thank you for your answer. I saw the reports that Tipco did on the tin content of the cans, but they are slightly contradictory, because in one report the tin content increases homogeneously during shelf life, in the other report this value increases only a bit at the end of shelf life. So the two studies are not telling the same; besides one is telling exactly what I experienced, that is that the tin content increases proportionally during the shelf life. Moreover after 36 months the tin is over 200 ppm, that is European limit. For me, I think that tin increases during shelf life, but it depends on the initial quantity, which may depend on the age of the tin, and on its grade of oxidization.
Anyway, we will check the tin quantity in that lot in six months time, to evaluate the tin concentration.
Only a question: don't the suppliers check the tin content of their lots, at least on some samples? Do they only trust the statistic evaluation they did?
Please tell the suppliers to keep at least 4 or 5 samples per lot produced, in case of any complaints or problems