doubly-substituted species can implicate the presence of a particular reaction taking place. Photochemistry occurring in the atmosphere has been shown to alter the abundance of 18O2 from equilibrium, as has photosynthesis. Measurements of 13CH3D and 12CH2D2 can identify microbial processing of methane and have been used to demonstrate the significance of quantum tunneling in the formation of methane, as well as mixing and equilibration of multiple methane reservoirs. Variations in the relative abundances of the two N2O isotopologues 14N15N18O and 15N14N18O can distinguish whether N2O has been produced by bacterial denitrification or by bacterial nitrification. Multiple substituted isotopologues Biochemical applications Multiple substituted isotopologues may be used for nuclear magnetic resonance or mass spectrometry experiments, where isotopologues are used to elucidate metabolic pathways in a qualitative (detect new pathways) or quantitative (detect quantitative share of a pathway) approach. A popular example in biochemistry is the use of uniform labelled glucose (U-13C glucose), which is metabolized by the organism under investigation (e. g. bacterium, plant, or animal) and whose signatures can later be detected in newly formed amino acid or metabolically cycled products. Mass spectrometry applications Resulting from either naturally occurring isotopes or artificial isotopic labeling, isotopologues can be used in various mass spectrometry applications. Applications of natural isotopologues The relative mass spectral intensity of natural isotopologues, calculable from the fractional abundances of the constituent elements, is exploited by mass spectrometry practitioners in quantitative analysis and unknown compound identification: To identify the more likely molecular formulas for an unknown compound based on the matching between the observed isotope abundance pattern in an experiment and the expected isotope abundance patterns for given molecular formulas. To expand the linear dynamic response range of the mass spectrometer by following multiple isotopologues, with an isotopologue of lower abundance still generating linear response even while the isotopologues of higher abundance giving saturated signals. Applications of isotope labeling A compound tagged by replacing specific atoms with the corresponding isotopes can facilitate the following mass spectrometry methods: Metabolic flux analysis (MFA) Stable isotopically labeled internal standards for quantitative analysis See also Mass (mass spectrometry) Isotope-ratio mass spectrometry Isotopomer Clumped isotopes Isotopocule References External links Fractional abundance of atmospheric isotopologues, SpectralCalc.com Isotopes