Magnetic stir bars are widely used for reactions in solution. They are generally coated with PTFE (polytetrafluoroethylene), which is considered inert and easy to clean.

Valentine P. Ananikov and colleagues, Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, have studied magnetic stir bars used in a lab working on synthesis and catalysis. The team observed visual changes in used stir bars compared with new ones, e.g., discolorations, scratches, dents, and cracks. They used field emission scanning electron microscopy (FE-SEM) to study the surfaces of new and used stir bars and found that the PTFE coating on the used bars is damaged on a microscopic level.

Energy-dispersive X-ray spectroscopy (EDX) elemental analysis showed that new bars do not contain metals, but used bars can be contaminated with transition metals from previous reactions, such as palladium, in the form of nanoparticles. To confirm this result, the team performed a single Pd-catalyzed reaction using a new stir bar. They found Pd contamination on the surface of the stir bar after only one use.

The researchers also tested if a used, contaminated stir bar can influence the results of catalytic reactions. They performed a Pd/C-catalyzed Suzuki–Miyaura reaction either with the usual catalyst and a new stir bar, without any catalyst but with a used stir bar, or without any catalyst and with a new stir bar as a control. They found no conversion in the control experiment, but significant conversion in some “catalyst-free” reactions with used stir bars.

According to the researchers, these results mean that the inertness of used magnetic stir bars can no longer be taken for granted. Control experiments with new stir bars should be conducted, especially in catalysis research. In addition, other PTFE-coated lab equipment could be subject to the same effect.

• Phantom Reactivity in Organic and Catalytic Reactions as a Consequence of Microscale Destruction and Contamination-Trapping Effects of Magnetic Stir Bars,
  Evgeniy O. Pentsak, Dmitry B. Eremin, Evgeniy G. Gordeev, Valentine P. Ananikov,
  ACS Catal. 2019.
  https://doi.org/10.1021/acscatal.9b00294