Operation Manual____________________________________________________________Techniques
_____________________________________________________________________________________________
13-1
13.
TECHNIQUES
PROBE DEPTH
Immerse the probe tip not less than 1 to 1½ times the tip diameters into the solution,
without touching the bottom. This depth rate applies to the ½" horn; immersion depth
can be less for larger horns and may have to be more for smaller probes used at higher
intensity.
FOAMING AND AEROSOLING
Insert the tip deep enough below the surface of the liquid to insure there is no aerosoling
or foaming. Aerosoling and foaming generally occur when the tip is not immersed far
enough into the solution. Lowering the power and increasing sonication time will usually
reduce aerosoling; in severe cases, use an aerosol cap or sealed atmosphere treatment
chamber. In organic materials, protein release from cell material acts like a wetting
agent and tends to promote foaming. Lowering the tip in the solution, increasing power,
and reducing solution temperature will normally prevent foaming. For severe foaming:
•
Use a narrower processing vessel
•
Use a vessel with an irregular inner surface
•
Increase effective viscosity by concentration or by adding glass beads.
Once foaming occurs, shut off power or reduce it below cavitation level before proceeding. It
may be necessary to use a centrifuge or high vacuum to reduce a tenacious foam. If foam
persists, the sample may have to be discarded.
VISCOSITY LIMITATIONS
Viscous solutions and highly concentrated liquids can be difficult to sonicate. If the liquid is
so thick that it will not pour or circulate easily, or if 25 micron glass powder will not settle to
the bottom quickly, it is too thick and cannot be processed effectively.
KEEPING SAMPLES COOL
Intense ultrasonic processing causes the liquid temperature to elevate especially with small
volumes. Since high temperature reduces cavitation, the liquid should be kept as cold as
possible. This can be accomplished by immersing the sample vessel in an ice-salt-water-
alcohol bath, or by using a water-jacketed processing vessel through which very cold water
or alcohol is circulated. To minimize temperature elevation, use the PULSAR™ Cycle Timer
or pulse manually in short, quick bursts.
FREE RADICAL FORMATION
Trace free radicals produced by ultrasonics will usually have little or no enzyme activity. The
free radicals may be H
2
O
2
, O
3
as well as many other molecules and ions produced from air
or water. If this kind of reaction is suspected, it can easily be avoided by using CO
2
or N
2
atmosphere. A CO
2
atmosphere is quickly obtained by placing a pellet of dry ice in the
solution before processing and let it "bubble out". A CO
2
atmosphere stops luminescence in
the cavitation bubble for the same reason it prevents it in a vacuum tube. It is this
luminescence which causes the water and air molecules to break up producing the radicals.
This CO
2
procedure, as simple as it is, again is ordinarily unnecessary.
