Accepted answer
60 ÷ 3 = 20 mg/mL. Concentration is vial content divided by diluent volume, so 60 mg of peptide in 3 mL of bacteriostatic water gives 20 mg/mL. On a U-100 barrel one unit is 0.01 mL, so one unit of this solution carries 0.2 mg — 200 µg. That is the number to write on the label, because you will not reconstruct it from memory at an awkward moment.
The relevant detail is that full dissolution of a well-lyophilised cake should take under a minute with gentle swirling and no agitation. If it takes ten minutes, the cake is either over-dried, partially collapsed, or the peptide has already aggregated.
Check the barrel marking, not your memory of it.
Reading a lyophilised cake
| Appearance | Interpretation | Action |
|---|
| Intact opaque puck, proud of base | Cycle ran correctly | Proceed |
| Slumped to one side | Shipped before fully dry, or vibration | Usually usable; note it |
| Glassy translucent film | Collapse above glass transition | Test before use |
| Melt-back ring at stopper | Thermal excursion in transit | Test before use |
| No visible cake at all | Very low fill, or nothing there | Weigh it; query the supplier |
If the material arrived warm and it was lyophilised, test it and proceed on the result.
The 28-day beyond-use convention for a multiple-withdrawal preserved preparation derives from USP compounding chapters, which set it on microbiological risk rather than chemical stability.
Do the arithmetic twice, ideally with someone else doing it independently.
edited 26 Aug 2026 by e_dziedzic — expanded the table to cover the lower concentration
6Thank you — the worked example is what makes this usable. – tabular_nums 9 months ago add a comment