How to Eliminate Contaminants from Magnetic Stirrers

There is no lack of news articles announcing product recalls due to contamination and associated  health risks.  Sources of contamination are many and can include improper cleaning of equipment used in processing.  This post will discuss preventive measures to be taken to eliminate contaminants from magnetic stirrers – commonly used tools in R&D labs to develop new formulations.

What is a Magnetic Stirrer?

Those of you familiar with this equipment can skip to the following sections.

Magnetic stirrers – also called magnetic mixers offered by CAT Scientific – are benchtop units designed to do just what you’d expect – stir or mix small samples.  Some are designed to perform their function at ambient temperature, others called magnetic hotplate stirrers can heat samples to very high temperatures.

These laboratory mixers have a single moving part – a motor driven rotating magnet in the base that can be controlled by speed and time.  Flasks or beakers placed on the mixer plate contain the magnetic  mixing bar.  The rotating magnet in the base turns the free mixer bar magnet (also called a flea) to performing the stirring action.

The very simplicity of a magnetic stirrer as compared to other stirring and mixing equipment is a great advantage when it comes to cleaning and contamination control.  There are two main parts involved – the mixing bar and the sample container.  Contrast this to  homogenizers and overhead mixers that require special care to remove residues from generator assemblies and stirring shaft impellers.

The disadvantage of magnetic stirrers compared to homogenizers and overhead stirrers is that they do not work well with samples of high viscosity.   That’s because there is no physical connection between the rotating magnet in the base and the free mixer bar magnet.

How to Clean Magnetic Stirrers

We should define “clean” here as thorough removal of contaminants followed by sterilizing or disinfecting depending on your organization’s procedures.  “Thorough removal” means just that because contaminants that remain on surfaces may not be removed by sterilizing or disinfecting.

In fact, depending on the contaminant, the heat may cause the contaminants to adhere more tenaciously to surfaces.

This is why simple shapes may be the simple solution to contamination control. Few mixing tools are simpler in shape than a magnetic mixer stir bar.

Mixer bars come in several configurations, the length and shape of which defines the sample volume and objective of the process.  For more detail on this please see our post on selecting stirring bars.

Here, however, we’re looking at contamination issues.

The key to easy cleaning is the simple shape of the bar vs. homogenizer generator assemblies and overhead stirrer impellers, and that they are encased in coatings that keep the media being processed from contacting the magnet itself.   Check to see that your stirring bars are FDA approved.  Common coating material choices include Teflon®, Pyrex® glass and plastics.

Establish a Cleaning Process

Wear gloves for this operation and use caution especially if you are working with hot equipment.

  • At the end of a mixing cycle remove the mixing bar and thoroughly wash it immediately to avoid samples from adhering to the surface.  This is particularly important if you’ve conducted stirring at high temperatures where samples can quickly dry on the hot stir bar surface.
  • We repeat this important point: Always keep in mind that sample residues adhering to surfaces will not be removed by sterilizing or disinfecting.
  • Use brushes to scrub the bars in a disinfecting solution as the next step after gross contaminants have been removed.
  • If you batch sterilize or disinfect keep stirring bars immersed in a disinfecting solution until the next step.
  • At the end of the process protect them from picking up local contaminants.
  • Oh yes, and don’t forget to clean and sterilize the beakers and flasks before starting a new experiment.

Ultrasonic Cleaners for Contaminant Removal

A benchtop ultrasonic cleaner can be a great help in achieving magnetic mixer contamination control goals.  These are especially useful after mixing ingredients that strongly adhere to the flea and to the container.

After rinsing residues from mixing magnets, beakers and flasks immerse them in a suitably sized ultrasonic cleaner filled with a biodegradable cleaning solution (formulas available for tasks at hand).  Cavitation action produced by the cleaner creates billions of microscopic vacuum bubbles that implode on contact to blast loose and carry away even the most tenacious contaminants.

This step is followed by disinfecting or sterilizing as recommended by professional and trade associations.

We at CAT Scientific hope this post is helpful. If you have questions on selecting and operating magnetic mixers please contact us for the answers.

Bob Wilcox

Bob Wilcox has represented CAT Scientific’s family of homogenizers, magnetic stirrers, liquid handling and related laboratory equipment since 2002 when Staufen, Germany-based CAT Ingenieurbüro M. Zipperer GMbH established operations in North America. Bob oversees CAT Scientific laboratory apparatus sales and service organization from the company’s headquarters in Paso Robles, CA. He also is in charge of the parent company’s line of JetCat jet turbines, turboprop, and helicopter power plants for hobbyists’ radio controlled fixed wing and helicopter model aircraft. -- Earlier in Bob’s career he was involved in visual and special effects as well as camera and electronics supervisory responsibilities for the motion picture and television industry.

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