The capture of externally produced highly charged ions in a Penning trap and the observation of their cyclotron frequencies is described. The ions were produced in an electron beam ion source at a 2.8 kV potential. Injection was made possible by a pretrap, in which these ions were first captured at 2.8 kV and then lowered down to ground potential which allowed transfer (at 1 kV) and capture in the measurement trap at 0 V. The first mass comparisons were made between different charge states of the same nuclei, that is, O8+/O7+, Ar14+/Ar13+ and Ar17+/Ar16+. Comparison s between different nuclei, Ar-13+,Ar-14+,Ar-15+,Ar-16+/H-2(2+), were also made. First tests of accuracy indicates uncertainties smaller than 10(-8).