This paper is an attempt to generalize from my own perspective the information held by nitrogen isotope ratios of major cellular components such as amino acids and chlorophyll, based on results with which I have been deeply involved. Particular attention is paid to compounds that are essential for vital activities as primary metabolites. Cells have an autonomous mechanism, sometimes called homeostasis, that controls the flow of substances within the cell. The nitrogen isotopic ratio of the compound i constituting the cell can be generalized as δ¹⁵N_i=δ¹⁵N_plant +Δ_i(TP-1)+γ_i, where TP represents the trophic position, and Δ_i and γ_i are the trophic discrimination factor and the intracellular ¹⁵N distribution of compound i, respectively. Food web analysis is one of the rapidly expanding applications of the above equation. Such studies are related to such diverse topics as the relationship between environment and ecosystem, the assessment of nitrogen resources, the tracking of fish migrations, and the reconstruction of ancient human diets. The nitrogen isotopic ratio of chlorophyll, which is synthesized from amino acids and has a light-harvesting function in photosynthesis, is useful for studying the nitrogen cycle associated with primary production in the modern and recent ocean. The nitrogen isotopic ratio of porphyrins, the geological derivatives of chlorophyll, provides accurate and precise information on the nitrogen cycle through geologic time. This information may provide constraints not only on the dynamics of nitrogen at the Earth’s surface, but also on the degassing of nitrogen from the Earth’s interior. The importance of isotopic ratios of molecules deeply involved in life from the complex mixtures of nature has been reaffirmed, and they are becoming increasingly important in Earth environmental research.

To achieve position-specific isotope analysis (PSIA) of carbon and hydrogen in hydrocarbons in natural gas, various methods have been developed, including NMR spectrometry, high-resolution isotope ratio mass spectrometry, chemical and enzymatic degradation, or on-line pyrolysis coupled with isotope ratio mass spectrometry. PSIA has been applied to propane in natural gas from petroleum basins and on-land hot springs, as well as to propane generated by laboratory pyrolysis experiments to elucidate the origins of natural gas. Site-preference (SP) value, defined as the difference of δ¹³C values between terminal and central C-atom positions of propane shifts from positive to negative as the maturity of natural gas increases. This change in SP value suggests alterations in the source organic matter of propane associated with thermal maturity or the effects of thermal cracking of propane itself. Position-specific δ¹³C value of propane can also be effective indicators for detecting and quantitatively assessing biodegradation of natural gas. In case that natural gas has abiotic origin, the SP value of propane has a potential to identify different polymerization mechanisms. Joint efforts from different laboratories will be needed to establish a propane standard or to improve analytical methods to verify the accuracy of PSIA data.