Abstract: A composition adapted for mass spectral analysis is disclosed as are methods of its use in mass spectral analyses. A contemplated composition contains a mass spectrally-determinable amount of each of (i) at least one analyte to be assayed and (ii) a standard compound. Each of the molecules of the standard compound contains one or the other of a pair of two stable isotopes of the same element that differ in molecular weight by at least two atomic mass units. Those two isotopes are present in the molecules of the standard compound in a predetermined ratio that is other than the naturally occurring ratio of those isotopes.