Compound tags and shifting agents are provided that find use in ion mobility spectrometry (IMS), mass spectrometry (MS), or a combination of IMS and MS, and which can substantially increase separation of multiple components in complex samples and facilitate quantitative and multiplexed analyses. In some cases, the compounds include a linker and a normalizing group, each including a structural unit and separated by a cleaveable group, and a crown ether. Also provided are methods for analyzing peptides in a sample. In some cases, the method includes coupling the compound to peptides which include a terminal guanidinium moiety capable of forming an intra-molecular complex with the crown ether.