Accepted answer
At 1 mg/mL a 1 mg dose is 1 mL — 100 units on a U-100 barrel — and no needle gauge changes that number. Gauge changes three other things: how long the draw takes, how much stays behind in the hub, and how much rubber you core out of the stopper. On the 30G scale a larger number is a finer needle, so a 30G needle is fine enough that a viscous solution draws slowly and a hurried draw pulls bubbles. If you are drawing 100 units at a time, the dead space matters more than the bore: a fixed-needle barrel loses microlitres, a luer hub loses tens of them, and at 1 mg/mL each microlitre is 1 µg.
The relevant physics is the fourth-power dependence of flow on radius, which makes small gauge differences enormous in practice.
Typical outer diameters: 21G is about 0.82 mm, 23G about 0.64 mm, 25G about 0.51 mm, 29G about 0.34 mm and 31G about 0.26 mm. The gauge number and the diameter move in opposite directions.
For injecting, 29G to 31G is the usual range and the difference in perceived discomfort between them is small. Needle length matters more than gauge for comfort at these volumes.
Needle gauge to outer diameter correspondence is standardised and published; the inverse relationship between gauge number and diameter is the reason for the counter-intuitive labelling.
Angle the bevel and insert gently to avoid coring the stopper.
Minor: the filter membrane chemistry matters as much as the pore size for adsorption. – a_lindgren 8 months ago 8I have added the label-the-vial suggestion to my own notes. Obvious in hindsight. – RP_C18 6 months ago add a comment