French researchers from Pau investigated the content and speciation of tin and mercury in red wines from many countries by using inductively coupled plasma mass spectrometry (ICP-MS) and gas chromatography coupled to ICP-MS.
As one of the most consumed beverages, wine is heavily regulated by quality standards worldwide. Metallic and organometallic compounds are regulated because of their influence on the quality of the wine (metals are influencing the taste) or the health risk for the consumer. Metals and their compounds can reach the wine because of
- uptake by the plant from contaminated soil or groundwater,
- deposition on the grapes from atmospheric pollutants or pesticide application,
- contamination during the winemaking or storage by apparatus (tanks, tubing, pumps, filters) or auxiliary means (filter media, clearing agents, etc.).
Organometallic compounds can also be produced during the winemaking by natural biotransformation. Some of the relevant species have been studied before, namely alkyllead compounds (Lobinski et al. 1993) and organoarsenic species (Vacchina et al. 2018).
The new study
A group of French researchers from Pau now developed methods based on the use of gas chromatography coupled to ICP-MS for the determination of organotin compounds (OTCs) as well as methylmercury. The researchers aimed at the analysis of red wine samples coming from different parts of the world, to gain an overview of the situation with respect to these organometallic species. A set of 122 samples of red wines were collected with provenance information covering all continents and nearly 50 years of vintage.
For total metal analysis by ICP-MS, red wine samples were digested with nitric acid in polypropylene tubes at 85°C on a heating block.
For mercury speciation analysis, methylmercury was determined by isotope dilution. 1 ml of red wine was spiked with 20 µL of Methyl201
Hg (20 µg Hg/L).
For tin speciation analysis, OTCs were quantified by external calibration with internal standard correction. Therefore, 1 ml of wine was spiked with 50 µL of tripropyltin (TPrT) chloride solutions (10 µg Sn/L). The internal standard was necessary to correct for the interfering effect of the wine matrix on the derivatization yield.
The following derivatization protocol was similar for both methods. Briefly, the pH of the sample was adjusted at 4.5 and ethylation of the species was performed by adding 1 mL of isooctane and 1 mL of NaBEt4 (1 % w/v) solution. The mixture was vigorously shaken for 20 min and the organic layer was collected for analysis by GC-ICP-MS.
For validation of the methods, a pooled wine sample was spiked at two levels, the lower level at close to the double of the natural content, the second level at 5 times higher concentrations. The recoveries ranged from 86% to 113% for both spike levels. The detection power of the two methods were roughly one order of magnitude better than those reported in the literature. In the worst cases, a LOQ of 0.15 µg Sn/l was obtained for triphenyltin (TPhT) and trioctyltin (TOcT), while 0.1 µg Hg/L was obtained for methylmercury (MeHg+).
With respect to OTCs, the most frequently found compounds were the methylated species, mainly as dimethyltin (DMT). The monomethylated species was present in about a quarter of the samples. In about 50% of the samples, monobutyltin (MBT) was present. Phenylated tins were not found. There was no value surpassing the maximum tolerated concentration set by OIV (200 µg/L), and only few species surpassing the lower level of 5 µg/l.
The relation of methylated tin species to total tin concentration was dependent on the wine region. While the degree of methylation was low in European wines, it was higher in American wines.
Methylmercury was not detected in any sample regardless of total Hg concentration.
The authors concluded, that the found species and their concentrations should not cause any harm for habitual consumers.
The original study:
, E. N. Epova, S. Bérail, B. Médina, O. F. X. Donard, F. Séby
, Tin and mercury and their speciation (organotin compounds and methylmercury) in worldwide red wine samples determined by ICP-MS and GC-ICP-MS
, Food Addit. Contam. Part B, (2020) DOI: 10.1080/19393210.2020.1734669
Used techniques and instrumentation:
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