A technology for sequencing single proteins would expand our understanding of biology and improve the detection and treatment of disease. Approaches based on fluorosequencing, nanopores, and tunneling spectroscopy are under development and show promise. However, only mass spectrometry (MS) has demonstrated an ability to identify amino acids with minimal degeneracy. We envision sequencing a protein by fragmenting it and delivering its constituent amino acids into a mass spectrometer in sequential order, but existing ion sources employ a background gas that scrambles the spatial ordering of ions and degrades their transmission. Here we report an ion source comprising a glass capillary with a sub-100 nm diameter pore that emits amino acid ions from aqueous solution directly into vacuum. Emitted ions travel collision-less trajectories before striking a single-ion detector. We measured unsolvated ions of 16 different amino acids as well as glutathione and two of its post-translationally modified variants.