Editor’s note: Dr. Daniel Kutscher, Applied Analytical Technology, Thermo Fisher Scientific, contributed to this blog post.
Heavy metal contamination of fish
While toxic metals such as arsenic, cadmium or lead are concerning in a variety of sample types, they exist only in the lower trace range in fish, whereas the element mercury is the most problematic heavy metal, primarily as a consequence of contamination in marine ecosystems.
Following release into the environment, commonly as inorganic mercury (elemental or Hg2+), aquatic microorganisms may biomethylate mercury and hence change its chemical form (often referred to as species) to methyl mercury, which is of great toxicological concern. Methylmercury is highly bioavailable and may also cross the blood-brain barrier in humans. Methylmercury also forms stable chemical bonds with thiol groups ꟷ for example in free cysteine residues ꟷ often found in proteins. This bond is extremely strong and cannot be broken, for example, by heating. According to the literature, methyl mercury constitutes 90% of the total mercury in tuna fish. Although people could be exposed to mercury from foods, drinks and even air, eating contaminated fish is the most important cause of mercury poisoning, according to the U.S. Environmental Protection Agency report. [1]
Which fish accumulate methylmercury?
The degree of mercury accumulation in fish depends on their position in the food chain, to a lesser extent also on the fishing area, but especially on age at time of catch. Fish that occupy a lower position in the food chain and are comparatively fast-growing are less contaminated. These include well-known edible fish such as cod, saithe, hake and herring. Fish from aquacultures are usually free of mercury or show only extremely low levels of heavy metals. However, large, old predatory fish in particular occupying a high trophic position at the end of the food chain, can have increased levels of mercury as a result of years of accumulation (age accumulation). These include large specimens of fish species such as swordfish, sharks, marlin, spearfish, and large tuna.
Other toxic metals, such as lead, do not usually play a role in fish and fishery products; only in mussels and mussel products have higher levels been determined in individual cases, but the applicable maximum levels were not exceeded. [2] In some cases, increased levels of cadmium have been detected in squid products. Cadmium in squid is mainly concentrated in the innards. The reason for the occurrence of elevated cadmium levels may be contamination of the edible parts with offal, and thus an expression of improper processing.
Tests at the IFF Cuxhaven
In the years 2020 to 2022, out of 1,299 tests for the legally regulated elements mercury, lead and cadmium, 11 results were above the legal maximum value (6 occurrences mercury and 5 cadmium).
The lower number of complaints regarding mercury (compared to previous years) is due to the fact that large predatory fish species that are conspicuous in this context ꟷ such as swordfish, shark, marlin and spearfish ꟷ are now less readily available on the market due to the historical higher levels of contamination. As a result, fewer samples are sent in for testing by food control authorities.
The complaints in detail
Mercury (Hg)
1 sample of tuna: Hg 1.29 mg/kg (HW* 1.0 mg/kg)
1 sample of shark: Hg 1.0 mg/kg (HW* 1.0 mg/kg)
2 samples swordfish: Hg 1.16; 1.38 mg/kg (HW* 1.0 mg/kg)
1 sample black marlin: Hg 1.79 mg/kg (HW* 1.0 mg/kg)
1 sample bream: Hg 0.31 mg/kg (HW* 0.3 mg/kg)
* HW = legal maximum value according to Regulation (EU) 2023/915 as amended
How is mercury in fish analyzed?
Simply measuring the total amount of an element in a sample does not provide the complete picture in the case of elements that may be present in different chemical forms, e.g., mercury. Arsenic may also be found in different chemical forms depending on the sample type. Whereas rice mostly contains the inorganic species (As (III) and As (V)), fish and seafoods again accumulate high amounts of arsenic, but in the form of organic molecules, predominantly arsenobetaine (the arsenic analog of trimethylglycine), which is significantly less toxic.
To determine a sample’s safety or toxicity, there is a need to identify and quantify the different species of an element that might be present in a sample. The method of choice for methyl mercury is often gas chromatography (GC) coupled with inductively coupled plasma mass spectrometry (GC-ICP-MS).
Methyl mercury is analyzed coupling a GC to an ICP-MS, as shown in Picture 1.
By seamlessly linking GC and ICP-MS systems, even applications with the most demanding detection limits can be easily implemented. The interface of the GCI series enables simple, reliable and dynamic sample measurement down to the sub-ppt range.
Learn more about Thermo Fisher`s solutions for speciation analysis by watching the following video.
Or watch the free webinar about speciation.
Sensitive detection of trace elements with ICP-MS
When sensitivity is key, inductively coupled plasma mass spectrometry (ICP-MS) is the preferred analytical solution for trace element determination down to ppt levels. Modern ICP-MS instruments offer comprehensive interference removal and innovative solutions to increase sample throughput. Single quadrupole ICP-MS systems, such as the Thermo Scientific™ iCAP™ RQ ICP-MS, are commonly used in many laboratories, combining consistent interference removal with short measurement times, enabling robust, high-throughput analysis. Interference removal can be accomplished by using a collision/reaction cell (CRC) to remove the most common polyatomic interferences.
A more advanced way to overcome more challenging interferences is to use triple quadrupole ICP-MS (TQ-ICP-MS) instrumentation, such as the Thermo Scientific™ iCAP™ TQ ICP-MS, which offers routine removal of all types of interferences, including doubly charged ions. Triple quadrupole ICP-MS is a viable tool for the routine analysis of environmental water samples, such as groundwater, surface water and wastewater. The Reaction Finder method development assistant enables fast method development for interference-free analysis even for inexperienced users.
Please ask us about special offers for ICP-MS and ICP-MS/MS for a limited time at analyze.eu@thermofisher.com.
References
[2]