genuine question about units that I am slightly embarrassed to ask
genuine question about units that I am slightly embarrassed to ask. If this has been answered properly somewhere, link me and I will delete. The failure modes I have actually seen posted here, in order of frequency. One: reconstituting at a different volume than intended and not writing it down, so the next draw is…
The arithmetic in full: concentration is mass over volume, so 10mg into 3mL gives 3.33mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 1mg is 1 divided by 3.33 millilitres, which is 30 units.
The lyophilised cake displaces a small volume, so the true concentration is very slightly lower than the arithmetic suggests. It is a fraction of a percent and it is dwarfed by fill tolerance.
Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.
Spent 9 weeks confused about this until somebody drew the table for me. 5mg, 3mL, 1.67mg/mL, 1mg is 60 units. That is the whole thing.
Aim the stream down the wall of the vial rather than into the cake. Reconstitution is a solubility problem, not a mixing problem, and mechanical shear does nothing helpful.
I would not round that figure. At 10mg/mL a rounding error of two units is a real fraction of the intended amount.