. Both FI methodologies were validated using two fish tissue certified reference materials, NRC DOLT-2 and NRC DORM-2, obtaining results in good agreement with the certified values, and were applied to the analysis of two real mussel tissue samples.

Comparison of slurry sampling and microwave-assisted digestion for calcium, magnesium, iron, copper and zinc determination in fish tissue samples by flame atomic absorption spectrometry.

The development of a slurry sampling method for the determination of calcium, copper, iron, magnesium and zinc in fish tissue samples by flame atomic absorption spectrometry is described. In comparison with microwave-assisted digestion, the proposed method is simple, requires short time and eliminates total sample dissolution before analysis. Suspension medium was optimized for each analyte to obtain quantitative recoveries from fish tissue samples without matrix interferences. Nevertheless, iron recoveries higher than 46% were not found. Treatment of samples slurried in nitric acid by microwave irradiation for 15-30s at 75-285W permitted to achieve efficient recoveries for calcium, iron, magnesium and zinc. Further improvement in the matrix effects for iron determination was accomplished by the use of an additional step of short microwave-assisted suspension treatment. However, standard addition method was required for calcium and copper determination, being necessary hydrochloric acid as suspension medium for the last one. Although copper could not be determined in the certified reference material using microwave-assisted digestion, the accuracy of the slurry sampling method was verified for all the investigated analytes. Detection limits were 22.8+/-8.0, 0.884+/-0.092, 5.07+/-0.76, 35.5+/-0.7 and 1.17+/-0.04mugg(-1) for calcium, copper, iron, magnesium and zinc, respectively. The standard deviations obtained using slurry sampling method and microwave-assisted digestion were not significantly different, and the mean relative standard deviation of the over-all method (n=3) of the slurry sampling method for different concentration levels was below 12%.

A Fractional Factorial Design Applied to the Optimization of Microwave- and Ultrasound-Assisted Acid Leaching Methods for Heavy Metals Determination in Sludges by Flame Atomic Absorption Spectrometry

Microwave-assisted extraction, ultrasound-assisted extraction and microwave-assisted digestion were compared for the determination of chromium, copper, lead and zinc in sludge samples by Flame atomic absorption spectrometry (FAAS) in terms of accuracy, precision, sample throughput and manipulation risk. The complete destruction of the matrix required the use of high temperature, elevated pressure and concentrated acids and the introduction of several time consuming steps, such as a cooling step of the digester and an evaporation step of the concentrated acids. The accelerated extraction procedures using dilute acids as extractants were optimised in order to reduce the treatment time and avoid the risks associated to the use of pressurised digesters. The capabilities of the Plackett–Burman saturated factorial design were explored to study the effect of various parameters on the extraction efficiency. The applicability of both extraction methods for sludge samples analysis of different origin was shown using principal component analysis and cluster analysis.

Monomethylmercury behaviour in sediments collected from a mercury-contaminated lagoon

Surface sediments and sediment cores were collected in a mercury (Hg)-contaminated lagoon, namely Largo do Laranjo – Ria de Aveiro, Portugal and analysed in order to establish the monomethylmercury (MeHg) behaviour in this kind of environment. In surface sediments, this compound was only detected in one place (13.2 ng g−1 (dry weight)). In this site, it was determined one of the lowest redox potentials (22 mV), indicative of oxic/anoxic conditions, which favours Hg methylation by enhancing the sulphate-reducing bacteria activity. However, the MeHg percentage obtained was low, namely less than 0.1% of the total Hg. This is probably due to Hg deposition with organic matter and iron oxyhydroxides, decreasing Hg availability to methylation. At the deeper layers, MeHg was also determined, reaching 46.4 ng g−1 (dry weight) and representing less than 0.1% of the total Hg. The higher MeHg percentages were observed near the surface, where Hg seems to be faster methylated as a result of the lower sulphide concencentrations that render bioavailable the inorganic Hg. At depth the low MeHg percentages obtained are due to the formation of HgS and to the adsorption of Hg to iron monosulphides.

Incidence of the Winemaking Technique on Metal Content and Phenolic Composition of Red Wines

The influence of different winemaking techniques usually used to elaborate red wines (Ganimede, prefermentative maceration, pectolytic enzymes and tannins addition, oak chips addition, delestage, and conventional procedure) on the metal content was evaluated. Linear discriminant analysis was also used to characterize the respective wines based on the metal content. By using iron, manganese, and calcium as chemical descriptors, the six winemaking techniques were perfectly discriminated. As the phenolic composition is an important quality factor in red wines, a correlation study between phenolic compounds and metals was also performed. A good correlation has been found for iron, calcium, and potassium with total polyphenols, tannins, and flavanols reactive. Iron, zinc, calcium, and potassium showed a significant correlation with anthocyanin compounds. Furthermore, higher concentrations of iron and calcium, but lesser ones of potassium, permitted to obtain red wines with a greater chromatic intensity.The influence of different winemaking techniques usually used to elaborate red wines (Ganimede, prefermentative maceration, pectolytic enzymes and tannins addition, oak chips addition, delestage, and conventional procedure) on the metal content was evaluated. Linear discriminant analysis was also used to characterize the respective wines based on the metal content. By using iron, manganese, and calcium as chemical descriptors, the six winemaking techniques were perfectly discriminated. As the phenolic composition is an important quality factor in red wines, a correlation study between phenolic compounds and metals was also performed. A good correlation has been found for iron, calcium, and potassium with total polyphenols, tannins, and flavanols reactive. Iron, zinc, calcium, and potassium showed a significant correlation with anthocyanin compounds. Furthermore, higher concentrations of iron and calcium, but lesser ones of potassium, permitted to obtain red wines with a greater chromatic intensity.

Effect of Organic Matter on Determination of Reactive Mercury in Contaminated Waters

Dissolved reactive mercury was determined in water samples from Ria da Aveiro, a coastal lagoon contaminated by mercury. Concentrations increased with acid dilution to a maximum of 193% in samples containing organic matter with high aromaticity, as inferred from absorbances at 250, 280 and 365 nm. Laboratory experiments with potassium hydrogenoftalate and humic acids solutions proved the influence of aromaticity in complexing mercury, how acid dilution protonates the aromatic Hg-complexes and consequent release of labile forms of mercury to solution. On the basis of these results it is proposed that natural waters from mercury contaminated ecosystems should be analysed without dilutions, otherwise additional studies should be performed to access the protonation of mercury organic complexes.