Accurate masses of neutron-deficient nuclides close to $Z = 82$

Mass measurements with the Penning trap mass spectrometer ISOLTRAP at ISOL\-DE/CERN are extended to non-surface ionizable species using newly developed ion beam bunching devices. % Masses of $^{179-197}$Hg, $^{196,198}$Pb, $^{197}$Bi, $^{198}$Po and $^{203}$At were determined with an accuracy of 1$\cdot$10$^{-7}$ corresponding to $\delta m$\,$\approx$\, 20\,keV. Applying a resolving power of up to 3.7$\cdot$10$^6$ ground and isomeric states of $^{185,187,191,193,197}$Hg were separated. First experimental values for the isomeric excitation energy of $^{187,191}$Hg are obtained. A least-squares adjustment has been performed and theoretical approaches are discussed to model the observed fine structure in the binding energy.