Units of Measurement for Physical and Chemical parameters kg/m3%o(by volume or by mass), pg/L, ng/L, ug/L, mg/L, g/L, ppb, ppm, mol/L, eq/L, meq/L The concentration of trace constituents is usuall expressed as micrograms per liter (ug/L) or nanograms per liter(ng/L) For dilute systems, one liter of sample weighs approximately one kilogram, the units of mg/L or g/mare interchangeable with ppm parts per billion"(ppb)and"parts per trillion"(ppt)are used interchangeably with ug/L and ng/LUnits of Measurement for Physical and Chemical Parameters kg/m3 ,%(by volume or by mass), pg/L, ng/L, µg/L, mg/L, g/L, ppb, ppm, mol/L, eq/L, meq/L . The concentration of trace constituents is usually expressed as micrograms per liter (µg/L) or nanograms per liter (ng/L). For dilute systems, one liter of sample weighs approximately one kilogram, the units of mg/L or g/m3 are interchangeable with ppm. "parts per billion" (ppb) and "parts per trillion"(ppt) are used interchangeably with µg/L and ng/L