USGS40 was prepared by Qi et al.  from commercially available L-glutamic acids. Its recommended δ13C value was determined in an international calibration .
(R/I/C) Classification assigned to the property value for analyte (Recommended/Information/Certified)
QI, H., COPLEN, T.B., GEILMANN, H., BRAND, W.A., BÖHLKE, J.K., Two new organic reference materials for δ13C and δ15N measurements and a new value for the δ13C of NBS 22 oil, Rapid Commun. Mass Spectrom. 17 (2004) 2483-2487.
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.