• Atomic Weights of the Elemets 2009 (IUPAC)

• Advanced Chemistry Development

• Galactic Industries

• Isotope Project at Berkeley Laboratory

• MassBank

• MINT

• NIST/EPA/NIH

• PNNL Biological MS Data & Software

• Stan Pesticide Library, @ Agilent

• Tables of Isotopic Masses & Natural Abundances (Wikipedia) search for "Natural Abundance", "Isotopes" & individual elements

• WebElements Periodic Table

• Wiley Registry

History of NIST/EPA/NIH Mass Spectral Data Base. Heller, S.R., "The histroy of the NIST-EPA-NIH Mass Spectral Database," Today's Chemist at Work, 8, pages 45-50, (February 1999). Publication of the predicted molecular properties of 12 million chemical structures, see Advanced Chemistry Development and Chemical Abstracts Service.