In order to analyze the generation process of ions by electrospray, the effect of concentration of solute (polyethylene glycol; PEG) was investigated. The size distributions of PEG particles and ions generated by the electrospray were measured by a Differential Mobility Analyzer (DMA) at various concentration of PEG solution. In addition, we proposed a model to explain the droplet breakup and ion emission process. In the experimental condition of the present study, sufficient ion generation was not occurred at high concentration (> 1.0 wt%) because of the precipitation of the solute as solid nanoparticles. On the other hand, the amount of generated ion increased at low concentration of the solution (< 1.0 wt%) because the large droplets also contributed to produce ions.