Atomic masses of the neutron-rich isotopes 121–128Cd, 129;131In, 130–135Sn, 131–136Sb, and 132–140Te have been measured with high precision (10 ppb) using the Penning-trap mass spectrometer JYFLTRAP. Among these, the masses of four r-process nuclei 135Sn, 136Sb, and 139;140Te were measured for the first time. An empirical neutron pairing gap expressed as the odd-even staggering of isotopic masses shows a strong quenching across N ¼ 82 for Sn, with a Z dependence that is unexplainable by the current theoretical models.