# Symantics of parts per (million, billion, etc)

I have always thought that parts per million meant that in a solution of one million particles, one of those particles is the particle that you are looking for its concentration in ppm. However looking at the formula ppm=(mass solute/mass solution)* 1 million, it seems like it may be meaning one part of mass per mass of solution? Doing a example calculation for one atom of mercury in 999,999 molecules of water, I get about 11 parts per million and not the 1 ppm. It seems like the ppX changes with molar mass of the X. Am I missing some key concept here or do I have the right idea. Thanks

(1 atom Hg)(mol Hg/Avogardros#)(20.59g/mol Hg)= 3.3309E-22g Hg = variable A

(999,999 molecules H2O)(mole H2O/ Avogardros#)(18.015g/mol H2O)= 2.9915E-17g H2O=variable B

ppm=(moles solute/moles solution)*10^6 ppm=A/(A+B) *10^6 = 11.13ppm

In chemistry however, $ppm$ is mostly used as concentrations of dilute solutions in water and sometimes used interchangeable with $mg/l$. The notion of "one particle per million particles of solvent" can be used for solid-solid solutions or initial understanding of the term.