The invention in general relates to methods of compressing an ion packet inside an IMS based device for achieving high IMS resolving power, while maintaining the peak resolution. When a relatively broad ion packet in gas phase, is subjected to a nonlinear potential profile with a decreasing electric drift field; the ions in the high field region move faster than the low field region resulting in peak bunching. While ions drift over a long drift section with a constant electric field of a mobility device, they separate out based on their mobilities. However, individual peaks also broaden due to diffusion. In the present method, by using non-constant electric fields (DC) the peak broadening can be overcome. Due to nonlinear potential profile, the ions in different regions move with different velocities. Therefore when applied suitably, these fields can be used to bunch together ions in a broad peak to a narrow packet. When the different mobilities are sufficiently separated, the bunching is applied locally; this processes will not negatively affect the time separation (resolution) characteristics. When applied to close mobilities or peaks not well separated, some loss in time resolution may occur. This method can be used in any general mobility device, but is particularly attractive in SLIM devices where ion transport is practically lossless, facilitates long drift lengths, provides sufficiently resolved peaks to be effectively bunched.