|How is ppm related to concentration?
ppm means parts-per-million. But what parts? The amount of particles in a suspension can be expressed as the total volume OR total mass of particles in a unit volume of water AND THESE TWO NUMBERS WILL ONLY BE THE SAME IF THE DENSITY OF THE PARTICLES IS 1 g/cm3.
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. For mineral grains (clay, silt and sand sizes), this will typically be 2.65 g/cm3. For example, a sample with a volume concentration of 25 µl/l will have a mass concentration of 25*2.65 = 66 mg/l.
To convert from ppm by mass to ppm by volume, divide by the density of the particles. For example, a sample with a mass concentration of 100 mg/l will have a volume concentration of 38 µl/l.
When ppm is used as a measure for the suspended particle concentration, it is therefore very important to specify if the concentrations are ppm BY VOLUME or ppm BY MASS, to facilitate comparisons with data where the concentrations are reported in µl/l or mg/l.