Sample-dispensing methods. (a) Atomizer spray, using Bernoulli's principle to generate very small sample droplets. (b) Electrospray nozzle, which uses ionization of the sample to generate fine mists of droplets at the output channel without the use of gas pressure. (c) Inkjet nozzle, using a piezoelectric element to generate acoustic vibrations in the sample at a fixed frequency, leading to uniform droplet formation. (d) Gas-assisted spray, where sample is continuously pumped through the channel and broken up by gas pressure at the exit, which generates small droplets as well as accelerating the spray towards the grid. Gas-dynamic virtual nozzles (GDVNs) are conceptually similar and enable greater aerosol control through more sophisticated manufacturing (Klebl, Monteiro et al., 2020).