Microfluidic devices provide substances to a mass spectrometer. The microfluidic devices include first and second surfaces, at least one microchannel formed by the surfaces, and an outlet at an edge of the surfaces. Some embodiments also include a tip surface with one or more surface features for helping guide substances from the outlet of the device toward a mass spectrometer. In some embodiments, the surface feature(s) includes a groove, which may be hydrophilic along all or part of its length. Hydrophilic surfaces and/or hydrophobic surfaces may also help guide substances out of the outlet and/or toward the mass spectrometer. In some embodiments, the outlet and/or the tip surface is recessed back from an adjacent portion of the edge. A source of electrical potential can help move substances through the microchannel, separate substances and/or provide electrospray ionization.