We present early time , high resolution spectroscopy of three GRB afterglows : GRB050730 , GRB050922C and GRB060418 .
These data give us precious information on the kinematics , ionization and metallicity of the interstellar matter of GRB host galaxies up to a redshift z \sim 4 , and of intervening absorbers along the line of sight .

In particular , we find that the GRB surrounding medium is complex , featuring a clumpy environment in which gas is distributed in regions of space with different distances from the GRB explosion site .
The absorption spectra show that elements are present both with high and low ionization states , and even forbidden , fine structure levels are commonly observed .
These features allow us to evaluate the physical parameters of the absorbing gas .
In details , the density of the gas regions lie in the range n = 10 \div 10 ^ { 6 } cm ^ { -3 } , and the temperatures are of the order of T = 10 ^ { 3 } \div 10 ^ { 4 } K .

The metallicity of the GRB host galaxies is computed using the hydrogen absorption features .
We find undersolar abundances for our GRBs , namely , Z _ { odot } \sim 10 ^ { -3 } \div 10 ^ { -2 } .
However , Z _ { odot } can be underestimated since the H column presents large uncertainties and dust depletion has not been taken into account .
The latter effect can be taken into account using as metallicity indicators Zn and Cr , which tend to remain in the gas phase .
We find metallicities higher than the previous values and in agreement with other measurements for GRB host galaxies .

Finally , the observed [ C/Fe ] ratio for GRB050730 ( z \sim 4 ) agrees with values expected for a galaxy younger than a Gyr undergoing bursts of star-formation .
In addition , the [ C/Fe ] ratio evaluated component by component can give informations on the relative distances of the components from the GRB explosion site , since Fe dust is more efficiently destroyed than graphite ; inversely , if the distance of the shells from the centre were known , we could obtain a powerful tool to investigate the dust depletion in GRB host galaxies .