In a recent interesting post, (Is 1 ppb equal to 1 μg/kg?) it was pointed out that IUPAC advises to abandon parts per million, parts per billion quantities and instead it suggests to employ micromole/ mole for ppm and nanomole/mole for ppb. An example is given on page 98 of the Green Book "The volume fraction of helium is 20 ppm".
May be I am missing a point or I see a problem here with the moles in the denominator of these suggested replacement definitions.
a) If we analyze Zr in a rock and find 5 mg Zr in a 20 gram rock, how would one apply the mole definition because one cannot define the moles of a rock. We have to stick to the classical definition:
ppm =10^6 * (mass of analyte / mass of sample).
b) How can one express ppm of Ar in air, and how should we define the moles of air? If we wish to generalize this case, what if our solvent system consists of several components.
The ambiguity I feel in micromole/mole definition is the mole in the denominator. Are we referring to the moles of the solvent or moles of solvent plus solute.
If this replacement is analogous to molal concentration, where the denominator is clearly definied to be kg of solvent not the kg of solvent plus kg of solute, then IUPAC should clearly define it.