See the related link for a conversion chart.
ppm
It is not possible to do so. The term "ppm" refers to parts per million and it requires that the two sets of measurement are the same specified units. For example, a half-and-half mixture by volume, of helium and neon would contain 500,000 ppm of each. But by mass, the same mixture would comprise 165,824 ppm of helium against 834,176 ppm of neon.
ppb is not a weight, but a ratio. "parts per billion"
Parts per Million and Parts per Billion. Parts of a solute per Million parts of the Solvent.
It is a mechanical mixture because of its many parts. :)