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Techniques for Optimizing Results
Probe Depth
Immerse t he probe tip 1.5 times the tip diameter into t he solution, w ithout touching the
bottom. For example, t he ½ " horn should be immersed ¾ " below the liquid surface.
Immersion depth can be less for larger horns and may have to be more for smaller probes
used at higher intensity.
Foaming and Aerosoling
Aerosoling and foaming generally occur w hen t he tip is not immersed f ar enough into the
solution. Low ering the t ip in the solution, decreasing pow er, and reducing solution
temperature w ill normally prevent foaming. Low ering the pow er and increasing sonication
time w ill usually reduce aerosoling; in severe cases, use an aerosol cap or sealed
atmosphere treat ment chamber. In organic materials, protein release f rom cell material
acts like a w etting agent and tends to promote foaming. For severe foaming:
Use a narrow er processing vessel
Use a vessel w ith an irregular inner surface
Increase effective viscosity by concentration or by adding glass beads.
Once foaming occurs, shut off pow er or reduce it below cavitation level before proceeding.
It may be necessary to use a centrifuge or high vacuum to reduce tenacious foam. If foam
persists, the sample may have to be discarded. If f oaming continues t o be a problem, an
indirect sonication device (such as a Cup Horn) may be a better option.
Viscosity Limitations
Viscous solutions and highly concentrated liquids can be difficult t o sonicate. If the liquid
is so thick that it w ill not pour or circulate easily it is too thick and cannot be processed
effectively.
Keeping Samples Cool
Intense ultrasonic processing causes the liquid temperature to elevate especially w ith small
volumes. High temperat ures reduce cavitation so the liquid should be kept as cold as
possible. This can be accomplished by immersing the sample vessel in an ice-salt-w ater-
alcohol bath, or by using a w ater-jacketed processing vessel w ith cold w ater circulation. To
minimize temperature elevation, use a pulse mode.
Free Radical Information
Trace free radicals produced by ultrasonics w ill usually have little or no enzyme activity.
The free radicals may be H
O
, O
as w ell as many other molecules and ions produced from
2
2
3
air or w ater. If this kind of reaction is suspect ed, it can easily be avoided by using CO
or
2
N
atmosphere. A CO
atmosphere is quickly obtained by placing a pellet of dry ice in the
2
2
solution before processing and let it " bubble out" . A CO
atmosphere stops luminescence
2
in the cavitation bubble for the same reason it prevents it in a vacuum tube. It is this
luminescence w hich causes the w ater and air molecules to break up producing the radicals.
This CO
procedure, as simple as it is, again is ordinarily unnecessary.
2
Rev. 6-12
24
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