Lab Tip: Why Stock Solutions made from Powders or Solids Should Always be Filtered
When making solutions in the lab, it is advisable to always filter any solution made by dissolving powders or solid materials in a solvent, usually water or buffer. Although the chemicals themselves may be of high purity, often there are particles of dust, plastics, and other materials from the packaging process or from the laboratory environment.
We noticed plastic particles like those shown below in a buffer made by dissolving high-purity biotech grade TRIS. The buffer was to be used for microscopic examinations, so obviously the particles had to be eliminated. We explored several possibilities and realized there were microscopic particles of plastic in the original powdered or crystalline reagent.
I always pass a solution made from powders through a filter, typically with a pore size of 0.4 microns. This is small enough to remove debris and even most cells, including some bacteria. All bacteria can be removed with a 0.2 micron filter to produce sterile solutions.
Many materials are used to make filters, and the choice of material is another topic itself. However, if a solvent other than water is used be sure that the filter material is compatible with the non aqueous solvent.
Small volumes can easily be passed through a syringe filter and collected directly in the storage container or dispensed as aliquots.
Larger volumes can be passed through presterilized filter units with containers. This is convenient but costs must be considered. For example, one advantage of presterilized devices is they can be used for solutions that cannot survive autoclaving.
Or a traditional reusable assembly:
Source
In summary, all stock solutions made from solids or powders should be filtered, even if no contamination is visible to the naked eye. All subsequent dilutions of the stock solution will also benefit from the filtration step.
Unfiltered solutions may contain not only plastics or airborne debris, but also microorganisms that could lead to decomposition and contamination of the solution.
Boom!
Respect, and little envy, for the highly specialized knowledge you have.
Thanks. I always reminded my students that I gained my experience through making mistakes. If they made a mistake, that's ok...they'll always remember an important lesson just like I did.