Gradients of voltage, pressure, temperature, and salinity can transport objects in micro- and nanofluidic systems by well known mechanisms. This paper explores the dynamics of particles in a viscosity gradient...

We report a nanopore ion source for mass spectrometry that delivers ions directly into high vacuum from aqueous solutions. The ion source comprises a pulled quartz capillary with a sub-100~nm opening. Ions escape an electrified meniscus between the liquid and high vacuum by ion evaporation and travel along collisionless trajectories to the ion detector. We measure mass spectra of 16 different amino acid ions as well as post-translationally modified variants of glutathione in unsolvated states using the nanopore ion source. The emitted current is composed of ions rather than charged droplets, and more than 90% of the current can be recovered in a distant collector. The nanopore ion source could enable more sensitive proteomic analyses. It circumvents the sample loss mechanisms inherent to conventional electrospray ionization (ESI), where charged droplets are sprayed into a background gas that scatters ions and degrades their transmission.