This 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 (K Blaum et al. 2004 Phys. Rev. Lett. 91 260801). This is important if you want "to place constraints on aspects of the weak interaction that are not included in the Standard Model".