The Penning trap mass spectrometer at the University of Washington is described along with the relevant detection mechanisms and the systematic shifts associated with the finite energies in the normal modes of the trapped particle. The cyclotron frequency for the particle-of-interest is compared with the corresponding frequency of a single trapped carbon ion (usually C⁶⁺). Upon correcting for lost electrons and their binding energies, the relative mass ratio then becomes the atomic mass for the particle-of-interest. As a recent example, the value of the electron's "atomic mass" has been measured to be M_e = 0.000 548 579 911 7(17) u and the corresponding proton–electron mass ratio becomes m_p/m_e = 1836.152 664 6 (58).