Delta notation
Delta notation expresses a small isotope ratio difference from a standard in parts per thousand, written δ.
Isotope ratios vary only slightly between natural samples, so scientists express them as differences from an agreed standard rather than raw ratios. In delta notation, written with the Greek letter δ, the difference is given in parts per thousand, or per mil, shown with the symbol ‰.
For example, δ18O is the deviation of a sample's oxygen-18 to oxygen-16 ratio from that of standard mean ocean water, multiplied by a thousand. A value of +5‰ means the sample has 0.5 per cent more oxygen-18 relative to oxygen-16 than the standard. Similar notations are used for hydrogen (δD), carbon (δ13C) and nitrogen (δ15N).
In meteoritics, delta values are everywhere. Organic matter in primitive chondrites often has very high δD and δ15N values, sometimes thousands of per mil, which suggests it formed in extremely cold environments such as the outer disc or the molecular cloud that preceded the Sun.
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