Mass Spectrometry is the study of isotopes and their behaviour. Mass Spectrometry differentiates isotopes on the basis of their mass to charge ratio. Isotopic studies are useful, because; although the isotopes have the same electron structure, and hence the same chemical properties, the different masses of the isotope, cause variations in isotopic abundance to develop as they undergo chemical reactions and diffusion. The degree to which these variations exist depend on the pathways and the type of chemical reactions that occur. Consequently the measurement of isotopic abundances, is a useful diagnostic tool, to quantify and identify these reaction pathways and the underlying processes.
Two natural abundance isotopes
Two commonly studied and used natural abundance isotopes are 15N and 13C. These two stable isotopes are significant in the measurement of biological and natural processes, because they are an integral part of the nitrogen and carbon cycling within the environment. Isotopic abundances can be expressed in many ways; but for natural abundance stable isotopes the delta or per mil notation is commonly used, because it is able to express the small differences in isotope abundance that occur within these natural systems.
Additionally since the mass spectrometry is measuring small changes in concentration, the necessity to remove instrumental errors is also addressed by using the delta notation. The delta notation system uses the ratio of the minor isotope to the major isotope of the sample and compares it to a the same isotope ratio of the reference standard.
This means that all the isotope values also have to state what they are being compared to, for example a delta 15N air of +10 means that your sample is enriched in 15N by 10 per mil when compared to the International standard air. A negative delta means that it would be depleted of that isotope when compared to the standard.
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