  A $^{238}$U projectile beam was used to create cadmium isotopes via
abrasion-fission at 410 MeV/u in a beryllium target at the entrance of the
in-flight separator FRS at GSI. The fission fragments were separated with the
FRS and injected into the isochronous storage ring ESR for mass measurements.
The Isochronous Mass Spectrometry (IMS) was performed under two different
experimental conditions, with and without B$\rho$-tagging at the dispersive
central focal plane of the FRS. In the experiment with B$\rho$-tagging the
magnetic rigidity of the injected fragments was determined by an accuracy of
$2\times 10^{-4}$. A new method of data analysis, using a correlation matrix
for the combined data set from both experiments, has provided mass values for
25 different isotopes for the first time. The high selectivity and sensitivity
of the experiment and analysis has given access even to rare isotopes detected
with a few atoms per week. In this letter we present for the $^{129,130,131}$Cd
isotopes mass values directly measured for the first time. The Cd results
clearly show a very pronounced shell effect at $N=82$ which is in agreement
with the conclusion from $\gamma$-ray spectroscopy of $^{130}$Cd and confirms
the assumptions of modern shell-model calculations.
