Development of microfluidic droplet shooter and its application to interface for mass spectrometry

•A microfluidic device for shooting uniform picoliter droplets into gas phase with trajectory control was developed.•The principle of droplet shooting utilizing two-step gas-phase flow focusing was proposed and experimentally verified.•A branched and stepped hydrophobic microchannel was designed and fabricated by top-down glass fabrication technologies.•Generation of aqueous and organic droplets in the volume range of 3.61 to 24.9 pL at kilohertz frequencies was achieved.•The device was applied to an interface for mass spectrometry, and 100 % sample injection for high-sensitivity was achieved. Gas/liquid microfluidics has developed for high-efficiency mass transport, mixing and reaction in various applications of chemical analysis and synthesis. However, ejection of uniform liquid droplets floating in gas phase from the microchannels with trajectory control is still challenging. In this study, we developed a method for shooting picoliter droplets in a controlled direction that utilizes microscale gas-phase laminar flows and applied it to a mass spectrometry (MS) interface. The principle of droplet shooting using two-step gas-phase flow focusing was proposed. A microfluidic device with a branched and stepped hydrophobic microchannel was fabricated by top-down glass fabrication technologies. We verified the shooting of droplets of aqueous and organic solvents in the volume range of 3.61–24.9 pL at kilohertz frequencies. A model that considers the capillary force and the drag force was verified to establish a design guideline. Utilizing the developed microfluidic droplet shooter, we demonstrated high-efficiency sample transport to a single quadrupole mass spectrometer. We verified a sample injection rate of approximately 100 % by ejection of picoliter droplets into an MS injection aperture of a 400-μm diameter and achieved detection of caffeine with 3 times higher sensitivity than conventional electrospray ionization interface with sample dispersion.