Abstract: A method for identifying a biological analyte that is affected by a stressor is disclosed in which two substantially identical biological samples are provided, with a first sample being a control sample and a second sample being an experimental sample. The control sample is grown with a nutrient having an isotope of a first atom, whereas the experimental sample is grown with a nutrient having a second isotope of the first atom. The experimental sample is grown with a stressing agent and regimen. The samples are admixed, and the formed composite is mass spectroscopically assayed for analyte peaks. The ratio of first isotope to second isotope is determined for the peaks, as is a sample median isotopic ratio. The ratio for assayed analyte peaks is compared with the median ratio. An analyte whose isotopic ratio significantly deviates from the median ratio is an analyte affected by the stressing agent.

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.