Cannula Transfer: What You Need to Know

Cannula transfers allow precise and controlled fluid transfers in the lab setting. They’re used to prepare samples, run experiments, and conduct research and analysis. Here’s how to perform the three basic variations of this process and some special precautions to keep in mind.

Cannula Transfer Techniques in the Lab

There are three ways to perform a cannula transfer. But before you get started, here’s what you’ll need:

• Cannula needle
• Flask of acetone
• Lab coat
• Fume hood
• Nitrile gloves
• Safety glasses
• Two air-free flasks

Because you’ll be dealing with a double-sided needle, it’s imperative to maintain situational awareness throughout the procedure to prevent needlesticks. Before starting any of the three techniques, you’ll need to take the following steps:

1. First, you’ll need to set up your two-flask system. The starting flask contains the solution you want to dispense, and the receiving flask (or frit or round bottom) is where you intend to transfer it.
2. The receiving flask must be set at the same level as or lower than the starting flask since potential energy is a significant factor in cannula transfers.
3. Securely clamp both flasks.
4. Place a rubber septum on each flask and put them both under high positive pressure.
5. To avoid trapped air in the septum cavity, “burp” the septa.
6. Remove the cannula from the oven, ensuring it’s hot. Work quickly after removing the cannula to prevent condensation from forming on the metal.
7. Hold the flask steady and pierce the cannula into the starting flask so it’s still above the solution. You should be able to hear gas flowing through the cannula.

From this point, proceed to your preferred transfer method, cleaning the cannula immediately following the procedure. It’s also a good idea to remove the rubber septa and replace them with greased glass stoppers to prevent air from leaking into the flasks. Be sure to burp any stoppers you replaced throughout the producer to avoid tiny air pockets from interfering with the reaction.

Positive Pressure

To use positive pressure for your cannula transfer, you’ll start by inserting the cannula and a bleed needle into the receiving flask. The bleed needle should be the same gauge or smaller than the cannula. Next, be sure to close any other inlets to the flask and complete the following steps:

1. Lower the needle in the starting flask into the solution to start the cannula transfer. If it doesn’t start within a few seconds, you can lower the receiving flask or raise the height of the starting flask to get it going.
2. Next, transfer the required amount of solution or solvent to the receiving flask and lift the cannula tip out of the liquid in the starting flask just enough to stop the transfer (while keeping it in the cannula). It may take a few seconds for any remaining solution to clear out of the cannula.
3. If there’s a nitrogen inlet in the glassware (such as with Schlenk flasks), open it.
4. Take out the bleed needle from the receiving flask first, followed by the cannula.
5. Pull the cannula from the starting flask and carefully place it to the side.

Negative Pressure

To do a cannula transfer with negative pressure, raise the starting flask as high as possible over the receiving flask after inserting the cannula into the receiving flask. Next, close the inlets on the receiving flask and leave the starting flask open to the line. Press the cannula lower than the solvent in the starting flask.

To start the transfer, insert a bleed needle into the receiving flask. As with the positive pressure transfer, this bleed needle should be the same gauge or smaller than the cannula. Once the transfer begins, remove the bleed needle while opening the receiving flask to the line. This will slow down but continue the rate of transfer. You can also control the rate by raising or lowering the starting flask.

When you achieve the desired transfer amount, lift the cannula tip out of the solution but not out of the cannula. Then, remove the cannula from the receiving flask, followed by the starting flask, and set it aside for cleaning.

Siphon

The last option for cannula transfer is via siphoning. For this method, follow these steps:

1. Put the cannula into the receiving flask.
2. Lower the cannula tip into the solution in the starting flask.
3. Place the starting flask under a dynamic flow of nitrogen.
4. Gently apply vacuum pressure to the receiving flask to kickstart the transfer.
5. Continue intermittently applying vacuum pressure to transfer the desired amount.
6. Open the receiving flask to dynamic nitrogen to stop the transfer.

It’s important to note that you don’t need a lot of vacuum pressure to move the solution. Too much vacuum can make the solution bump, evaporate, and freeze inside the cannula. Siphoning requires careful monitoring and a light touch with the vacuum.

Best Practices for Optimal Transfers

It’s crucial to select the correct gauge cannula for your application. A medium gauge cannula is sufficient to transfer solvents. If performing transfers on a suspension, a thick gauge is needed. Small gauge cannulas are best for layering solvent because they enable slow transfer to protect the layer.

After inserting the cannula into the starting flask, never submerge it in the solution. This will cause the liquid to start spraying onto the fume hood.

Sometimes, it’s difficult to hear the flow of gas when working under low pressure. In this case, you can place the cannula tip close to a liquid to see if ripples form on the surface to confirm gas flow. However, you should never point the cannula at your ear or face to listen for gas flow.

Always direct the needle away from your body to avoid potentially life-threatening lab accidents.

Cleaning the Cannula

While you may be tempted to save clean-up for another day, you should never leave a dirty cannula in the lab. Taking a few extra minutes to clean the cannula properly will keep the lab environment safe and preserve the integrity of your equipment.

Cannulas may be cleaned using water and acetone to flush out any remaining solution. The basic steps for cleaning a cannula that was used to transfer pure solvent are as follows:

1. Place a rubber septum on an Erlenmeyer flask, connecting the side arm to a water aspirator.
2. Stick the cannula tip into the rubber septum.
3. Insert the other end of the cannula into a flask with acetone and flush it for a few seconds.
4. Wipe the outside of the cannula with an acetone-dipped wipe to remove any remaining residue.
5. After the cannula is completely dry, place it back in the oven.

For cannulas that transfer a metal-containing solution, rinse with acetone, followed by a water rinse. Then, rinse with a 1 M HCL solution, water again, and finally acetone. Ensure that the final rinses run clear; otherwise, repeat the process.

If transferring solutions that contain only organic ligands, rinse them with the solvent in which the ligand was dissolved, followed by acetone. If the solution was a suspension, rinse with water, acetone, and hexanes.

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References

Cannula Transfer. The Schlenk Line Survival Guide. 2024.

Section 5.10 Title: Cannula Transfers. University of Wisconsin-Madison. November 1, 2019.