What does one ppm represent?
A thousandth of a gram (0.001 g) in 1000 ml is equal to one ppm, while one gram in 1000 ml is equal to 1000 ppm. One thousanth of a gram is equal to one milligram, and one liter is equal to 1000 milliliters, hence 1 ppm is equal to 1 mg/liter.