In the present paper the emission of intact insulin quasi-molecular ion [M+H]+ during laser ablation (MALDI) is studied. It was observed in particular that the insulin TOF molecular peak increases as the laser power increases. The DE-Vestal method for the initial velocity measurements was improved theoretically including the distance (d0) until the free expansion regime can be considered. According to the present analysis, the v0 parameter given by the DE-Vestal method is interpreted as the initial velocity that the desorbed ion would have if no collision occurs in the plasma. The improved method interprets v0 as the "final" initial velocity, i.e., as the velocity that the desorbed ions have when the plasma free expansion starts and, effectively, collisions no longer occur. The new method allows also the determination of d0, the distance to the solid when the free expansion starts. The data fitting shows that the distance (d0) has a linear dependence on the laser's intensity. Extrapolation of these values gives I = 0.69 G W cm-2 as the minimum energy density necessary to produce high density plasma during the insulin ions desorption when using alphaCHCA matrix.
