How are the mass-based ppm and ppb units defined?

1. Parts per million and parts per billion are units for very low solute concentrations. 2. The mass-based definitions take the ratio of solute mass to solution mass. 3. That ratio is multiplied by 10^6 to give ppm and by 10^9 to give ppb. 4. They work exactly like a percentage, which is a part per hundred, only on a far smaller scale. 5. The denominator is the solution mass, not the solvent mass alone. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.4 Other Units for Solution Concentrations_