  Isochronous mass spectrometry (IMS) of heavyion storage rings is a powerful
tool for the mass measurements of short-lived nuclei. In IMS experiments,
masses are determined through precision measurements of the revolution times of
the ions stored in the ring. However, the revolution times cannot be resolved
for particles with nearly the same mass-to-charge (m/q) ratios. To overcome
this limitation and to extract the accurate revolution times for such pairs of
ion species with very close m/q ratios, in our early work on particle
identification, we analyzed the amplitudes of the timing signals from the
detector based on the emission of secondary electrons. Here, the previous data
analysis method is further improved by considering the signal amplitudes,
detection efficiencies, and number of stored ions in the ring. A sensitive
Z-dependent parameter is introduced in the data analysis, leading to a better
resolution of $^{34}$Ar$^{18+}$ and $^{51}$Co$^{27+}$ with A/Z=17/9. The mean
revolution times of $^{34}$Ar$^{18+}$ and $^{51}$Co$^{27+}$ are deduced,
although their time difference is merely 1.8 ps. The uncorrected, overlapped
peak of these ions has a full width at half maximum of 7.7 ps. The mass excess
of $^{51}$Co was determined to be -27332(41) keV, which is in agreement with
the previous value of -27342(48) keV.
