Applications of acoustic levitation

As shown by the applications developed so far, the characteristics of acoustic levitation make it especially suitable for use in analytical and bioanalytical chemistry — however, the earliest applications focused on the determination of mechanical and physical properties of materials such as specific density, viscosity and surface tension [93,115,116]. Ohsaka et al. developed a method for determining the viscosity of highly viscous liquids (particularly, undercooled liquids, which exist at temperatures below their freezing points [117]). Weiser and Apfel used acoustic levitation to measure mechanical properties such as density, compressibility and sound velocity in biological materials [71]. 

The earliest applications of acoustic levitation in analytical chemistry were concerned with the development of various steps of the analytical process. Thus, Welter and Neidhart [72] studied the preconcentration of n-hexanol in methanol by solvent evaporation and the liquid-liquid extraction of n-hexanol from water to toluene in a levitated droplet, which they found to be efficient when using GC-FID with n-pentanol as internal standard. Solvent exchange of fluorescein from methylisobutyl ketone to aqueous sodium hydroxide was also accomplished. Sample concentration in an acoustically levitated droplet prior to injection into a CE equipped with an LIF or UV detector has also been accomplished [73,118]. The target analytes (namely, dansylated amino acids) were concentrated in the levitated drop and a limit of detection of 15 nM — much lower than the 2.5 pM achieved by hydrodynamic injection without preconcentration — was achieved following CE separation and quantification. For this purpose, 36000 sample droplets 2.3 pi in volume each were sequentially positioned in the acoustic Ievitator and evaporated. This example illustrates the potential of acoustic levitation for coupling to any type of detector for micro- or nanotrace analyses. 

Derivatization of target analytes has also been performed in acoustically levitated droplets for the determination of mono-, di-, tri- and tetrabutyltin [119]. The target analytes were extracted simultaneously from acetate buffer to hexane and derivatized using NaB(C2H5)4. Then, the organic phase was transferred for separation—determination by GC-AES. The results were comparable to those provided by conventional derivatization. 

Acoustic levitation has also been used for microtitration with absorptive and fluorescent indicators. The addition of titrant was efficiently controlled via a piezoelectric micropump. This application testifies to the possibility of using this technique in routine laboratories where sample and (or) reagent availability may be a limiting factor [90]. 

A portable Raman spectrometer coupled to an acoustic levitator (Fig. 8.11) has been used for environmentai monitoring and taxonomic identification of microorganisms in their native environments from spectra for iiving ceiis. The high signai-to-noise ratio achieved by suspending the ceiis in air without attenuation from surfaces provides an advantage over conventionai Raman measurements in a conventionai cuvette [120]. 

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