Isotope-ratio mass spectrometry isotopic make up of the sample is obtained. For example, carbon isotope ratios are measured relative to the international standard for C. The C standard is produced from a fossil belemnite found in the Peedee Formation, which is a limestone formed in the Cretaceous period in South Carolina, U.S.A. The fossil is referred to as VPDB (Vienna Pee Dee Belemnite) and has C:C ratio of 0.0112372. Oxygen isotope ratios are measured relative the standard, V-SMOW (Vienna Standard Mean Ocean Water). It is critical that the sample be processed before entering the mass spectrometer so that only a single chemical species
Standard atomic weight publishes "abridged" values (up to five digits per number only), and for the twelve interval values, "conventional" values (single number values). Symbol "A" is a relative atomic mass, for example from a specific sample. To be specific, the standard atomic weight can be noted as , where (E) is the element symbol. The abridged atomic weight, also published by CIAAW, is derived from the standard atomic weight reducing the numbers to five digits (five significant figures). The name does not say 'rounded'. Interval borders are rounded "downwards" for the first (lowmost) border, and "upwards" for the "upward" (upmost) border. This