Abstract: Methods and compositions for determining the amount of chemical cross talk r isotopic scrambling, induced by an analytical procedure on a test sample using a reference standard composition containing a predetermined ratio of isotopes is disclosed. The method includes subjecting a reference sample which comprises a predetermined ratio of labeled isotopes to an analytical procedure; determining the degree of isotopic scrambling in the reference sample, subjecting a test sample to the identical analytical procedure; the degree of isotopic scrambling caused by the analytical procedure to the reference sample provides an indication of the presence of chemical cross-talk caused by the analytical procedure on the test sample. In preferred aspects, the cross talk is determined using mass spectral analysis and the reference sample comprises an algae mixture having a predetermined ratio of carbon-12 to carbon-13 isotopes.