Accepted answer
5 ÷ 1 = 5 mg/mL. Concentration is vial content divided by diluent volume, so 5 mg of peptide in 1 mL of bacteriostatic water gives 5 mg/mL. On a U-100 barrel one unit is 0.01 mL, so one unit of this solution carries 0.05 mg — 50 µg. That is the number to write on the label, because you will not reconstruct it from memory at an awkward moment.
Stated carefully, the answer depends on what you want your measurement resolution to be, and that is a real trade-off rather than a preference. More diluent gives you more syringe marks per dose and therefore less rounding error; it also gives you a larger volume to keep cold and a longer period over which the solution has to remain within specification.
On re-freezing something that thawed in transit: if it arrived as a lyophilised solid that warmed but never got wet, re-freezing costs you nothing except the thermal cycle.
Dead space by syringe type
| Configuration | Dead volume | Loss at 5 mg/mL | Over 20 draws |
|---|
| Fixed-needle insulin syringe | 3–5 µL | 15–25 µg | 0.3–0.5 mg |
| Low-dead-space, detachable | <2 µL | <10 µg | <0.2 mg |
| Standard luer-lock + 30G | 35–60 µL | 175–300 µg | 3.5–6 mg |
| Luer-lock + 21G drawing needle | 70–100 µL | 350–500 µg | 7–10 mg |
It helps to be literal here: tilting the vial to pool solution in the corner before the final draw, and giving it a minute to drain down the walls, genuinely recovers ten to twenty microlitres.
Published data on syringe dead space in the context of injection-equipment programmes quantifies low-dead-space designs as retaining under 2 µL against 35 µL or more.
Do the arithmetic twice, ideally with someone else doing it independently.
5Confirming: I did the wrong thing here once and got exactly the predicted result. – kwn_analytical 8 months ago 4Minor: the filter membrane chemistry matters as much as the pore size for adsorption. – Dr_Marek_Zielinski 6 months ago add a comment