The rapidly whirling magnets used to stir mixtures in laboratories the world over can carry contaminants that confound experimental results.

Stirring bars — little magnetic beads coated in plastic — are often used for months or years. Valentine Ananikov and his colleagues at the Zelinsky Institute of Organic Chemistry in Moscow scanned bars with an electron microscope to investigate the effects of these long periods of use. They found that the bars’ polytetrafluoroethylene coating developed cracks, dents and scrapes. Worse still, those flaws trapped metal atoms and carried them from one experiment to another.

Routine cleaning of the stirring bars didn’t help. The researchers looked at 60 beads from various laboratories, and found only one that was free of contamination. Among the contaminants were palladium, platinum and gold, all of which can act as catalysts to speed up chemical reactions.

In tests, brand new stirring bars did not affect reactions, but used bars had unpredictable effects that probably depended on the nature and amount of contamination. The authors recommend using fresh beads to get the best results if avoiding contamination is important.