What are we measuring and reporting?
Ammonia contains atoms of nitrogen (N) and hydrogen (H). Some analysis may report Total Ammonia, as described above, but other analysis refer to the quantity of nitrogen in the ammonia molecule only. In this case the analysis units are presented as total ammonia nitrogen (TAN), which is the sum of NH3 -N + NH4 -N. Results reported in the two forms are not the same. It is necessary to know the percentage of nitrogen in the ammonia molecule to compare the two units. The atomic weight of nitrogen is 14 and that of hydrogen is 1; thus the molecular weight of NH3 is 14 + 3 = 17. Nitrogen is 14/17 or 82% of the weight of the ammonia molecule. The difference is much more important for nitrite and nitrate. Therefore, one (1) mg/L of NH3 is the same thing as 0.82 ppm of NH3 -N. Obviously, we need to know in what "units" we are reporting, so that we can compare values, etc.