Schlenk Lines
Moving solutions on a Schlenk line
A cannula transfer or addition allows one solution to be transferred to another Schlenk tube using a Schlenk line. There are various ways this can be achieved depending on the requirements of the reaction and personal preferences. The general principle is that a cannula is inserted into one solution, and the pressure in that flask is increased compared to the receiving flask, which results in the solution being transferred to the second flask.
Cannulae come in a variety of different forms. Historically cannulae were constructed from metals (typically stainless steel), but nowadays disposable plastic cannulae are more commonly used. A variety of needle gauges (diameters) are used which changes the rates of addition. Metal cannulae come in a variety of lengths, whereas plastic cannulae are cut to the desired length as required. Most cannulae feature two bevelled (sloped, pointed) ends, which enable them to be inserted through septa as required. However, plastic cannulae are not very sharp (even when cut with a bevelled end) so only try to insert them through a pre-pierced septum.
Cannula transfer
The method below details one approach for the transfer of a solution which has been prepared in a Schlenk tube into another solution in a second Schlenk tube using a disposable plastic cannula.
Obtain a length of disposable cannula tubing. The length required is generally about 2.5 times the length of the Schlenk tube. The cannula will need to be able to run from the bottom of one Schlenk tube, out the top, across in a loop and into the receiving Schlenk tube.
The ends of the cannula should be cut to have a sharp, bevelled end which will be needed to pierce through septa.
Insert each end of the cannula through an appropriate septum for each of the Schlenk tubes. The ends of the cannula need to be through the septa, but not so far that they would end up in the solutions in the Schlenk tubes.
Ensure both the receiving and donating Schlenk tubes are connected via the double oblique taps and side arm taps to the gas manifold.
Remove the stopper from the donating Schlenk tube and quickly insert the septum (which is now pierced with the cannula). There should be a gas flow from the manifold, through the Schlenk tube and out of the cannula (check you can feel the flow – the rate of gas flow through the oil bubbler should decrease or stop).
Allow the cannula to be purged with gas for a few moments to ensure that any air has been displaced.
Insert the other septum into the receiving flask, after removing the stopper/helmet. As both flasks are open to the gas manifold, there should be no net flow of gas through the cannula.
Close the side arm tap on the receiving Schlenk tube and insert a needle into its septum (next to the cannula). This should result in a flow of gas from the donor flask, via the cannula into the receiving flask, and exit via the needle.
Carefully push the end of the cannula into the donor solution and the solution may begin to transfer through the cannula into the receiving Schlenk tube.
The solution can be encouraged to transfer by partially closing the gas exit from the oil bubbler using a finger which will divert the gas flow through the next easiest route which should be the cannula to needle exit. Care should be taken to not build up too much pressure on the line.
Cannula filtration
A cannula filtration is carried out in a similar way to a cannula transfer. The cannula is modified to place a filter on one end, so that only the liquid is transferred and any solid material remains in the original flask. The filter is constructed as follows:
Prepare a cannula of suitable length, with one sharp, bevelled end and the other with a blunt end.
Use a small filter paper to fold over the blunt ended cannula opening and tightly wrap the filter paper around the cannula. Secure the filter in place using Teflon tape.
The filtration is carried out in a similar fashion to cannula transfer, ensuring the filter end of the cannula is placed into the donor Schlenk tube. The liquid will be transferred from the starting Schlenk tube to an empty tube leaving behind any solids as a residue.
Cannula washing
Where the residue is the product of interest, filtrations are usually followed with solvent washing of the solid residues. These are carried out as follows:
The cannula for the filtration should still be in place connecting the residue Schlenk tube to the tube of filtrate. Raise the filter end of the cannula from the bottom of the Schlenk tube towards the top of the flask.
Add in the desired solvent for washing the residue and agitate the flask to ensure the residue is washed with the solvent.
Lower the filter back into the flask and begin transferring the wash solvent by closing the gas bubbler in the same way as for cannula transfer.
Repeat the washing steps as required.
Finishing a cannula transfer
For both cannula filtration and washing, it is likely that significant product is stuck to the filter paper attached to the cannula. It is often prudent to cut the cannula as the septum is removed in order to leave behind the filter paper in the Schlenk tube in order to avoid losing material. A typical procedure to complete a transfer would be:
Remove the exit needle from the receiving Schlenk tube.
Reopen the sidearm tap on the receiving Schlenk tube so that the flask is open to the gas manifold.
Remove the cannula from each septum.
If a cannula filter is fitted, cut the tubing close to the filter end and allow it to drop into the Schlenk tube. You should do this as you are replacing the septum with a stopper (otherwise the cannula will still be gripped by the septum), but only allow it to drop into the Schlenk tube if you have not exposed it to air.
Ensuring each flask is open to the gas manifold, replace the septa with stoppers.