This reply is from a lab microwave vendor; since you're now officially
forewarned, no flames, please! :-)
That said, for me, the usual suspect is the paraffin step. From the
inception of microwave processing, this has always been the most
problematic process, due to paraffin's non-polar nature. Pre-heating
paraffin to the operating temperature of the protocol has always been
extremely important; if the temperature of paraffin is raised by more
than a degree or two when tissue is present, there is a strong
potential for artifacts, inconsistency, and other problems.
Here's why there's a critical difference between merely maintaining
paraffin temperature, and raising paraffin temperature in the
microwave. Paraffin does not absorb significant microwave energy;
microwaves pass through wax almost as if it weren't there. So the
energy has to be absorbed by "something," hence EBS' recommendation
for a Pyrex processing container; Pyrex does absorb small amounts of
microwave energy, transferring it to paraffin, maintaining a given
temperature. Things get a little more delicate when using, for
example, disposable plastic containers instead of Pyrex, making
precise pre-heating even more critical.
Pre-heating produces good, consistent results, since maintaining
temperature does not expose patient samples to undue energy, but
RAISING the temperature is another matter, requiring long wait times
and, significantly, LOTS of excess energy. Where does this excess
energy go? All too often, patient samples, which, even after fixation,
dehydration, and clearing, are probably the most polar materials
present. In effect, tissue can take the entire "hit."
Since most melted paraffin in today's A.P. lab is somewhere around
60'0C, if lab personnel skip the pre-heating step, they are using the
microwave to heat paraffin 14 - 24'0C. Besides taking a VERY long
time, this can expose patient samples to WAY too much energy. There's
also an unfortunate temptation to raise power output of the microwave
since it's "taking too long," which may well be the worst thing to do.
I know -- since we all use microwaves outside the lab, this whole
concept is very counter-intuitive, which is why all EBS' microwaves
have sported door warning labels exhorting users to pre-heat paraffin.
Preheating paraffin to the operating temperature does require
additional time, and equipment such as an oven or additional paraffin
pot, assuming availability of a unit that can be set hot enough, plus
sufficient space and electrical power. On the other hand, early
attempts at microwave heating of paraffin resulted in questionable
solutions like the addition of marble chips, glass marbles, glass
discs and other more exotic (and expensive) materials.
SHAMELESS COMMERCE WARNING: At EBS, we recognized the many benefits of
the ability to microwave paraffin like any polar reagent: time
savings, convenience, and standardization of procedures. So we
developed PolarHeat(TM) (patent pending) disposable paraffin heating
sheets. When exposed to microwave energy, a PolarHeat sheet, properly
placed and immersed in paraffin, provides safe paraffin heating at a
speed comparable to water or alcohol. Although significant tine
savings are realized, more importantly, the sheets afford protection
to irreplaceable tissue samples, "mission critical" in the context of
patient care. Versions are available to fit popular EBS containers and
racks; free sample packs are available. See
for more information, or feel free to contact me directly.
Microwave Product Manager
Energy Beam Sciences, Inc.
29-B Kripes Rd.
East Granby, CT 06026
Tel: 800.992.9037 x 341
I skate to where the puck is going to be, not to where it's been.
- Wayne Gretsky
You must be the change you want to see in the world.
- Mahatma Gandhi
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