35-S contamination of Thermal Cyclers

Wed Feb 9 13:49:23 EST 1994

Are you sure that your thermal cycler has been contaminated.  We recently
did some work involving 35-S in our thermal cycler.  After using the machine
we did alcohol swabs in our isotope area and those taken from the PCR wells
came back as hot.  For our swab tests we use a piece of filter paper dipped in
ethanol and swab the test area.  For a negative control we do a scintillation
count of a piece of filter paper dipped in ethanol.  After getting a high
reading on the PCR machine post 35-S, which persisted after a thorough cleanup
I ran some other controls which consisted of a filter paper dipped in ethanol
with a couple of drops of the mineral oil we put in the thermal cycler wells.
These controls came back at a level 3-4 times the background level of the
simple negative control.  This suggests that mineral oil may give a false
positive reading in a scintillation counter.  Has anyone else noticed the same

Barry Campbell
Molecular Ecology Laboratory
Department of Biology
Queen's University in Kingston
Ontario, CANADA K7L 3N6

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