MJ Research DNA Engine Dyad PTC-220, Operation Manual, Page: 35 / 157

4-15
Operation
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, 0.2ml tubes, and 0.5ml 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.
0.2ml 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 perme-
able to the
35
S breakdown product. Either way, there have been reports of
35
S passing
through the walls of 0.2ml tubes of several different brands during thermal cycling. No
data are yet available on radioactive contamination with polypropylene microplates.
0.5ml Polypropylene Tubes
Contamination problems are rarer with this type of tube, but instances have been re-
ported.
The Solution
1. Substitute the low-energy beta emitter 33 P in cycle sequencing. 33 P nucleotides are not
subject to the same kind of chemical breakdown as 35 S nucleotides, and they have not
been associated with volatile breakdown products.
Appendix 4-B Safety Warning Regarding
Use Of
35 S Nucleotides