Assessment of Mercury Level in Local Polish and Imported Honeys with Use of Direct Mercury Analyzer

Honey is made by either bees from the nectar of plants or the secretions of plant-sucking insects foraging on trees. The raw material for honey production accumulates various chemical components, including heavy metals, and their level is closely related to their content, primarily in soil, but also in water and air (Murashova et al., 2019; Yaqub et al., 2020; Fisher et al., 2022). Consequently, when bees collect honeyflow, they retain most of the pollutants in their bodies, acting as a biofilter, but the remainder passes into the honey (Dżugan et al., 2018; Borsuk et al., 2021). As a result, not only bees, but also honey is used to monitor the cleanliness of the environment (Dżugan et al., 2017; Murashova et al., 2019; Yaqub et al., 2020).

Mercury (Hg), a heavy metal included in the top ten pollutants, has been rarely studied in honeys, but its occurrence can be dangerous for bees and consumers. Mercury cations are toxic, mainly due to their highly oxidizing nature and interaction with sulfhydryl groups which are a component of proteins including cell membranes proteins and enzymes. Such an interaction causes the loss of cell-membrane integrity and inhibition of most enzymatic processes in the body (Vieira et al., 2014). Mercury induces changes in cellular mitochondria, disrupts oxidative phosphorylation and electron transport, and leads to lipid peroxidation and cell death. It also reduces the concentration of glutathione, cysteine and homocysteine, causing oxidative stress, and lowers the activity of ion pumps and ATP synthesis, causing disturbances in ions distribution in the cellular and extracellular space (Brodziak-Dopierała et al., 2021). Methylmercury, the organic form, readily crosses the blood-brain barrier through large transporters of neutral L-type amino acids and, due to its high affinity for lipid structures, accumulates in the brain where leads to neurotoxicity (Creed et al., 2019). Chronic mercury poisoning leads to various neurodegenerative diseases and death (Ferreira et al., 2015; Fischer et al., 2022), and for this reason it is important to control its concentration in foodstuff, including honey.

Hg content in honey is mostly determined by the use of the vapor generation technique coupled to atomic absorption spectrometry (CV AAS), fluorescence spectrometry (CV AFS), inductively coupled plasma optical emission spectrometry (CV ICP-OES) and inductively coupled plasma mass spectrometry (CV ICP-MS) (Ferreira et al., 2015). However, these techniques are solution-based, which means that if the sample is solid it has to be digested before it is introduced to the instrument, and because of the high volatility of mercury, this operation leads to a loss of analyte and poor recoveries if the sample is not digested correctly. The solution is to carry out measurements through direct mercury analysis, a technique for determining total mercury directly in solid, liquid and gas samples, using the principle of thermal decomposition, amalgamation, and atomic absorption (Ferreira et al., 2015). Thus, in this study the usefulness of the direct method of Hg determination with the Direct Mercury Analyzer (DMA) was checked using honeys of different botanical and geographical origin.

The results of the analysis of Hg content in the tested honey samples measured with a DMA-80 automatic mercury analyzer (Direct Mercury Analyzer) are presented in Tab. 1. Among the tested samples in 21 honeys (46%) mercury was not detected. The Hg level in tested honey samples was strongly diversified and ranged from 0.1041 to 0.8119 μg/kg.

The level of mercury contamination of tested samples in terms of honey variety was also observed to highly varied (Fig. 1). Among all tested varieties, coniferous honeydew honey exhibited the highest mercury content (0.463 μg/kg) and buckwheat honey the lowest (0.051 μg/kg on average). However, the observed varietal differences were not statistically significant (p>0.05) probably due to a great variation within a single variety.

Fig. 1.

In the current study, the mercury level was also analyzed taking into account the origin of the sample, and such data clustering was presented in Fig. 2. Honey was divided into Polish (n=20), European (n=12) and blends of honeys from Europe and outside the Europe (n=13). Such grouping of the results showed that honeys from Poland were characterized by the lowest levels of mercury while honeys from European countries (Spain, Italy, Bulgaria, Ukraine) by the highest. Nevertheless, due to the great inner-group variation, no statistically significant differences were found between the groups (p>0.05).

Fig. 2.

Hg content in all tested honey samples was far below the maximum residues permitted in obligatory regulation. The EU Commission Regulation 2018/73 from January 16, 2018 amending Annexes II and III to Regulation (EC) No 396/2005 of the European Parliament and Council specifies that the maximum mercury content in honey in must be 10 μg/kg. Brodziak-Dopierała et al. (2021) who assessed the Hg content in the Polish honeys using DMA found that for all tested samples the range of changes was 0.02–1.55 μg/kg and the arithmetic mean of all samples, regardless of the variety, was 0.37 μg/kg. These results are in line of our observations, and more importantly in our previous study Hg concentrations in all samples of local honey measured by means of the ICP-OES method were found to be under the detection level (<1 ppm) (Dżugan et al., 2018). However, Madras-Majewska et al. (2014) conducted similar research with ICP-OES and the Hg concentration in honeys from Polish apiaries was on average 0.27 μg/kg, which was similar to the values obtained in the present research.

Brodziak-Dopierała et al. (2021) used DMA and determined similarly to the present study that the Hg highest concentration in Polish honeys was 1.02 μg/kg for honeydew honeys successively 0.52 μg/kg for linden honeys, rape, acacia, multifloral and buckwheat honeys. Nevertheless, the levels measured by these authors were higher than in the present study. Moreover, in contrast to present study, differences in Hg content among individual honey types were statistically significant (p<0.05). In our previous research, we found exceptionally high Al levels in honeydew honeys compared to other varieties (Dżugan et al., 2017) and speculate that it is due to its specific origin: the production of honeydew by plant sucking insects, or the specificity of the cellular tree saps from which honeydew comes. However, the Hg values obtained for honeydew honey were still about twenty times lower than the applicable limits.

The Hg contamination of honeys has been reported by foreign authors. Astolfi et al. (2021) tested the mercury level in Italian honeys with the same technique and had values ranging between 0.91 and 3.37 μg/kg. Vieira et al. (2014) compared the usefulness of CV AAS method and DMA in the determination of Hg concentration in Brazilian honeys and found that all thirty-five tested honey samples exhibited Hg concentrations below the LOQ for both methods (2.5 μg/kg for DMA, and 60.0 μg/kg for CV AAS). Toth et al. (2016) studied the Hg content in honeys from eastern Slovakia by AMA mercury analyzer and learnt that honey from the city of Košice had 0.081 μg Hg/kg and honey from the rural areas of Rozhanovce had 0.079 μg Hg/kg. Moreover, no statistically significant differences between the concentration of Hg in honeys from urban and rural areas were found. All these studies confirm that the Hg content in honey measured by DMA is well below the limits and poses no risk to the health of consumers (Fisher et al., 2022).

The above results confirm the usefulness of DMA for Hg determination in honey. The possibility to feed the DMA device a “raw” sample of the tested material significantly improves and speeds up the analysis, as it neither requires a large amount of sample and nor generates high final dilution factors. The cold mercury vapor generated by the pyrolytic decomposition of the sample is captured by the gold sorbent, forming an amalgam. Decomposition products, such as nitrogen or sulfur oxides, halides and others, which may interfere with the absorbance measurement, are previously eliminated on the catalyst. The remaining products of combustion are washed away by the carrier gas (oxygen), and mercury vapor is released into the optical path once by the heating of the amalgamator furnace. This solution allows increased selectivity and removal of possible interference (Vieira et al., 2014; Brodziak-Dopierała et al., 2021).

The main advantage of DMA analysis is the avoidance of a sample preparation. Microwave-assisted digestion is most commonly used for preparing honey for mineral composition determination (Ferreira et al., 2015; Astolfi et al., 2021), but this requires large sample masses and reagent volumes, often leading to high final dilution factors and a consequent increase in the method detection limits (Astolfi et al., 2020; Grainger et al., 2020). In literature, many studies have quantified Hg concentrations in honey matrices with atomic absorption spectroscopy and inductively coupled plasma-mass (ICP-MS) or optical emission spectrometry (ICP-OES) (Dos et al., 2010; Nawrocka et al., 2016; Oroian et al., 2016; Dżugan et al., 2017; Fisher et al., 2022), while the direct mercury analyzer (DMA) is a rather new method rarely used in honey analysis, despite being fast and requiring little operator input (Vieira et al., 2014; Astolfi et al., 2021; Brodziak-Dopierała et al., 2021).

According to the obtained results, all analyzed honeys contained Hg in an amount not exceeding the applicable mercury standard of 10 μg/kg and posed no risk to the health of consumers. Moreover, no statistically significant differences depending on the variety or geographical origin were found. Finally, it was proved that DMA could be used as a quick, precious, and time-saving technique for Hg determination in honey in contrast to the ICP MS/OES methods which require specific sample preparation through wet mineralization which involves high dilution, generating excessively high levels of quantification.