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

Just wondering what's the plural form of "noun of noun". Should the 's' be added to the first noun or the second, or both?

Example: we will examine the 'concentration of heavy metal' in this paper.

concentrations of heavy metals. Does this indicate each heavy metal has more than one concentration?

concentrations of heavy metal. Only one type of heavy metal, and has many concentrations?

concentration of heavy metals. Does this indicate there are many types of heavy metals and each has only one concentration?

I found the 3 types of plural forms all exist in academic papers.

Solution 1:

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.

Solution 2:

"Concentration of heavy metals" Concentration is the amount of a substance in a solution. Therefore, concentration of heavy metals should refer to a SPECIFIC AMOUNT of a substance in a solution of the metals. However, "Concentrations of heavy metals" may refer to an UNSPECIFIC AMOUNT of a substance in a solution of the metals. This's my understanding as a Chemist. Thanks.