A U-100 insulin syringe is graduated in units defined by insulin concentration: 100 international units of U-100 insulin occupy 1 mL, so the barrel markings run 0 to 100 across one millilitre and one unit equals 0.01 mL. On a 0.5 mL barrel the same 0.01 mL per unit holds, with markings to 50.
Why U-100 units matter in peptide research
The critical point is that a unit is a volume mark, not a quantity of peptide. It only carried a peptide meaning for insulin because insulin was standardised at 100 units per millilitre. For any reconstituted research peptide, what sits in ten units depends entirely on the concentration produced at reconstitution. A worked example: a 5 mg vial in 2 mL gives 2.5 mg/mL, so 10 units (0.10 mL) contains 250 mcg and 1 unit contains 25 mcg. Reconstitute the same vial in 1 mL instead and each unit holds 50 mcg.
That is why the vial concentration, not the unit count, should be recorded on the label. Syringe selection also involves gauge and barrel size — see 31G 1 mL syringes, 29G 0.5 mL syringes and the syringe selection guide. All items are supplied for laboratory research use only.