  The minuteness of the nuclear binding energy requires that mass measurements
be highly precise and accurate. Here we report on new measurements $^{29-33}$Mg
and $^{26}$Na performed with the {\sc Mistral} mass spectrometer at {\sc
Cern}'s {\sc Isolde} facility. Since mass measurements are prone to systematic
errors, considerable effort has been devoted to their evaluation and
elimination in order to achieve accuracy and not only precision. We have
therefore conducted a campaign of measurements for calibration and error
evaluation. As a result, we now have a satisfactory description of the {\sc
Mistral} calibration laws and error budget. We have applied our new
understanding to previous measurements of $^{26}$Ne, $^{26-30}$Na and
$^{29,32}$Mg for which re-evaluated values are reported.
