A method for assaying phenotypic similarity or dissimilarity between organisms is disclosed in which a composite sample of admixed first and second samples is provided. The first, standard sample contains average concentrations of compounds of molecular mass less than about 1000 AMU present in the organism species. The second, assay sample contains compounds of having a similar molecular mass present in the organism whose phenotype is to be assayed. The constituents of both samples are (i) in a liquid medium and (ii) each compound of a sample has the same, first and second respective amounts of first and second stable isotopes of a first atom. The composite sample is mass spectroscopically analyzed for analytes, with the ratio of first to second isotope being determined for each analyte, along with a composite sample median ratio. The ratios for each analyte are compared to the median, with outlying ratios indicating dissimilarity.