Use the SPI Supplies Brand PTFE laboratory funnels for use under the most adverse conditions. Their chemical resistance can be thought of as being nearly "total" since there are so few compounds that will attach the PTFE used in the production of these funnels. And unlike glass, there is nothing that can leach out and cause contamination in an ultra-pure material being passed through the funnel. And just as the "non-stick" properties of PTFE are well known when applied to a cooking utensil, similarly, nothing will want to stock to the very smooth walls of the PTFE funnels.
