Methods and systems are provided for detecting analytes in a gas phase sample. An ion mobility spectrometer is provided for detecting analytes having an excess amount of dopant in its separation region. In an embodiment, the dopant is added directly to the separation region, such as with a drift gas or by diffusion, thereby providing excess dopant that dominates subsequent cluster formation and maintenance. Excess dopant in the separation region minimizes or reduces interfering signals associated with unwanted substances, such as water vapor, that are introduced to the IMS. In an aspect, the invention provides IMS systems and methods having increased sensitivity and reliability for analyte detection.