  Isochronous mass spectrometry (IMS) in storage rings is a successful
technique for accurate mass measurements of short-lived nuclides with relative
precision of about $10^{-5}-10^{-7}$. Instabilities of the magnetic fields in
storage rings are one of the major contributions limiting the achievable mass
resolving power, which is directly related to the precision of the obtained
mass values. A new data analysis method is proposed allowing one to minimise
the effect of such instabilities. The masses of the previously measured at the
CSRe $^{41}$Ti, $^{43}$V, $^{47}$Mn, $^{49}$Fe, $^{53}$Ni and $^{55}$Cu
nuclides were re-determined with this method. An improvement of the mass
precision by a factor of $\sim 1.7$ has been achieved for $^{41}$Ti and
$^{43}$V. The method can be applied to any isochronous mass experiment
irrespective of the accelerator facility. Furthermore, the method can be used
as an on-line tool for checking the isochronous conditions of the storage ring.
