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.

Introduction

Mercury has been recognized as one of the most toxic heavy metals present in the environment. However, total mercury concentration yields little information about its toxicological and biogeochemical behavior, which depends on the specific chemical form [1]. It is well known that methylmercury is more toxic than inorganic mercury [2]. The anthropogenic sources of methylmercury are usually rare, but it is naturally formed in sediments by bacterial methylation of inorganic mercury [3]. Methylmercury can then be bioaccumulated and biomagnified in the food chain [4]. Hence, the consumption of contaminated seafood, especially predatory fish and marine mammals, with methylmercury represents a potential hazard to human health [4]. Furthermore, the analysis of sediments and fish tissues permits monitoring of mercury levels in water, since mercury present in contaminated waters is accumulated in both environmental compartments.

As a consequence, not only total mercury determination, but also methylmercury determination, is needed in order to know the toxicological and environmental impact of mercury. The most commonly used technique for mercury determination in biological and environmental samples is cold vapor-atomic absorption spectrometry (CV-AAS) due to its analytical abilities [5]. Some methods have been published on mercury speciation by CV-AAS without previous chromatographic separation, based on either the use of several reducing agents with different reducing power, such as sodium borohydride and stannous chloride, or the possible oxidation of organomercury species to inorganic mercury previous to the reduction to elemental mercury.

In the first method, inorganic mercury is selectively determined using stannous chloride in acid medium as reducing agent due to the inability to reduce organomercury compounds [6], [7], [8]. Furthermore, total mercury determinations are carried out using sodium borohydride due to its power to reduce both inorganic and organic mercury species when the sensitivity of the species does not differ significantly [8]. However, some authors have obtained different sensitivity for methylmercury and inorganic mercury in several matrices when sodium borohydride is used as reducing agent for total mercury determination [8], [9], [10], [11], [12], [13]. The formation of methylmercury hydride (MeHgH) instead of elemental mercury may be the cause of the different behavior of both species [14], [15], [16], [17]. A paper has recently been published based on the use of sodium borohydride for inorganic mercury determinations [18]. This problem is solved by the oxidation of organomercury compounds to inorganic mercury, using combinations of strong acids (hydrochloric, sulfuric and nitric acids), oxidants (hydrogen peroxide, potassium permanganate, potassium dichromate, potassium persulfate, potassium bromide–potassium bromate), elevated temperatures, ultraviolet irradiation, microwave exposure and sonolysis, previous to reduction to elemental mercury [9], [18], [19]. Other methods are also based on selective reduction of inorganic mercury using stannous chloride in acid medium as reducing agent and the determination of total mercury with the same reducing agent after the oxidation of organomercury compounds as mentioned above [9], [10], [20], [21], [22]. Organomercury species, mainly methylmercury, are determined by difference between total and inorganic mercury concentrations.

Some workers report that sodium borohydride allows the determination of total mercury without prior oxidative treatment in batch systems [23]. The lower concentration of sodium borohydride used and the shorter reaction time required in flow injection systems (FI) compared to those involved in batch systems could be the cause of the low sensitivity obtained for methylmercury in relation to inorganic mercury sensitivity in FI-CV-AAS systems [8], [9], [10], [11], [12], [13], [18], [19].

In this work, a new methodology is proposed for mercury speciation that permits differentiation between inorganic mercury and total mercury in a flow injection mercury system (FIMS) using sodium borohydride as the sole reducing agent. Inorganic mercury was selectively determined with a low concentration of sodium borohydride as the reducing agent. Similar sensitivities were obtained for methylmercury and inorganic mercury when high concentrations of sodium borohydride were used for total mercury determination, avoiding the need for oxidative treatment. Mercury speciation solely based on the use of different concentrations of sodium borohydride has not been previously reported in the literature. All FI parameters were optimized and the FIMS was characterized in relation to its analytical properties. This methodology was then applied to mercury speciation in water samples and validated by the analysis of two fish-tissue certified reference materials (CRMs), two sediment CRMs and one fish-tissue control sample. Furthermore, the effect of different extraction methods on the precision, accuracy and sensitivity of inorganic mercury and total mercury determinations was studied in the solid samples mentioned above. The combination of the proposed FI methodology with microwave-assisted extraction (MAE) methods permitted the rapid and simple determination of inorganic mercury and total mercury in sediments and fish tissues. Methylmercury could be determined by difference between total mercury and inorganic mercury concentrations.