What is an “exact mass” and why is it useful?
The exact mass can be used to confirm an elemental composition (i.e., the number of carbons, hydrogens, nitrogens, etc) in a reasonably small molecule (i.e., less than about 1,000 Da on the Micromass Q-Tof API in the Keck Laboratory) produced perhaps from the chemical synthesis of a potential chemotherapy drug. In this instance, one calculates the theoretical “exact mass” by summing up the masses of all the elements expected to be in the compound. This calculated mass is then compared to the experimentally measured mass to determine if they agree within the expected 5 parts per million (ppm) specification of the Q-Tof API – which generally would be acceptable for publication. Measuring the exact mass in these cases is relatively easy because one knows what the answer should be before beginning the mass spectrometry. The reverse process is more difficult. In this case the elemental composition of the sample is not known so one begins by measuring the mass of the sample. Now the working
