Hydrogen–deuterium exchange observed in angiotensin I, m/z 648.85, with nanoESI using a theta-capillary. The extent of deuteration increases by pulling the capillary further back from the inlet of the mass spectrometer. (A) Δt = 34 μs, TIC: 2.6 x 106; (B) Δt = 133 μs, TIC: 8.4 x 104; (C) Δt = 185 μs, TIC: 8.0 x 103; and (D) Δt = 319 μs, TIC: 5.6 x 103; where Δt is the average dwell time for droplets in air before they enter the mass spec-trometer, and TIC is the total ion current.