Chemistry: The Molecular Science (5th Edition)

1 ppm= 1 part per million= $\frac{1}{10^{6}}=\frac{10^{-3}}{10^{3}}= \frac{10^{-3}\,g}{10^{3}g}$ ( Multiplying and dividing by gram does not make any difference). Prefix for $10^{-3}$= milli or m Prefix for $10^{3}$=kilo or k Then, 1 ppm= $\frac{10^{-3}g}{10^{3}g}=\frac{1\,mg}{1\,kg}$