Editing Molecular mass (section)

=== Mass spectrometry ===
{{main|Mass spectrometry}}
In mass spectrometry, the molecular mass of a small molecule is usually reported as the [[monoisotopic mass]]: that is, the mass of the molecule containing only the most common isotope of each element. This also differs subtly from the molecular mass in that the choice of isotopes is defined and thus is a single specific molecular mass out of the (perhaps many) possibilities. The masses used to compute the monoisotopic molecular mass are found in a table of isotopic masses and are not found in a typical periodic table. The average molecular mass is often used for larger molecules, since molecules with many atoms are often unlikely to be composed exclusively of the most abundant isotope of each element. A theoretical average molecular mass can be calculated using the [[standard atomic weight]]s found in a typical periodic table. The average molecular mass of a very small sample, however, might differ substantially from this since a single sample average is not the same as the average of many geographically distributed samples.