A physicsweb.org article states that an international team working at the ISOLTRAP mass spectrometer at CERN has determined the masses of two isotopes of argon (³²Ar and ³³Ar) with the highest precision ever. This is important if you want "to place constraints on aspects of the weak interaction that are not included in the Standard Model".1

• 1. Masses of Ar-32 and Ar-33 for Fundamental Tests,
  Blaum, K., Audi G., Beck D., Bollen G., Herfurth F., Kellerbauer A., Kluge H. - J., Sauvan E., and Schwarz S.
  , Phys. Rev. Lett., 12/2003, Volume 91, Number 26, p.260801, (2003)