A first nutrient medium and a second nutrient medium, said first nutrient medium containing predetermined amounts of first and second isotopes of a first atom within a nutrient, and said second nutrient medium substantially identical to said first nutrient medium but containing different predetermined amounts of said first and second isotopes of said first atom within said nutrient compared to said first nutrient medium, said first and second isotopes being other than H or D, wherein said first nutrient medium and said second nutrient medium are suitable for use in a method for identifying an analyte of a biological organism sample that is affected by a stressor comprising the steps of: (a) providing two substantially identical biological organism samples, the first sample being a control sample and the second sample being an experimental sample, (b) growing the experimental sample in said second nutrient medium with a stressing regimen containing a stressing agent and maintaining said stressing regimen for a time period sufficient for said experimental sample to grow; (c) growing the control sample in said first nutrient medium with a regimen substantially identical to the stressing regimen but lacking the stressing agent and maintaining said regimen for a time period sufficient for said control sample to grow; (d) admixing the two samples to form a composite biological sample; (e) mass spectroscopically analyzing said composite biological sample for analyte peaks; (f) determining the ratio of first isotope to second isotope for each analyzed analyte peak; (g) determining the composite biological sample median isotopic ratio; and (h) comparing the ratio of first isotope to second isotope for each analyzed analyte peak with the composite biological sample median isotopic ratio, an analyte whose isotopic ratio significantly deviates from the composite biological sample median isotopic ratio being an analyte affected by the stressing agent. (a) providing two substantially identical biological organism samples, the first sample being a control sample and the second sample being an experimental sample, (b) growing the experimental sample in said second nutrient medium with a stressing regimen containing a stressing agent and maintaining said stressing regimen for a time period sufficient for said experimental sample to grow; (c) growing the control sample in said first nutrient medium with a regimen substantially identical to the stressing regimen but lacking the stressing agent and maintaining said regimen for a time period sufficient for said control sample to grow; (d) admixing the two samples to form a composite biological sample; (e) mass spectroscopically analyzing said composite biological sample for analyte peaks; (f) determining the ratio of first isotope to second isotope for each analyzed analyte peak; (g) determining the composite biological sample median isotopic ratio; and (h) comparing the ratio of first isotope to second isotope for each analyzed analyte peak with the composite biological sample median isotopic ratio, an analyte whose isotopic ratio significantly deviates from the composite biological sample median isotopic ratio being an analyte affected by the stressing agent.