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.