NBS 19 was prepared by I. Friedman, J. R. O’Neil and G. Cebula  of the U.S. Geological Survey. The reference material NBS 19 (in literature also referred to as TS-Limestone) was obtained from a single slab of white marble of an unknown origin. It was crushed by H. Friedrichsen, University of Tübingen, Federal Republic of Germany. The fraction between 177 μm and 325 μm was bottled . The reference material NBS 19 is intended for 13C/12C and 18O/16O isotope-ratio analysis of carbonates and is used to define the VPDB scale . Therefore these δ-values bear no associated uncertainty. In 2006 new guidelines were published for δ13C calibration, and recommend additionally the use of an assigned δ13C value for LSVEC reference material to obtain an improved two-point calibration .
(R/I/C) Classification assigned to the property value for analyte (Recommended/Information/Certified)
[#] by definition (VPDB scale)
FRIEDMAN, I., O'NEIL, J., CEBULA, G., Two new carbonate stable isotope standards, Geostandards Newsletter 6 1 (1982) 11-12.
HUT, G., Consultants' group meeting on stable isotope reference samples for geochemical and hydrological investigations, Report to the Director General, International Atomic Energy Agency, Vienna, Austria (1987).
COPLEN, T.B., BRAND, W.A., GEHRE, M., GRÖNING, M., MEIJER, H.A.J., TOMAN, B., VERKOUTEREN, R.M., New guidelines for 13C measurements, Anal. Chem. 78 7 (2006) 2439-2441.