Do I have to inject air first?

For small amounts you can often get away without. For anything approaching a millilitre it makes the difference between a clean draw and a fight.

How much air should I inject?

Roughly the same volume you intend to remove. It does not need to be exact.

Why is my syringe full of bubbles?

Almost always the needle tip is near the surface rather than deep in the liquid, or you are drawing too fast, or you skipped the air.

Filling4 min read

Drawing from the vial

Inject a little air into the vial before drawing, equal to the amount of liquid you want out. Without it you are pulling against a vacuum, the plunger fights you, and the syringe fills with bubbles instead of liquid.

Why the syringe fights you

A sealed vial has a fixed amount of air in it. Take liquid out and that air has to expand to fill the space, which drops the pressure inside. The lower the pressure gets, the harder the atmosphere pushes back on your plunger, and the more likely air is pulled through the needle instead of liquid.

Putting air in first pre-empts the whole problem. It is the difference between a smooth draw and a fight.

The method

Draw air equal to what you want out

Want 1ml of liquid? Pull the plunger to 1ml with the needle in the air first.

Insert through the stopper, vial upright

Push the air in while the vial is the right way up, so the needle tip is in the air space and not blowing bubbles through the liquid.

Invert, tip below the surface

Turn the vial upside down and make sure the needle tip sits well inside the liquid, not near the surface where it can catch air.

Draw slowly

Slowly enough that no bubbles form at the tip. If bubbles appear, you are pulling too fast or the tip is too shallow.

Clear any air before withdrawing

With the vial still inverted, push small bubbles back into the vial and draw again. This is much easier now than after you have taken the needle out.

Draw a touch more than you need, then push the excess back before withdrawing. It gives you room to clear bubbles without ending up short.

Questions

Do I have to inject air first?: For small amounts you can often get away without. For anything approaching a millilitre it makes the difference between a clean draw and a fight.
How much air should I inject?: Roughly the same volume you intend to remove. It does not need to be exact.
Why is my syringe full of bubbles?: Almost always the needle tip is near the surface rather than deep in the liquid, or you are drawing too fast, or you skipped the air.