The plural of "noun of noun", three types of combination

To me,

• "concentration of heavy metals" means "the overall concentration of all heavy metals taken as a collective in a single sample"
• "concentrations of heavy metals" means either "the collection of the individual concentrations of each heavy metal in a single sample" or "the collection of the concentration(s) of heavy metal(s) in multiple samples"
• "Concentrations of heavy metal" seems to mean only "the collection of the concentration(s) of heavy metal(s) in multiple samples." Like @Lawrence above, I'm surprised at the use of the singular in this case, though, given the way "heavy metal" is usually used to describe a particular elements membership in a class. Maybe if it were referring to the musical genre?

I am not a chemist.