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Innovative technique detects mercury in tuna fish

By combining electricity and chemistry, a group of Universidad Nacional de Colombia (UNal) researchers developed a technique which identifies minimum quantities of mercury in canned tuna. The technique is also applicable to pharmaceutical production and for water quality control.

With this new procedure, researchers may quantify very low mercury levels in canned tuna. “We can detect 0.1 parts per million and in some samples up to 0.1 parts per billion of mercury,” said Universidad Nacional de Colombia (UNal) Chemists and Ph.D. in Sciences Óscar Rodríguez Bejarano, who developed a verified an innovative technique to detect mercury in tuna known as “anodic dissolution voltammetry".

Bejarano has been working on producing electric power using chemical combinations for 15 years and his interest is to standardize a new technique coincided with that of this colleague, Universidad Santo Tomás Professor César Quiñón, who was looking to quantify the amount of mercury in tuna but had not succeeded because the methods to determine trace and ultra-trace levels of mercury was very expensive. Techniques such as atomic absorption –which identifies mercury using light, not electricity– requires equipment costing over US $30 million, while using the electrochemical technique, which is not only simpler to implement, but costs up to five times less.

Tuna testing

The first step to develop this technique consisted in choosing several different brands of canned tuna (six cans each) from three large supermarkets in Bogotá.

The samples were taken to the laboratory to extract oil and liquid within its content. Later it was placed in a glass container (electrochemical cell) and mixed with nitric and hydrochloric acid, a procedure prior to electrical analysis which implied the use of three electrodes.

“When we applied negative voltage (-0.5 and 0.2 volts) to the electrodes, mercury adhered to the electrodes forming a metallic layer. The explanation of this behavior is that the substance gained electrons and adhered to the electrochemical cell. Therefore at higher current levels, greater amounts of mercury adhered,” claims Rodríguez.

Repeating the process, the results verified the discovery of heavy metals, particularly mercury. “Two of the canned tuna brands exceeded a part per million which is the maximum amount allowed by Colombian regulations for human tuna fish consumption, and inclusively y one particular brand surpassed 4 ppm” added Rodríguez.

Further reading (in Spanish) Encuentran altos niveles de mercurio en atún enlatado

Further reading (in Spanish)Grasa del atún, cebo para el mercurio

Foolproof method

The electrical-analytical procedure was tested on numerous occasions, leading to the following conclusions, which showed that the tests were:

  • Sensible: detecting mercury variations in different tuna samples.
  • Accurate: It doesn´t matter how many times the method is applied, it will always produce the same results.
  • Exact: it registers real amounts in comparison to standard mercury samples.

One of the most interesting aspects of the new technique is that it may be applied to natural or residual water and honey samples in order to identify other metallic elements such selenium, or sulphur, which are present in minerals, drugs, nutritional supplements and organic compounds such sildenafil citrate, the active ingredient of Viagra®; Omeprazole, proton-pump inhibitors (PPIs) which diminishes stomach acid production and vitamin C.

According to the researcher, they can analyze between 40 and 50 of the 91 natural elements of the chemical periodic table; the only condition is that they can be oxidized (lose electrons) or reduced (gain electrons) over the surface of an electrode.

Due to the low costs and ease of use, the technique may be applied for tuna quality control by the health authorities; for pharmaceutical products and water quality control as well as other quality control processes.

Despite the enormous usefulness in the analysis of a great amount of ions or molecules, electroanalytical methods are not very popular in Colombia, a situation which Rodríguez cannot really provide an explanation.  “Generally speaking it is not a very popular method among Colombian chemists, just a few groups are devoted to its research,” he added.

Permanent observation

Tuna is a fish which is usually found in the Atlantic and Pacific as well as the Indian Oceans, weighs approximately 100 kilos and is around 2 meters long. It is a natural predator of other fish and shellfish such as lobsters and shrimp among others and for having a high impact of the food chain as it is also a preyed on by larger species of fish.

Although the prior process produces a rich transmission of nutrients, it also produces high mercury levels; therefore the species is closely monitored by the Colombian health authorities in order to avoid exceeding the allowable concentration of mercury by valid regulations.

The last case of excess mercury in tuna in Colombia was this past August 24th  when the National Food and Drug Oversight Institute (INVIMA, for its Spanish acronym) discovered excessive levels of mercury in 3,270 cans of tuna, which ultimately were withdrawn from the market and alerted the Colombian general public to refrain from consuming particular batches of canned tuna.

Although the entity at the moment stated that consuming a sole portion of tuna in these conditions, “does not imply a direct health impact” it did warn that the risk increases in pregnant women and with permanent and prolonged consumption.

Health effects

“The importance of detecting mercury in tuna lies in the adverse effects the metal produces on human beings, which includes damaging the central nervous system, loss of hearing, vision, consciousness and even death,” said UNal Toxicology master´s holder Juan Manuel Sánchez Londoño, in his thesis “Evaluation of mercury concentration in several brands of canned tuna, marketed in the city of Cartagena de Indias”.

However, according to information from the Ministry of Agriculture (quoted in the same thesis), tuna consumption has increased in Colombia. Estimates are that annual tuna consumption is close to 93.6 million tuna cans, equal to 2.6 cans per person, of which 77% is provided by the Colombian industry.

“Therefore it is important to detect small particles of mercury in tuna with sophisticated detection methods so the authorities can control compliance with established levels for the product”, said Sánchez.

Consejo Editorial