Cannula filtration is a convenient and routine method employed in Schlenk line chemistry that can be used for removing unwanted solids or isolating precipitated/crystalline product.
Preparing the Filter Cannula:
Step 1: Wrap a flat-ended cannula with a 5 cm strip of PTFE tape.
Step 2: Carefully fold a 125 mm diameter glass-fibre filter ensuring that the filter is flat with the end of the cannula. (This is quite fiddly and takes practice!) Regular Whatman filter paper works fine too and can be cut into squares of an appropriate size.
Step 3: Wrap a second 5 cm strip of PTFE tape over the filter to secure it to the cannula, making sure that the end is not covered by PTFE tape.
Step 4: Store the filter cannulae in an oven prior to use.
Performing a Cannula Filtration:
Step 1: Pierce the sharp end of the prepared filter cannula through the bottom of a rubber septum and pull it along so that the filter end of the cannula is close to the bottom of the septum.
Step 2: Replace the glass stopper with the rubber septum/filter cannula under a flow of inert gas. Purge the cannula for 15-30 seconds before inserting the cannula into an empty Schlenk flask that has previously been cycled onto the Schlenk line.
Step 3: Insert a bleed needle into the septum on the receiving flask and close the tap to establish a pressure differential. Lower the filter cannula down into the liquid. Increasing the gas pressure and lowering the height of the receiving flask can help to speed up the filtration.
Step 4: Once all filtrate has been transferred, raise the filter cannula above the residual solids and allow any remaining filtrate to pass through the cannula. If necessary, add additional solvent to the transfer flask to wash or extract the solids (either by cannula transfer or syringe) and repeat step 3.
Step 5: Open the receiving flask back to the inert gas and remove the bleed needle from the septum. Remove the filter cannula from the septum of the receiving flask and then from the transfer flask along with the septum, replacing it with a greased glass stopper. Replace the septum on the receiving flask with a greased glass stopper. Clean the cannula immediately after use to prevent blocking. The solids can now be dried under vacuum (if desired) or discarded/quenched, and the filtrate is ready for further manipulations.