  The recent detection of shock-precursors toward the very young L1448-mm
outflow offers us the possibility to study the grain chemistry during the first
stages of the shock evolution, constraining the molecules ejected from grains
and the species formed in gas phase. Observations of key molecules in the grain
chemistry such as SiO, CH3OH, SO, CS, H2S, OCS, and SO2 toward this outflow are
presented. The line profiles and the derived abundances show three distinct
velocity regimes that trace the shock evolution: the preshock, the
shock-precursor and the postshock gas. The SiO, CH3OH, SO, and CS abundances
are enhanced with respect to the quiescent gas by 1 order of magnitude in the
shock-precursor component, and by 3 orders of magnitude in the postshock gas.
The derived SiO and CH3OH abundances are consistent with the recent ejection of
these molecules from grains. Since H2S is only enhanced in the shock-precursor
component, and OCS and SO2 are undetected, SO and CS are the most abundant
sulfur-bearing species in the grain mantles of L1448-mm. The ejection of mainly
SO and CS rather than H2S or OCS from grains, suggests that the sulfur
chemistry will depend on the chemical "history" of the grain mantles in
outflows and hot cores.
