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Mass spectrometry imaging (MSI) performed under ambient conditions is a convenient and information-rich method that allows for the comprehensive mapping of chemical species throughout biological tissues with typical spatial resolution in the 40-200 μm range. Ambient MSI methods such as desorption electrospray ionization (DESI) eliminate necessary sample preparation but suffer from lower spatial resolution than laser-based and vacuum techniques. In order to take advantage of the benefits of ambient imaging and to compensate for the somewhat limited spatial resolution, a secondary orthogonal separation nested in the imaging scheme was implemented for more selective discernment of tissue features in the spectral domain. Differential mobility spectrometry (DMS), an ion mobility-based separation that selectively transmits ions based on their high-to-low electric field mobility differences, can significantly reduce background chemical interferences, allowing for increased peak capacity. In this work, DESI DM-MSI experiments on biological tissue samples such as sea algae and mouse brain tissue sections were conducted using fixed DMS compensation voltages that selectively transferred one or a class of targeted compounds. By reducing chemical noise, the signal-to-noise ratio was improved 10-fold and the image contrast was doubled, effectively increasing image quality.

The methodology for obtaining accurate ion-neutral collision cross section (Ω) values for peptides and proteins using periodic focusing ion mobility spectrometry (PF IMS) is presented. A mobility dampening factor (represented by the term α) is introduced to account for the relative increase in ion-neutral collisions in PF IMS compared to uniform field ion mobility spectrometry (UF IMS) for equivalent operating conditions. The results show that α may be easily quantified both theoretically and empirically for a specific PF IMS design operating at a given pressure based upon the charge state of the analyte. By simply incorporating an α term into traditional UF IMS expressions, accurate Ω values were obtained with excellent agreement (≤4% difference) compared to UF IMS measurements found in the current literature.