The L part is assumed to have density of 1 (kg/L) and therefore be 1kg mass (mostly water). 1 kg = 1 trillion ng. Because the vast majority of the sample is wa

The L part is assumed to have density of 1 (kg/L) and therefore be 1kg mass (mostly water). 1 kg = 1 trillion ng. Because the vast majority of the sample is water the number of nanograms of the substance per nanograms of water is approximately the number of parts per trillion. As you get into higher concentrations of solute you have to consider density of the solute and water as separate parts. In the ng/L or ug/L range you can hand wave the L as 1 trillion parts (by mass) and the ng as the parts out of a trillion in the full volume of sample.