Mass measurements on 3 3 , 3 4 , 4 2 , 4 3 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 Ar : T 1 / 2 = 174 ms ). A mass accuracy of about 10 − 7

( δ m ≈ 4 keV ) was achieved for all investigated nuclides. The isobaric multiplet mass equation was checked for the A = 33 , T = 3 / 2 quartet and found to be inconsistent with the generally accepted quadratic form.