Breakdown of the Isobaric Multiplet Mass Equation at $\mathit{A}=33\mathrm{}$, $\mathit{T}=3\mathrm{}/2$

Mass measurements on ${}^{33,34,42,43}\mathrm{Ar}$ were performed using the Penning trap mass spectrometer ISOLTRAP and a newly constructed linear Paul trap. This arrangement allowed us, for the first time, to extend Penning trap mass measurements to nuclides with half-lives below one second ( ${}^{33}\mathrm{Ar}$: $T_{1/2}=174\mathrm{}\mathrm{ms}$). A mass accuracy of about ${10}^{-7\mathrm{}}$ $\left(\delta m\approx 4\mathrm{keV}\right)$ was achieved for all investigated nuclides. The isobaric multiplet mass equation was checked for the $A=33\mathrm{}$, $T=3/2\mathrm{}$ quartet and found to be inconsistent with the generally accepted quadratic form.