Approaches to acoustic levitation

This section discusses the most interesting acoustic levitation approaches for use in analytical chemistry and describes devices for sample positioning in the Ievitator and reagent delivery, ultrasonic levitators and the coupling of acoustic levitators to detection and separation systems. Also, it discusses future prospects for acoustic levitation. 

The importance of sample delivery is frequently overlooked in the studies on acoustic levitation, even though precision in the analyses relies heavily on an appropriate choice of the sample volume. Delivery systems are closely related to the nature of the sample (liquid, suspension, solid or gas). Any type of system e.g. a micropipette, capillary, or microsyringe) can be used to position a drop from a liquid or slurry sample in an ultrasonic Ievitator. 

One of the problems conoerning mioropumps is caused by the formation of gas bubbles inside the pump head, which can be avoided by adjusting the viscosity and surface tension of the solution (e.g. by adding 2-5% m/v glycerol [90]). 

There is a viscosity limit for liquids to be ejected by the dispenser that depends on the nozzle dimensions. Thus, liquids of viscosity higher than the dispenser limit exhibit poor accuracy in droplet formation and volume this problem, however, can be overcome by heating the dispenser to a maximum temperature of 100°C restricted by the particular 

One advantage of micropumps and flow-through microdispensers is that droplet evaporation can be controlled accurately by adding a solvent in order to keep the droplet volume constant, whioh is essential for quantification purposes. Because US increases the temperature of the medium, it can facilitate solvent evaporation. This requires using a system such as an imaging detector to continuously monitor the droplet volume in order to determine the evaporation rate during acoustic levitation. Both mioropumps and microdispensers oan be coupled via FI manifolds to other units for the development of different steps of the analytical process. 

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