Needles and syringes offer a convienient way to measure and add volumes of liquids into reactions. Their use is particularly prevelant with reactions which are being performed under a modified atmosphere, and this is reflected in the guide below.

The guidance below example assumes that a reaction flask has been set up under an inert atmosphere using balloon and septa techniques with the flask being under a balloon of nitrogen. The techniques may be easily adapted for the addition of liquids to other methods of air sensitive chemistry (eg Schlenk lines), for reactions under other gases (eg argon, hydrogen or oxygen) or for addition to reactions being carried out open to the atmosphere.

• 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 for drawing up liquids)

  • Whether a Luer lock or Luer slip needle is appropriate

  • The hazards for the compound (eg corrosive or pyrophoric)

  • How air sensitive a reagent is

• Remove both the syringe and needle from its packaging and attach the two together. At the stage you should not remove the sheath from the needle.

• You should keep the sheath in place until you are ready to start purging/drawing up. Unsheathed needles are a sharps hazard and there is the potential for needlestick injuries to occur.

• The sheath should be removed by holding the coloured part of the needle with one hand and loosening the sheath with the other. Then you can allow the sheath to slide off the needle 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 otherwise result in a needlestick injury.

Prior to drawing up liquids, it is usual to purge the syringe and needle from the atmosphere in the reaction flask. Needles and syringes are usually provided in sterile packaging, but whilst sterile, they have still been packaged under air. By purging the needle and syringe from the inert atmosphere in the reaction flask, this is further minimising the amount of air the reaction is being exposed to.

It is best practise to purge the needle and syringe from an assembled reaction flask, rather than purging using solvent or reagent bottles. Purging from solvent or reagent bottles adds to the number of needle insertions through their septa, which have a limited lifetime, and potentially introduce atmospheric air into these containers.

• At this stage, the sheath has been removed and the needle and syringe should still be held such that one hand is holding the base of the needle, and the other hand is holding the syringe barrel.

• Using the hand on the needle, guide the needle towards the centre of the septum and pierce the needle through into the flask. The needle should be pushed into the flask so that the base of the needle remains in the headspace above the liquid.

• The circle on the septum indicates the thin part of the septum through which the needle should be inserted.

• When inserting the needle through the septum and into the flask, take care to ensure the needle doesn't get stuck into the thick rubber sides of the septum.

• Once the needle is into the headspace, your hands should be repositioned such that one hand is holding the barrel of the syringe and the other hand is holding the syringe plunger.

• The plunger can then be extended to draw nitrogen into the syringe barrel. The syringe barrel wants to be filled with nitrogen, extending the plunger to its maximum extent without removing the plunger from the barrel.

• Once the syringe has been filled with nitrogen, hands should be repositioned again. One hand should be on the barrel of the syringe and the other holiding the needle just above the septum.

• The hand holding the needle should then guide the needle out of the septum.

• Hands should be repositioned such that one hand is holding the barrel of the syringe in a manner it which it can be used to expel the nitrogen from the syringe, and the other hand should hold the needle at the joint between the syringe and the needle.

• At the interface between the needle and syringe there is a pressure change, as the larger diameter syringe outlet connects into the narrower needle. With Luer slip needles, it is possible for this pressure difference to cause the needle and syringe to separate. Whilst Luer locks prevent this separation, it remains good practise to position your hands to hold this joint before expelling the syringe contents.

• The plunger can then be steadily depressed to expel the nitrogen from the syringe. This should be carried out in a controlled manner, holding the needle/syringe joint throughout.

At this point, the purging process should be repeated. Typically purges are carried out three times in total, but this can be adjusted appropriately.

Liquid solvents and reagents come in a variety of containers, and whilst there are some differences for drawing up liquids, most of the principles are common to the different designs. Some of the commonly encountered containers include:

Liquid solvents and reagents come in a variety of containers, and whilst there are some differences for drawing up liquids, most of the principles are common to the different designs. Some of the commonly encountered containers include:

• Round-bottomed flasks fitted with septum and balloon. These are particularly common for use with dried solvents.

• Manufacturer supplied bottles with integrated seals to maintain chemicals under an inert atmosphere. There are a number of different designs which are bespoke to particular manufacturers. Some common examples include:

  • AcroSeal® manufactured by Acros

  • Sure/Seal® manufactured by Aldrich

  • ChemSeal® manufactured by Alfa Aesar

• Simple reagent bottles containing liquids. When the lid is removed, these reagents are open to the air.

The instructions below continue from the end of the purging of the needle and syringe from a reaction flask.

• At this stage, the syringe and needle should be held such that one hand is holding the syringe barrel and the other hand in holding the needle which is attached to the syringe.

These steps show the drawing up liquid from a round-bottomed flask containing dried solvent. The same principles apply to chemical bottles with built-in seals. For open bottles, there are a few modifications to this procedure, which are explained in the guide linked below.

• At this stage of drawing up, the syringe should be filled to a greater volume than the volume required.

• The excess amount varies depending on syringe size, but typically drawing up around 20% extra is a reasonable rule of thumb.

• In the example below, we are aiming to transfer 10 mL of liquid. Note that around 12 mL has been drawn up.

As the liquid is drawn into the syringe, a gas bubble will form above the liquid. This comes from a combination of the dead-volume in the syringe and needle. This is the nitrogen present is the syringe and needle which is drawn into the syringe ahead of the liquid. It is also quite common to draw in some nitrogen from the reagent bottle, particularly if the needle is not very deep within the liquid.

• The nitrogen bubble will need to be displaced to enable the accurate measurement of the liquid volume.

• Hands should be positioned such that the thumb and forefinger on one hand are holding the needle/syringe joint. The other hand should be holiding the barrel, ready to expel the nitrogen bubble.

• The syringe is lowered by flexing the needle, until the syringe is fully inverted alongside the flask or reagent bottle.

• Once the gas bubble has been expelled, the excess volume can then be returned to flask. In order to read the measurements, the syringe graduations will need to be facing you.

• For Luer lock syringes, you cannot twist the needle in the syringe, as this would loosen the connection of the needle in the Luer lock. If the syringe needs repositioning, this needs to be achieved by returning the syringe to the vertical and flexing the needle again such that the markings are towards you.

• For Luer slip, the syringe can be carefully rotated in the needle without detaching the needle and syringe from each other.

• The final step in the drawing up process is to protect the liquid prior to transfer by drawing nitrogen back into the syringe. This ensures there is no liquid present in the needle, and the liquid in the syringe has a nitrogen layer protecting the liquid from any contact with the air.

To transfer the liquid, the needle syringe need to be removed from the solvent flask and inserted into the reaction flask.

• The needle/syringe connection should be held between thumb and forefinger with the rest of the same hand holding the syringe.

• The other hand should carefully hold the needle close to the septum. The needle should then be removed from the flask, keeping the syringe inverted and the needle forming an 'n' shape. Care needs to be taken to avoid the needle flicking as the needle is withdrawn from the septum.

To ensure that the correct volume is transferred into the flask, it is first nescessary to expel the air bubble from the syringe. This ensures that when the liquid is subsequently transferred, a 'dead-volume' remains in the syringe, which was accounted for during the measuring. Note that for small syringes, eg 1 mL, the dead volume can be significant (~20%), which could significantly alter reaction stoichiometry if transferred.

• The syringe and needle should still be in the same orientation as the transfer, with the syringe plunger at the bottom and the syringe exit uppermost.

• One hand should be transferred to hold the syringe/needle joint, and ensure these remain securely attached. The other hand should be placed on the syringe plunger to be able to expel the gas bubble.

• Carefully expel the gas bubble from the reaction.

Consider any specific instructions for the liquid addition, such as if the liquid needs to be added dropwise.

• Ensure the gas bubble has been expelled.

• Continue to expel the liquid into the reaction flask.

• The syringe does not need to remain inverted during liquid addition.

• Once the transfer has been completed, the needle and syringe should be removed from the reaction flask, with both parts placed straight into a sharps bin.†

Note: The syringe and needle will still contain a small volume of the liquid (the 'dead-volume'). For some compounds it may be nescessary to carry out additional waste disposal procedures for this dead volume prior to syringe / needle disposal.

†Disposal processes vary in different laboratories. Waste disposal for syringes and needles in York requires both parts to be placed into sharps bins, so there is no need to separate needles/syringes.