Measuring volumes using needles and syringes under a modified atmosphere

Needles and syringes offer a convienient way to add liquids to reactions which are being performed under a modified atmosphere. The below example assumes the addition of a solvent being maintained under an inert atmosphere and this being added to an empty flask which has been placed under an inert atmosphere using balloon and septa techniques. The below may be easily adapted for the use of reagents stored under inert atmospheres, or for the addition to other methods of air sensitive chemistry (eg Schlenk lines). Some notes are provided after the procedure to indicate some of the modifications which may be required.

Step by step process

The process below explains how to draw up a liquid into a needle held in a modified atmosphere. The below assumes drawing up solvent from a round bottomed flask which is held under a nitrogen atmosphere, with this solvent then being introduced into a reaction flask which has already been prepared using the guidance for setting up a reaction flask under an inert atmosphere.

Preliminaries

• Select an appropriate needle and syringe for use. Considerations may include:

  • Amount of liquid to be dispensed and the syringe volume

  • Length of needle required (generally long needles are used)

  • Whether a Luer lock or Luer slip needle is appropriate

Preparing the needle and syringe

• Remove both the syringe and needle from its packaging and attach the two together. For Luer slip needles, you should slide the needle onto the end of the syringe and ensure that it is tightly connected. For Luer lock needles you should screw the needle into the Luer lock. At the stage you should not remove the sheath from the needle.

• Once the needle is attached and you are ready to start purging/drawing up. the sheath can be carefully removed from the needle.

• This is acheived by loosening the sheath and allowing it to slide off by gravity.

• Once the sheath has been removed, the best way to hold the needle and syringe is to have one hand on the syringe, and one hand on the needle. This way you can't end up with your hands coming into contact with the needle which could result in a needlestick injury.

Purging the needle

• Purge the needle and syringe; holding the base of the needle, guide it into the centre of the septum and pierce the needle through. With the base of the needle out of the liquid, hold the syringe and lift the plunger, taking in nitrogen. Hold the base of the needle to lift it out of the septum and then expel the nitrogen out of the syringe. 

  • When pulling nitrogen into the syringe, hold onto the barrel with one hand, and the plunger in the other.

  • When expelling, hold the needle onto the syringe at the joint. This is the weak part where the incresed pressure on expelling may cause the syringe and needle to detach. Whilst this is unlikely when purging, it is a good habit to get into to ensure that you never expel from a syringe without holding the needle securely onto the syringe.

• Repeat the nitrogen flushing for a total of three times.

Drawing up liquid

• Take up liquid; hold the base of the needle and pierce through the septum, this time immersing the base of the needle in the liquid. Whilst holding the barrel of the syringe, slowly pull up the plunger taking up liquid. Take up more liquid than you need to measure out.

  • The septa will only pierce in the centre of the septum, inside the circle which is drawn. Outside this, the needle will get stuck in thick rubber. When you insert, ensure you are going through parallel to the sides of the setum, so that the needle doesn't get stuck in the solid rubber. Have a look at the design of another septum so you can see where to inject the needle through.

  • Note that the viscosity of the liquid will exert resistance for drawing up the solution, you may need to be patient for the material to be fully drawn into the barrel

• Get rid of air bubbles; supporting the needle-syringe attachment point, lift the needle partly out of the septum and invert the syringe. This will allow the air bubbles to travel to the top of the syringe. You may need to carefully rotate the syringe in the needle joint without detaching it, in order for the bubbles to be at the syringe exit point. Slowly expel the air bubbles whilst holding the needle/syringe joint in place and gently pushing on the plunger.

• Taking the correct volume; with the needle and syringe inverted, continue to expel the liquid until you have the correct volume you need for your experiment. Ensure you continue to hold the needle firmly onto the syringe Luer lock as you do this.

• Forming a nitrogen barrier; with the base of the needle still out of the liquid, pull the plunger back out again. This allows a layer of nitrogen gas to sit over the top of the liquid, protecting it from the atmosphere for when you transfer your liquid into your reaction flask.

• Transferring the liquid; Taking the syringe in one hand, with the needle/syringe joint held between your thumb and index finger, you can then take old of the base of the needle and remove it slowly from the septum. Ensure you have control of the needle to avoid it 'flicking' as you remove it from the septum.

• Pierce through the septum of the reaction flask with the needle. Hold the syringe and slowly expel the liquid into the flask, ensuring the needle/syringe joint is held securely to prevent the needle popping off the end of the syringe.

  • If you need an accurate volume, you should expel the nitrogen layer first, then expel the liquid. If the measure is less crucial (eg solvent), then you do not need to keep the needle and syringe inverted. The difference here is in that the syringe has a 'dead volume', in that the needle contains a small amount of solvent when emptied, unless the needle contents are expelled. The needle can be caused to expel by using the nitrogen cushion if it is pushed from the syringe after the liquid. For larger quantities this is insignificant, but on a 1 mL syringe this can be a significant change in the amount reagent being added.

• Hold the base of the needle to remove from the septum and carefully dispose of the needle and syringe into the sharps bin as soon as possible.

Additional Information

Non-air sensitive reagents

For reagents in solid form, it is usually easier to add them (along with the stirrer bar) before purging the flask. This way you don’t have to remove the septum later to add them in, which would expose the purged flask to the atmosphere.

For adding liquids which are not air sensitive, the approach taken may vary depending on the reaction requirements. There are step by step instructions in the 'Measuring volumes' tab. If adding non-air sensitive reagents to an air-sensitive reaction, it is good practice to still purge the syringe (from the reaction flask) before drawing up the reagent in order to minimise any introduction of atmosphere. It is also important not to draw up a cushion of air, which would be injected into the purged flask. Remember to clamp the bottle when drawing up the liquid, as it could easily tip over during the transfer.

Reagents stored under inert atmospheres

Some reagents need to be stored in an inert atmosphere to preserve reagent quality. They are usually unstable or will react with the air and water vapour. It would not be feasible to store them with a nitrogen balloon in them as this would require the balloon to be constantly changed. Typically, they are packaged under nitrogen or argon in a crown-cap bottle with a small rubber hole. When a needle is withdrawn from the hole it reseals to prevent moist air entering the bottle.

When removing liquid from the reagent bottle, it creates a vacuum where the liquid used to be which needs to be replaced with an inert gas instead of the atmosphere. This can be achieved by placing a nitrogen/argon balloon into the rubber seal prior to drawing up any liquid. Any liquid removed will be replaced with the inert gas.