SPI-Chem™ "Low Acid" GMA Kits Water Soluble
Some Frequently Asked Questions (and Answers)
This will be an on-going "work in progress", that is, we will keep adding
to our listing of Frequently Asked Questions as time goes on.
Send us your own
questions and/or comments that might be useful to others about to start
working with GMA.
Question: We are experiencing difficulty with our GMA.
After embedding our tissue in the GMA, the plastic is not getting hard
enough to cut. The plastic is sometimes milky white especially after
polymerization and the tissue within the block is too soft to cut. Do you
have any suggestions? We use SPI-Chem brand GMA.
Answer: The most likely cause of "too soft a block" is too much water
still in the tissue or pre-polymer. If the softness increases as you get
toward the tissue, then almost for certain it is water in the tissue.
The "milky white appearance" is probably due to one or more chemical other
than GMA and water in the plastic. The presence of a non-polar solvent
will cause this kind of effect for sure.
Our suspicions are that the tissue was not rinsed free of fixative, not
dehydrated properly, or the GMA was in some way contaminated. Of course
these same symptoms could arise if the GMA was not stored under proper
refrigeration on your end.
Question: Is there any one "best" recipe for working with plant
tissue at the light microscope level?
Answer: There is no one "standard" recipe because different users
want to optimize the final cured block to suit their particular samples
and objectives. However, it has been reported that the following recipe
does work well for plant material:
SPI-Chem Low Acid GMA Monomer: 95% volume/volume
SPI-Chem Polyethylene Glycol (PEG 200): 5% volume/volume
SPI-Chem Benzoyl Peroxide (catalyst): .15 to 1% wt/volume
Using more catalyst results in a much faster polymerization and harder
blocks, but if overdone, the blocks are brittle, with bubbles, and nearly
impossible to section. With more catalyst, polymerization is faster and
blocks are harder, but too much produces brittle blocks, sometimes with
bubbles. The PEG 200 can be varied through the range of 0-10% to achieve
desired results.
Question: Can the SPI-Chem Low Acid GMA be polymerized with heat?
Answer: Yes! However, oxygen must be excluded at all times. There
are a number of different choices for excluding oxygen, such as sealing the
GMA blocks in gelatin capsules,
BEEM® capsules,
or Micromolds™
(with one stacked on top to exclude oxygen). Another possibility for
those used to using
silicone flat embedding molds is to overfill the
cavities a bit and then place another identical mold on top; the capillary
action will form an effective seal against oxygen. And the mold being used
for the top seal still can be used as a mold.
Question: For heat polymerization, what would be a recommended
time and temperature combination?
Answer: There is no "absolute" combination of time and temperature
but we would suggest as a starting point, 60°C/140°F, especially
if using the sample recipe as indicated above.
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Sunday March 21, 2010
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