In the modern environment, the determination of total mercury content in food, plant, and animal tissue is not enough. Often, the analyst needs to differentiate between various inorganic and organic species, such as elemental mercury (Hg0), divalent inorganic mercury (Hg2+), methylmercury (CH3Hg+), and dimethylmercury ((CH3)2Hg). The reason is that the levels of toxicity can vary by orders of magnitude, where the organic form is much more toxic. Both natural and anthropogenic sources of mercury exist, including volcanoes, metals smelters, and seafood. During the biogeochemical cycle, mercury can undergo several transformations that can be natural, for example, some bacteria transform inorganic mercury into more toxic organic methylmercury, and some plants can absorb vapour mercury and transform its species.
The sample preparation and further digestion are complicated by the high volatility of mercury and its species. To avoid false negative results, the appropriate sample preparation and digestion procedures have to be performed.
A comprehensive review of analytical methods for mercury speciation analysis has been completed by a research team (Ferreira et al., 2015). Major methods for sample preparation and acid digestion techniques as well as different instrumentation have been tested and analysed for their advantages and disadvantages. Further, the scientists analysed different samples matrices, such as nuts, mussels, canned fish, rice, infant foods, fishes, honey, herbs, and milk.
The Direct Mercury Analyses , slurry sampling, and solid sampling were compared. The main advantages of such sample introductions included:
- reduction of error associated with acid digestion (lower risk of contamination from reagents, less error from sample mass and volume of solution),
- decreased the loss of mercury due to lower temperature of analysis, and
- better speciation analysis.
Therefore, a major disadvantage is small sample size, which can be a misrepresentation of the bulk material. To address this, the sample analysed has to be remarkably homogeneous.
The following digestion techniques were analysed:
- closed vessels and thermal heating
- "Cold finger" (a process when a glass tube is put on top of the digestion tube causing condensation of vapours a reflux)
- microwave heating
- Vigreux column
- microwave induced combustion (can be used for relatively big sample masses up to 500 mg)
Several detection techniques were considered, including:
- Neutron activation analysis(INAA): Although the technique is applicable, it was shown that during sample drying and irradiation, 23-49% of mercury can be lost and therefore pre-treatment with materials like L-cysteine is required.
- Cold vapour atomic absorption spectrometry (CV-AAS)
- Inductively coupled plasma mass spectrometry (ICP-MS)
- Gas chromatography mass spectrometry (GC-MS)
- Cold vapour atomic fluorescence spectrometry (CV-AFS)
- Flow injection analysis system cold vapour atomic absorption spectrometry (FIAS-CV-AAS)
- Hg analyser
Mercury speciation analysis was successfully performed using chromatography, atomic fluorescence, and emission spectrometry as well as mass spectrometry techniques. Overall, further method development is required to confirm standard procedures.
For more details on the analyses discussed, please refer to the original publication here.