Yeah, its Tin and Fluorine mixed to creat Stannous flouride, the point was that the Fluorine in parts per milliion affected the characteristics of the metal, i.e. poisoned it.
Re.

Stannous fluoride, also known as tin(II) fluoride, is the chemical compound with the formula SnF2. This colourless solid is a common ingredient in toothpaste. In the enamel, it converts apatite into fluoroapatite, which is more resistant to attack by acids generated by bacteria. Sodium fluoride and sodium fluorophosphate perform similarly. Stannous fluoride can be mixed with calcium abrasives while the more common sodium fluoride gradually becomes biologically inactive combined with calcium.[1] It has also been shown to be more effective than sodium fluoride in controlling gingivitis.[2]

ITS the weekend class dismissed....Woody