Most research peptides are supplied in one of two formats: a lyophilised (freeze-dried) powder in a sealed vial, or a pre-filled pen that already holds the peptide in solution. The compound can be identical. What changes is how it is stored, prepared, documented and priced. This guide sets out the practical differences so you can choose the format that suits your protocol.
What each format is
A vial contains the peptide as a dry powder or 'cake', sealed under vacuum or inert gas after freeze-drying. Before use, the lab adds a chosen diluent and volume to bring it into solution, which means the researcher sets the final concentration.
A pre-filled pen contains the peptide already in solution inside a sealed cartridge at a fixed, stated concentration. There is no reconstitution step: the measured volume is dispensed directly from the cartridge.
Stability and storage
Dry, lyophilised material is generally the more stable form. Water is what drives most peptide degradation (hydrolysis, deamidation, oxidation and aggregation), so removing it slows those reactions down. Unopened vials are typically stored cold and dry, with frozen storage common for longer periods.
A peptide in solution has a shorter stability window, so pens are usually kept refrigerated and used within a defined period once opened. Whichever format you hold, the storage guidance on the batch label and its documentation takes priority over any general rule.
Preparation and handling
Vials give you control. You choose the diluent and the volume, so you can work at the concentration your protocol calls for, and you can reconstitute only when you are ready to start. The trade-off is an extra preparation step, and every preparation step is a place where a calculation or handling error can creep in. Our reconstitution calculator does the concentration maths for you.
Pens remove that step. The concentration is fixed and stated, which reduces preparation variables between runs, but you lose the flexibility to change concentration or to store the material dry for long periods.
Documentation to check
Both formats should come with a Certificate of Analysis tied to the batch number on the label. For pens, it is worth confirming what the CoA covers: the purity of the peptide that went into the cartridge, and ideally the stated concentration of the filled product, not only the raw powder before filling.
Cost per milligram
Vials are usually the lower cost per milligram, because a pen's price also covers the cartridge and the filling step. When comparing suppliers or formats, divide the price by the milligrams of peptide supplied rather than comparing unit prices.
Which format to choose
A quick rule of thumb:
- Choose a vial if you need to set your own concentration, store material dry for longer, or work at a larger scale.
- Choose a pen if you want a fixed, stated concentration and fewer preparation steps between runs.
- Whichever you choose, match the batch number on the label to its Certificate of Analysis before you start.