It's weird to express a concentration using a weight. Should weight per unit of volume, for instance. However, this means that the following specifications will be given in gramme, milligramme or any multiple of these.
No, not "quantities", but precisely "mass" (or "masses ?).
The sentence would be something like:
"Concentrations expressed in mass of the analysed chemical elements" But then again, expressing a concentration by means of a mass alone is weird, be it in english or french.
If it helps, the whole sentence reads "Le tableau No.2 présente les concentrations exprimées en masse des différents éléments chimiques analysés, ainsi que les fourchettes de composition de l'acier 40CrMoV20".
Unfortunately I can't reproduce the table here, but it is headed "% (g/g)", with the above elements (Cr, Mo etc.) heading the columns.
At first I thought to use "concentrations of the different chemical elements analysed, expressed by weight, together with the composition margins (i.e. tolerances) of steel 40CrMoV20". Any thoughts?