Many of todays analytical problems are characterized through small sample volumes and can only be solved through a corresponding miniaturisation of the analytical instrumentation. Handling of small sample volumes is inherently difficult due analytical blanks, contamination, and sorption processes on the walls of the containers which are employed during analytical procedures. Acoustic levitation is a powerful tool for contact-less sample handling of solid, liquid, and certain gaseous samples. In addition, levitation permits a chemical pre-treatment such as enrichment, extraction, and derivatisation as well as combination with other analytical techniques such as optical spectroscopy or mass spectrometry. Acoustically levitated liquid and solid samples are typically in a range between 0.005 - 5 µl (diameter 0.2 - 2 mm respectively) and are suspended in a gaseous environment by a stationary ultrasonic field (nodes of a standing wave). Acoustic levitation avoids sample contamination and sorption processes by container walls, but suffers from evaporation and loss of solvents. To balance evaporation and condensation on levitated drops during the experiments, techniques of contact-less droplet size monitoring and solvent and reagent supply have been developed. The advantages of acoustic levitation are demonstrated for on line analysis of crystallisation in a levitated droplet. The evaporation and formation of crystals is observed via time-resolved X-ray diffraction at BESSY. In this way, a high-throughput analysis for polymorphs seems feasible. Future work will be devoted to adaptation of a binding assay with receptors, antibodies or enzymes. Special emphasis will be on homogeneous immunoassays such as fluorescence polarisation immunoassays (FPIA) or apo-enzyme reactivation immunoassay systems (ARIS) combined with laser-induced fluorescence (LIF). This offers the opportunity to study the kinetics of the bio-macromolecule interactions without any wall effects which can have a significant influence on the apparent performance of bioanalytical assays.
Many of todays analytical problems are characterized by small
sample volumes and can only be solved through a corresponding
miniaturisation of the analytical instrumentation. Handling of
small-sample volumes is inherently difficult due to contamination
and sorption processes on the walls of containers.
Acoustic levitation is a powerful tool for contact-free sample
handling of solid, liquid, and selected gaseous samples. In addition,
levitation permits a chemical pre-treatment such as enrichment,
extraction, and derivatisation as well as combination with other
analytical techniques such as optical spectroscopy. Acoustically
levitated liquid and solid samples are typically in a range between
0.005-5 µl (diameter 0.2-2 mm respectively) and are suspended
in a gaseous environment by a stationary ultrasonic field. Acoustic
levitation avoids sample contamination and sorption processes by
container walls, but suffers from evaporation and loss of solvents.
Nonetheless, it is of great impact for the characterization of small
samples. Exact determination of volume is inherently important to
find out the resultant concentration in evaporating droplets. Levitation
is a possibility to characterise the starting conditions of crystallisation
and polymerisation. Even for spectroscopic methods the
correct information on shape, size and concentration of samples
are indispensable.
To determine the size and volume of levitated samples, different
methods of contactless droplet size monitoring were developed and
compared in detail. They give the initial values for modelling of
evaporation effects. In addition, for balancing evaporation and condensation
on levitated drops during the experiments, non-contact
techniques of solvent and reagent supply have been tested.
Here, the advantages of acoustic levitation are demonstrated for on
line analysis of crystallisation in a levitated droplet. The evaporation
and formation of crystals is observed via time-resolved X-ray
diffraction using synchrotron radiation at BESSY. Wall-free
crystallisation in an acoustic levitator gives a tool to prove existing
models concerning crystallisation processes. Furthermore the
occurrence of polymorphic modifications depending on different
conditions can be detected in situ under different and adjustable
conditions.