Ion mobility spectrometry (IM) has become an important analytical tool over the past 25 years. Known also as plasma chromatography and ion chromatography, it has mainly been applied to the analysis of volatile organic compounds1, and used as a tool to probe the electronic states of ions.
Recently, ion mobility has been applied to the analysis of biomolecules employing electrospray ionization (ESI) and matrix-assisted laser-desorption ionization (MALDI) sources.These combined with mass spectrometry create a powerful tool in the analysis of proteins and peptides.
Ion mobility separates ions based on their mobility through a neutral target gas, usually helium. This separation is achieved by the migration of the ions in a linear field, E, through the buffer gas at a specific pressure, p. This is normally expressed as the combined term E/p. The figure below shows a basic diagram of a MALDI-IM-oTOF instrument used in our lab. Once the ions have eluted from the mobility cell, they are pulsed orthogonally into the time-of-flight (TOF) chamber and detected.
Ion mobility adds a degree of orthogonality to mass spectrometry due to the ion's interaction with a neutral target or buffer gas. This interaction for macromolecules leads to separation based primarily on the ion's volume. Ions having similar conformational forms will display a "trend line" with respect to mass-to-charge, making it easy to isolate peptides and proteins with conformational differences as outliers of these trend lines.