A compact apparatus for atomisation of fluid samples comprises a sonotrode (11), placed so that an ultrasonic wave emitted by the sonotrode is directed through a channel (25) in a separate channel device (21) and reflected by from the interface (26) in a high-low impedance transition zone (Tz), so that a standing wave is formed within the channel. A positive air flow through the channel, driven by a pressure differential at each end of the channel, interacts with the working fluid or slurry being delivered by a fluid delivery device (30) to atomise it. The speed of the air flow and the dispersal, homogeneity, and size of particles in the slurry sample can be controlled by varying the shape of the channel outlet.