Question: is ppm by mass or volume?
Answer: If ppm is expressed as THE VOLUME of particles to a unit volume of water, then ppm BY VOLUME is equal to µl/l. However, if ppm is expressed as THE MASS of particles in a unit volume of water, then ppm BY MASS is equal to mg/l. To convert from ppm by volume to ppm by mass, multiply by the density of the particles.