I am using magnetic stir bars to agitate solutions for 4 weeks at a time. I have noticed at the bottom of the glass bottles that white marks have accumulated where the stir bar spins. Even with soaking, thorough washing and scrubbing, acid washing, and sonicating, the marks remain. I suspect these marks are white PTFE that has eroded from the stir bars then stuck to the glass.
I have 2 questions:
Are the marks removable? I know PTFE is very resistant to most solvents, which makes removal seem difficult.
Is it necessary to remove these marks? Perhaps because PTFE is so resistant to solvents, it will not affect my analysis.
I am extracting ammonium from soil extracts. The soil extract is an acidified water solution, which I immediately add MgO to, in order to increase the pH to above 9. This pushes the NH4+ into the headspace of the bottle. Floating on the solution is a glass filter in a sealed Teflon packet. The ammonium ions diffuse through the Teflon, and react with citric acid on the filter to make an NH4Cl salt. This is then analyzed on an EA-MS for d15N.