  We present a new experimental method to correlate the isotopic composition of
intermediate mass fragments (IMF) emitted at mid-rapidity in semi-peripheral
collisions with the emission timescale: IMFs emitted in the early stage of the
reaction show larger values of $<$N/Z$>$ isospin asymmetry, stronger angular
anisotropies and reduced odd-even staggering effects in neutron to proton ratio
$<$N/Z$>$ distributions than those produced in sequential statistical emission.
All these effects support the concept of isospin "migration", that is sensitive
to the density gradient between participant and quasi-spectator nuclear matter,
in the so called neck fragmentation mechanism. By comparing the data to a
Stochastic Mean Field (SMF) simulation we show that this method gives valuable
constraints on the symmetry energy term of nuclear equation of state at
subsaturation densities. An indication emerges for a linear density dependence
of the symmetry energy.
