The lab was maintained under positive pressure to keep out the slightest hint of any kind of contaminant.
"Syndrome"
Thomas Hoover
Any other coloured patches seen on the spawn signify contaminant fungi in the spawn.
"Free from School"
Rahul Alvares
Even so, the radioactivity was so weak that the new element was identified "one atom at a time"; this is possible because its daughter element, fermium, spontaneously fissions and releases energy in greater bursts than any possible contaminant.
"A Brief History of Element Discovery, Synthesis, and Analysis"
Glen W. Watson