Determination of Mercury Content in Canned Tuna Brands - Literature review Example

? Determination of Mercury content in Canned Tuna brands using Inductively Coupled Plasma Mass Spectrometry Mercury (Hg) is one of the most harmful environmental pollutants due to its toxicity and is accumulation in aquatic organisms. This heavy metal is regarded as one of the most toxic element bearing great impact on human and ecosystem health. Due to population growth and urbanization, more human activities have significantly contribute to the elevated mercury levels in the environment. It has been reported that more than 2500 tons of mercury emissions occur annually through anthropogenic sources. The relative lethal nature of mercury is dependent on its chemical form, methyl mercury as one of the substances existing in the environment. Additionally, this chemical form of mercury is also toxic due to its high liposolubility. However, all the forms of mercury released in the ecosystem undergo biogeochemical transformation processes hence converted to methyl mercury. The main exposure of mono methyl mercury to humans is consumption of fish. Mono methyl mercury (MMHg) represents the major type of mercury in fish. This is because it has the capability of biomagnification in the food chains within the marine. Organomercury compounds may also find their way into the environment from both anthropogenic sources and from production by natural in situ biogenic modifications. Exposure to mercury leads to a variety of signs and symptoms including dizziness, allergy, vomiting and muscular weakness. Furthermore, its toxicity elicits impaired hearing and vision as well as depressed immune system. Eventually, its accumulation within the body leads to brain damage which consequently may lead to death. As it is the case with most anthropogenic, mercury finds its way into the aquatic environment in its inorganic form. Chlor-alkali industry is one of the major pollution source and gateway for mercury (Gao et al.). Consequently, monitoring of the mercury levels within the environment is not enough. Therefore, speciation analysis provides critical in useful information in the assessment of toxicity and health risks of mercury. Furthermore, such analysis helps in understanding the biogeochemical cycling of mercury compounds. As a result, enormous and concerted efforts have been made in developing reliable methods for mercury determination and speciation analysis in biological and fish samples. This review articulates with emphasis special attention to clean sample preparation, good storage sample procedures and performance of the different techniques (Lee et al). Mass spectrometry (MS) can be considered as one of the most important analytical techniques in the analysis of element concentration. It is a method that has also been employed for isotope analysis, surface characterization and structural examination of bioinorganic compounds. In an attempt to understand metabolic pathways of toxic and toxicology in general, strategies have been developed. These novel approaches are important in obtaining qualitative and quantitative information regarding the elements, element species and their interactions. A variety of methods for determination of mercury have been well captured and elaborated in literature. The cold vapor atomic absorption spectrometry (CV AAS) has been the most preferred method in the analysis of mercury especially in foodstuffs. This is because the method has an efficient speed in addition to its simplicity. Moreover, this technique possesses relative freedom from obstructions and also has low operational costs as well as high sensitivity. The high sensitivity is very evident when mercury vapor is pre-concentrated on an amalgamation comprising gold element. Another variant of atomic absorption spectrometry is the graphite furnace technique, commonly abbreviated as GF AAS. This method allows for direct determination of many metals at trace and ultra trace levels in solid samples. Advantages of this technique include high sample throughput and low sample requirement in terms of the sample mass for analysis. In this approach of mercury analysis and determination, a major limitation is high volatility. This is mainly experienced due to the volatile nature of mercury and its compounds. Essentially, there is the need for thermal stabilization so as to avoid analyte losses before the atomization procedure (Krata et al). This technique has been improved in several ways including introduction of various noble metals as well as their mixtures. Other procedures have seen the introduction of modifiers within the furnace and stabilization of mercury. These approaches are geared towards achieving high sensitivity and low recognition limits. Recent advances have seen the development of inductively coupled plasma mass spectrometry (ICP MS) for determination of mercury. This is because the method has excellent detection limits, less than 0.01µg 1?1. However, the method is unable to determine mercury under the normal routine operating conditions. This is majorly attributed to the adhering nature of mercury even at very minimal concentrations of between 1 and 5 µg 1?1 hence leading memory impacts. Such hyphenated systems developed to some extent for specific elemental speciation analysis are gaining research interest. In general, the use of ICP-MS for the identification of molecules is possible when the system is online or offline combined with a separation technique. This is only achievable when the ionization of the molecules does not retain any molecular information. In this case, the advantage of ICP-MS is that the molecular occurrence of the element of concern does not influence the response for a given element. This method displays limited matrix influence, thereby leading to a species-independent response hence stability of elemental species of the same element. The total content of the element to be determined before separation allows for the computation of the portion missed by separation technique. This technique also allows for the calculation of percentage of the individual peak recovered in comparison to the total amount (Meng et al). In quantitative process determination of mercury in fish, external calibration is commonly employed. Such requirements go hand in hand with matrix matched standards that help in reducing effects resulting from the matrix components. Although theoretically possible, in reality adjusting the matrix of the calibration standards to match those of samples is quite cumbersome. Additionally, the content of mercury in the sample is prone to undervaluing due to its extremely volatile nature. In general, it is well in the knowledge that isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP MS) has a high potential or routine analysis of trace elements. This is so especially in circumstances where the accuracy of the outcome is of major analytical importance. As a result, this technique has consistently been used in the certification of elements compositions in certified reference materials (CRMs). However, a few applications have been reported with regards to determination of mercury. This is despite the fact that the technique has been used extensively in the analysis of trace elements in a variety of matrices. The major problem associated with this method in mercury analysis is the predominance in memory effect. Ultimately, this impact largely increases blank counts and further compromises the analytical capability of the technique. Another difficulty related to the inductively couple plasma mass spectrometry in mercury analysis is mercury loss. This is usually the principal impediment during sample decomposition procedure brought about due to its volatility (Hight et al). The amalgamation of chromatographic separation techniques together with specific element detectors is a pragmatic approach or speciation analysis in this important metal. Inductively coupled plasma mass spectrometry and cold vapor atomic fluorescence spectrometry are the most powerful owing to their unmatched sensitivity. Therefore, for hyphenation purposes, the two most critical factors to be considered are separation strength and compatibility with the spectrometer. A majority of separation techniques can be directly joined to atomic spectrometric devices via commercial or conventional interfaces. In most analytical undertakings, gas chromatography is coupled to atomic spectrometry hence a routine technique in speciation analysis. The elevated separation power o gas chromatography and the excellent detectability of current atomic spectrometers become extremely vital in speciation analysis of mercury. Since gas chromatography is used in the analysis of volatile and semi-volatile compounds, the derivatization of mercury to volatile and thermally stable species becomes a significant step. The ethylation of mercury using aqueous solution of sodium tetraethyl borate (NaBEt4) is one of the well accepted methods for mercury speciation analysis. Proceeding ethylation, volatile mercury species can then be preconcentrated by SPME or directly subject to gas chromatography related sensing. However, under circumstances where EtHg + is one of the target analyte, both EtHg + and Hg 2+ can be transferred to the same product which is dimethylmercury. Consequently, the ethylation of mercury by aqueous sodium tetraethyl borate does not serve the role of distinguishing EtHg + and Hg 2+ in tandem. Alternatives to aqueous ethylation include the use of sodium tetraphenylboron (NaBPh4) and sodium tetrapropylborate (NaBPr4). Aqueous propylation of mercury by NaBPr4 faces the challenges of lack of commercial reagents and its unstable nature. Recently, Yan et al. were first to describe the use of butyl magnesium bromide as a derivatization reagent. This is helpful in avoiding the loss of species specific information of the analytes. Furthermore, the review reported the use of Pr3PbCl as an equivalent internally placed standard in order to improve the analytical precision and accuracy in mercury determination. Moreover, another variant of thermo-diffusion interface (TDI) has been designed and assembled for increased efficacy in the transportation of analytes derived from GC to ICP-MS. This major development has been successfully applied in the speciation analysis of mercury simultaneously. Major milestones have been achieved in the development of the GC-based speciation techniques. These are implementation of species specific isotope dilution and association of GC-ICP-MS /AFS. In such a combination, the preconcentration and matrix separation techniques are based on head-space SPME or purge and trap technologies (Meng et al ). The examination of stable isotopic fractionation of mercury is capable of providing a powerful tool for tracking mercury conversions in biogeochemical cycle. The amalgamation of GC with multicollector ICP-MS opens a new door or the analysis of species specific stable isotope of mercury. Conversely, limited analyte quantities and isotope ratio drift during analyte transient passage are limiting factors. This calls for development of efficient methods in obtaining precise and accurate species specific mercury isotope. A new method has been reported by Epov et al. where species specific isotope values can be obtained from consecutive GC transient signals. This is achieved by coupling gas chromatography with a commercially available inductively coupled plasma mass spectrometry. In order to gain optimal counting statistics, the use of isothermal temperature programs to broaden elution of mercury species is critical. Other crucial components include proper selection of peak integration window and the preconcentration of the real samples. High performance liquid chromatography (HPLC) is a powerful separation technique which offers competitive advantages compared to gas chromatography. This separation technique can be applied directly to non-volatile compounds of low and high molecular weights. This capability provides adaptability in analytes derived from anion exchange, reverse phase and size exclusion methods. Consequently, it can be easily on line coupled to an element specific detector for detection of mercury in biological and environmental samples. In studies involving mercury toxicity, metabolism and transmission in organisms, size exclusion chromatography (SCE) is used. This separation technique is connected to inductively couple plasma mass spectrometry for the sole purpose of mercury determination in such samples. SCE-ICP-MS is a highly superior technique in examining mercury binding biomolecules. Therefore, maintaining the species integrity during the analysis procedure is very paramount (Reyes et al.). In comparison to all the separation techniques, capillary electrophoresis is a relatively recent and evolving technique still under development. Nonetheless, this technique has already portrayed immense potential for mercury speciation analysis. Among its principal features include high separation efficiency, and minute sampling volume. On the contrary, its sensitivity, precision and reproducibility for mercury speciation are on the lower side compared to GC-HPLC related techniques. As a consequence of its small sample injection volume, the selection of almost similar detector is very critical. Deng et al. have reported the use of an on-line coupled CE cold vapor generation with electrothermal quartz. In this combination, CE –AAS mercury speciation is achieved at lower cost, simplicity in structure, and easy operation. Additionally, this technique offers a low dead volume, possesses good conductivity, good selectivity of AAS as well as good gas-liquid separation efficiency. In yet another strategy, Li et al. devised an attractive strategy for mercury throughput and faster speciation analysis. This was enabled by connecting the CE through a short column to inductively couple plasma mass spectrometer. In order to increase the nebulization efficiency, a micro mist nebulizer is incorporated. Furthermore, buffer contamination is reduced or eliminated by using a laboratory made removable SC-CE-ICP-MS interface. The use of capillary electrophoresis in studies involving real mercury speciation analysis is limited compared to gas chromatography and HPLC. Conversely, this technique remains promising as a powerful tool in provision of important information regarding the interaction of various mercury species with other biomolecules (Hight et al). Isotope dilution mass spectrometry is one of the most reliable analytical techniques used in mercury isotope analysis. This is because o its high sensitivity, and accuracy, little influence of matrix effects, loss of spiked samples on analytical results and shift of instrumental signals. Traditionally, thermal ionization mass spectrometry (TIMS) and spark source mass spectrometry (SSMS) have been applied for analysis o isotope ratio. However, recent studies have shown that determination of elements by isotope dilution inductively coupled plasma mass spectrometry has been advanced. The technique offers easiness in sample preparation, rapidness of analytical procedure and convenience of connection to another technique. Isotope dilution mass spectrometry is an analytical method based on the measurement of the isotope ratio in a sample by mass spectrometry. In this setting, the element’s isotopic composition has been changed by the addition of a known amount of a spike or an enriched isotope. Among the pre-treatment steps required in performing mercury species determination and isotope analysis include sampling and storage procedures. This is a critical stage because it assists in the maintenance of species information during the analytical process. It’s imperative to note that this step can take two strategies so as to achieve this one objective. In the first case, species preservation may keep the entire chemical species of interest unaltered during the analysis process. Secondly, the species may be quantitatively modified chemically into suitable derivatives for separation, accumulation and quantification. Pragmatically, both of these strategies are usually employed. In such an undertaking, the chemical stability and volatility of the analytes can be considered as crucial as all sampling procedures. However, during the storage step, a couple of setbacks may interfere with the analytical process. One drawback is degradation which is dependent on the chemical nature of the species. The degradation problem may also be influenced by biochemical processes such as the activity of enzymes especially in canned seafood. Temperature is also another important parameter in determination of chemical reactions; hence it can also dictate the rate of transformation of chemical species. Consequently, it is essential to reduce the temperature so as to decrease the rate of species transformation. Stabilization of the sample is also critical and is mainly conducted through drying. However, there is no standard method for this purpose. Lyophilization also simply referred to as freeze drying is one of the most commonly employed procedures in determination of trace elements. Freeze drying allows the removal of solvents and water from the sample through a sublimation process. Extreme caution should be taken so as to avoid loss of volatile compounds within the samples. Additionally, shock freezing of desired samples via gaseous phase in liquid nitrogen can be used as a safe method. It’s a preferred option that can be done at sampling site and also because it prevents changes in chemical species (Resano et al). Preparation of sample is a key step or accurate determination of mercury in various sample matrices. Preparation of sample is basically underpinned as a step undertaken with a view of manipulating it so as to modify the sample matrix. The main objective in sample preparation is to convert the sample into a more suitable form so as to enable proper and efficient analysis. Sample preparation is usually achieved by reducing the heterogeneity within the sample at molecular level. Conventional preparation of sample methods comprise of several steps with isolation of the analyte being the most critical. Problems associate to sample preparation include loss of samples and sample contamination. Further setbacks in this critical step comprise of long time required for leaching and large consumption of solvents. Strategies have been developed for sample preparation and are exemplified by extraction, solubilization, leaching, decomposition and ashing. In the determination of total mercury in a sample, there is the need for sample digestion prior to analysis. This procedure aids in the decomposition of organic compounds that are in the sample matrix (Marcia et al). Microwave-induced combustion, oxygen flask combustion as well as sample preparation with tetraethyl ammonium hydroxide (TMAH) solubilization have been used in mercury determination. Microwave-assisted acid digestion offers a rapid sample decomposition and high recovery of analyte. However, the advancement of on-line and automated sample preparation methods has offered promising options. These developments in sample preparation are aimed at reducing sample analytical time and simplify pretreatment process. Han et al. elaborated an automated pressurized digestion methodology on the basis of an electromagnetic induction acid assisted heating for on-line decomposition of solid samples. In such an advanced technique, there is achievement of high digestion efficiency due to the high pressures and temperatures. Leopold et al. further used the extraordinary oxidation power of bromine monochloride to develop a fully automated flow injection system for the transformation of mercury. Additionally, slurry sampling without complicated sample preparation process offers a good alternative for conventional digestion of many types of samples. Aqua regia solution can be used as the solid dispersion reagent for determination of mercury by cold vapor generation atomic fluorescence spectrometry. Another dispersing solution that can be used is hydrochloric acid which is mostly used in cold vapor inductively coupled plasma mass spectrometry (Reyes et al). For mercury speciation analysis, microwave or sonication induced acid leaching or alkaline dissolution methods are commonly preferred. This is because such techniques offer simple devices and high efficiency in terms of extraction. By using a concoction of mercaptoethanol, hydrochloric acid and L-cysteine, rapid sonication of seafood is achieved within 15 minutes. Rodriguez and colleagues established a fast sample preparation method for mercury species establishment. In their approach, microwave induced mercury extraction with TMAH was used. Only 10 minutes is required for the sample pretreatment procedure thereby saving a lot of time and hence high sample throughput. Compared to other microwave/sonication assisted mercury extraction protocols, the TMAH extractor is most efficient. This strategy offers simultaneous extraction of both ethyl mercury and methyl mercury at rapid rates and with minimal time consumption. The synchronized application of ultrasonic probe and ultrasonic bath in sample preparation can increase rate of extraction and improve efficiency in the extraction. In an attempt to reduce the time required for sample preparation, Torres et al. developed a total and inorganic mercury determination protocol. In this approach, samples are prepared as slurries in water instead of inorganic acids and further quantification is carried using CV-AAS. There is no requirement in standing time prior to mercury determination. Consequently, there is massive decrease in sample preparation time as well as reduction of probabilities in sample contamination. For measurement of total mercury in the samples, rapid oxidation of the sample analyte is undertaken using potassium manganate (KMnO4). The unprecedented emergence in metallomic research stipulates more gentle digestion so as to preserve complete molecular information. As such, enzymatic hydrolysis which has the ability of splitting analytes from macromolecules becomes an alternative method in mercury sample preparation. It involves the use of enzymes in sample solutions and these catalysts must not alter chemical forms of the elements. Lemes et al. came up with a technique to determine methyl mercury cysteine complexes, methyl mercury glutathione complexes as well as MeHgX and inorganic HgX in seafood. On further advancement, Chung et al. employed the use of pancreatin and hydrochloric acid digestion. This was specifically for the determination of both methyl mercury and ethyl mercury in fish muscles. This is achieved via gas chromatography-inductively coupled plasma mass spectrometry. However, this method of enzymatic hydrolysis suffers setbacks of long sample preparation time of between 5 to 24 hours. Digestion of the elements through enzymatic hydrolysis also has the drawbacks of portraying low recovery of the analytes especially for mercury determination (Centineo et al). Due to the extremely low concentrations of mercury in many samples and also because of is potentially complicated matrix, preconcentration is critical. This should be done prior to measurement and demands for sophisticated and sensitive detection platforms. In the process of separation or after the process, interfering compounds have to be removed or inactivated. These compounds can be categorized as those substances that are present in the sample and interact with metal species present in the cell or tissue. Sample cleanup is especially important in analytical separations of gas and liquid chromatography as well as electrophoresis. Solid matrixes and biological sample scan contain many compounds such as biomolecules and hydrocarbons. This makes it cumbersome to identify analytes present at lower concentrations when interfering species are present. Therefore, it is essential that sample cleanup be conducted prior to analytical measurement. Methods employed in sample cleanup have included Lyophilization, gel filtration, gel permeation chromatography and centrifugation. More methods used in cleanup are solid phase extraction (SPE), column chromatography and ultra filtration (Burger et al). Mester et al. reported the coupling of SPME with ICP-MS for mercury species determination. In this undertaking, they used an SPME utilized fiber which was coated with PDMS/DVD co-polymer. They tried both headspace extraction and liquid immersion on methyl mercury species solution that was previously saturated with sodium chloride. The method was based on the relatively high vapor pressure resulting from methyl mercury chloride. The introduction of sample into the inductively coupled plasma-mass spectrometer was achieved by unique thermal desorption interface. The coupling of graphite furnace to an inductively coupled plasma mass spectrometer can have significant improvements. Therefore, the use of electrothermal vaporization (ETV) ICP-MS has been studied for this case. Although this procedure may have satisfactory outcome in some occasions, it normally requires additional sample pretreatment and is limited to various sample types. A major consideration to put in place while developing direct sold sampling method is how to achieve a reliable and universal calibration procedure. One of the principal advantages of solid sampling by ETV-ICP-MS compared to other sampling techniques is the probability to carry out direct calibration. Additional strategies to accomplish this objective are well articulated by Belarra et al. However, when mercury is the target analyte, obtaining a selective vaporization is a very complicated requirement. Mercury species are usually volatile. Furthermore, mercury may be present in various chemical forms, thereby displaying varied volatilities. For example, MeHgCl sublimates at 167?C while HgO decomposes at 500?C, which further makes the improvement of a suitable temperature procedure complex. In principle and common among samples containing organic elements, a typical approach is to get rid of most matrix components during pyrolysis while retaining mercury using a suitable modifier (Kuban et al). Oscillating B 6•103 K produced by RF current in Coils. B Produces Oscillating e andAr+flow In a majority of the analysis, the mean atomic mass suffices when there is the need to convert between number and mass. Since mass spectrometry separates ions by mass and atomic number, it is possible to generate a signal for every isotope detected when the mass analyzer resolution is high enough. Therefore, measuring the isotope ratios especially in mercury and other elements is an important method for fingerprinting hence identifying the precise source (Serafimovski et al.). References Adams, D. Total mercury levels in tunas from offshore waters of the Florida Atlantic coast. Marine Pollution Bulletin , 2004; 659–667. Burger, J., & Gochfeld, M. Mercury in canned tuna: white versus light and temporal variation. Environmental Research , 2004; 239–249. Centineo, G., Gonzalez, E., Alonso, I., & Sanz-Medel, A. Isotope dilution SPME GC/MS for the determination of methylmercury in tuna fish samples. Journal of Mass Spectrometry , 2006; 77-83. Diez, S., & Bayona, J. M. Determination of Hg and organomercury species following SPME: A review. Elsevier-Talanta , 2008; 21–27. Gao, Y., Shi, Z., Long, Z., Wu, P., Zheng, C., & Hou, X. Determination and speciation of mercury in environmental and biological samples by analytical atomic spectrometry. Microchemical Journal , 2012; 1-14. Hight, S., & Cheng, J. Determination of methylmercury and estimation of total mercury in seafood using high performance liquid chromatography (HPLC) and inductively coupled plasma-mass spectrometry (ICP-MS): Method development and validation. Analytica Chimica Acta , 2006; 160–172. Krata, A., & Bulska, E. Critical evaluation of analytical performance of atomic absorption spectrometry and inductively coupled plasma mass spectrometry for mercury determination. Spectrochimica Acta , 2005; 345– 350. Kuban, P., Houserova, P., Kuban, P., Hauser, P., & Kuban, V. Mercury speciation by CE: A review. Electrophoresis , 2007; 58–68. Lee, S. H., & Suh, J. K. Determination of mercury in tuna fish tissue using isotopedilution- inductively coupled plasma mass spectrometry. Microchemical Journal , 2005; 233– 236. Marcia, M., Hartwig, C., Bizzi, C., Pereira, J., Mello, P., & Flores, E. Sample preparation strategies for bioinorganic analysis by inductively coupled plasma mass spectrometry. International Journal of Mass Spectrometry , 2011; 123– 136. Meng, W., Yu, Z., Feng, W.-Y., Ming, G., Bing, W., Jue-Wen, S., et al. Determination of Mercury in Fish by Isotope Dilution Inductively Coupled Plasma-Mass Spectrometry. Chinese Journal of Analytical Chemistry , 2007; 945–948. Nevado, J., Mart??n-Doimeadios, R. C., Bernardo, F., & Moreno, M. J. Determination of mercury species in fish reference materials by gas chromatography-atomic fluorescence detection after closed-vessel microwave-assisted extraction. Journal of Chromatography , 2005; 21-28. Resano, M., Gelaude, I., Dams, R., & Vanhaecke, F. Solid sampling–electrothermal vaporization–inductively coupled plasma mass spectrometry for the direct determination of Hg in different materials using isotope dilution with a gaseous phase for calibration. Spectrochimica Acta , 2005; 319– 326. Reyes, H., Rahman, M., & Kingston, S. Robust microwave-assisted extraction protocol for determination of total mercury and methylmercury in fish tissues. Analytica Chimica Acta , 2009; 121–128. Serafimovski, I., Karadjova, I., Stafilov, T., & Cvetkovic, J. Determination of inorganic and methylmercury in fish by cold vapor atomic absorption spectrometry and inductively coupled plasma atomic emission spectrometry. Microchemical Journal , 2008; 42–47. Read More