The generation of charge during hypervelocity impacts of microparticles provides one of the most sensitive methods for the detection and characterisation of dust particles in space. Linear time-of- flight (TOF) mass spectrometry provides an opportunity for investigating the thermodynamical properties of an impact ionisation plasma, e.g. the velocity distribution of the ions. The dependence of the plasma properties on the impact parameters were studied at the Heidelberg dust accelerator laboratory using impacts speed from 1 to 90 kms-1 and particle sizes form 30nm to 4μm. The results imply that the defining parameter of impact ionisation is either the impact velocity or the energy density and not the total impact energy.