  The validity of impact parameter estimation from the multiplicity of charged
particles at low-intermediate energies is checked within the framework of ImQMD
model. The simulations show that the multiplicity of charged particles cannot
estimate the impact parameter of heavy ion collisions very well, especially for
central collisions at the beam energies lower than $\sim$70 MeV/u due to the
large fluctuations of the multiplicity of charged particles. The simulation
results for the central collisions defined by the charged particle multiplicity
are compared to those by using impact parameter b=2 fm and it shows that the
charge distribution for $^{112}$Sn +$^{112}$Sn at 50 MeV/u is different
evidently for two cases; and the chosen isospin sensitive observable, the
coalescence invariant single neutron to proton yield ratio, reduces less than
15\% for neutron-rich systems $^{124,132}$Sn +$^{124}$Sn at $E_{beam}$=50
MeV/u, while the coalescence invariant double neutron to proton yield ratio
does not have obvious difference. The sensitivity of the chosen isospin
sensitive observables to effective mass splitting is studied for central
collisions defined by the multiplicity of charged particles. Our results show
that the sensitivity is enhanced for $^{132}$Sn+$^{124}$Sn relative to that for
$^{124}$Sn+$^{124}$Sn, and this reaction system should be measured in future
experiments to study the effective mass splitting by heavy ion collisions.
