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Appendix 4-B Safety Warning Regarding
Use of
35
S Nucleotides
Some researchers have experienced a problem with radioactive contamination when
using
35
S in thermal cyclers. This problem has occurred with all types of reaction vessels.
The Problem
When
35
S nucleotides are thermally cycled, a volatile chemical breakdown product forms,
probably SO
2
. This product can escape the vessel and contaminate the sample block of
a thermal cycler, and possibly, the air in the laboratory. Contamination has been reported
with microassay plates, 0.2 ml tubes, and 0.5 ml tubes.
96-Well Polycarbonate Microplates
These microplates present the largest risk of contamination. Polycarbonate is
somewhat permeable both to water and the
35
S breakdown product. This problem
is exacerbated when polycarbonate plates are held at high temperatures for long
periods of time, or when the plates are sealed for oil-free thermal cycling.
0.2 ml Polypropylene Tubes and Polypropylene Microplates
These tubes are manufactured with very thin walls to enhance thermal transfer. The
thin walls are somewhat fragile and can “craze” or develop small cracks when
subject to mechanical stress. Undamaged thin polypropylene tubes may also be
somewhat permeable to the
35
S breakdown product. Either way, there have been
reports of
35
S passing through the walls of 0.2 ml tubes of several different brands
during thermal cycling. No data are yet available on radioactive contamination
with polypropylene microplates.
0.5 ml Polypropylene Tubes
Contamination problems are rarer with this type of tube, but instances have been
reported.
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