Why dry, cold lyophilized peptide is stable, and what degrades it
Lyophilization removes the bulk water from a purified peptide solution and leaves a porous cake in which the molecules barely move. The reactions that degrade a peptide in solution, deamidation, backbone hydrolysis and oxidation, all need water as reactant or solvent. Remove the water and cool the cake, and a sealed container holds its certificate values for a long time.
The acylated side chain does not change this. The C18 diacid on semaglutide and the C20 diacid on tirzepatide, retatrutide and cagrilintide are chemically robust; what degrades is the peptide backbone.
What degrades a sealed lot
Moisture is the main enemy. A lyophilized cake is hygroscopic: once water gets in, molecular mobility rises and solution chemistry restarts. Deamidation of asparagine shows on mass spectrometry as a +1 Da shift and on reversed-phase HPLC as a new shoulder beside the main peak; an aspartate that closes to a succinimide ring loses water and appears at −18 Da.
Heat cycles accelerate every route, and the damage is cumulative. Light and oxygen drive oxidation of tryptophan and methionine, which appears as +16 Da on the mass spectrum and as an earlier-eluting HPLC peak. Cagrilintide adds a disulfide that can scramble if the cake is wet and warm.
Repeated opening is the route a buyer controls most directly. Each opening exchanges the headspace with humid laboratory air, and a cold container pulls in condensation. A vial opened once is at low risk; a bulk bottle opened weekly is not.
The cold chain from Guangzhou and what to inspect on arrival
Material leaves the Guangzhou site in a sealed primary container, crimped vials with stopper and flip cap or a sealed bottle for bulk powder, inside an inner carton that carries the lot label. The inner carton sits in an insulated box with gel packs or dry ice, inside a corrugated outer carton with the airway bill.
The dispatch photos on the order-photos page show this arrangement at the packing bench before courier collection: outer box, inner carton with lot label, cold packs or dry ice, and the courier label with the destination visible.
Arrival inspection
Open the parcel on the day it arrives. Check, in order: the outer carton for crush or water damage; the insulated box seal; the cold-pack state or any remaining dry ice; the inner carton lot label against the lot alias on your quote and on the COA; the primary containers for intact crimps, stoppers and tamper seals; and the count, remembering that 1 kit = 10 vials. Photograph the open carton and the lot label.
A thawed gel pack is a finding to log, not a reason to discard. A dry cake tolerates a transit excursion far better than any solution, and the inspection point is the cake itself: collapsed, discolored, sticky or visibly wet means something went wrong; a clean white cake behind an intact seal is the normal case. If the excursion concerns your QA, bring the first re-test forward.
Every lot manufactured by Pepti, Guangzhou carries the same lot alias on the courier paperwork, the inner carton and the certificate. If any of the three differs, stop and ask before storing the material.
Storage conditions for sealed lyophilized peptide
The condition that governs a lot is the one printed on its batch COA. The figures below are the commonly used conditions for sealed lyophilized peptide in research laboratories; they are context for reading the certificate, not a stability commitment by Pepti for any lot, and they stop applying once a container has been opened.
- −20 °C is the commonly used long-term condition for sealed lyophilized peptide. Prefer a manual-defrost freezer; a frost-free model warms periodically to clear ice.
- 2–8 °C is the commonly used short-term condition for material about to be opened or being worked through, such as the working set of vials from a kit.
- −80 °C is used where a site prefers it for reference material or long holds. The dry seal still matters more than the extra cold.
Dry, dark, undisturbed
Keep the primary container in a closed secondary bag with a desiccant pouch, in the body of the freezer rather than the door. Keep it dark, in amber glass or an opaque carton, to prevent photo-oxidation of tryptophan and methionine.
Minimize freeze-thaw by not taking the whole kit out to retrieve one vial. Split a kit on arrival into a reserve set that stays sealed at the long-term condition and a working set that moves to the short-term condition when needed; the reserve set is where a retained sample for a later re-test comes from.
The three dates on the certificate and what retest means
A Pepti batch certificate shows three dates: manufactured, tested and retest. The manufactured date is when the lot came off the final process step. The tested date is when the release results on the certificate were generated. The retest date is the point by which the holder should re-examine the lot before relying on it further.
Take the public record for PEP260718-SM20 as the worked example: semaglutide, 20 mg per vial, HPLC purity 99.6%, manufactured 2026-07-18 and tested 2026-07-28. The retest date on the same record is the trigger for the holder’s next examination, and 99.6% is the reference for any later HPLC result on that lot.
Retest is a trigger, not a promise
An expiry date says the material should not be used after that day. A retest date says the manufacturer’s knowledge of the sealed container under the stated condition runs to that day; beyond it the holder needs fresh data. If a re-test by HPLC and MS meets the original certificate, the holder may continue under its own QA policy and set a new internal retest point; if not, the lot is quarantined or discarded.
A lot that has been opened, aliquoted, stored warmer or shipped on has left the conditions the date was set for, and the holder’s own log of those events becomes the basis for deciding when to re-examine it.
As the date approaches
Decide early between re-testing and discarding; a laboratory result takes time. If the lot is worth keeping, book the HPLC and MS work so the report lands before the retest date, record the result against the lot alias and set the next internal retest point.
Lot segregation and the incoming log
Lot integrity is the ability to say, for any container in the freezer, which certificate describes it. It rests on one rule and one record. The rule: one lot per container and per storage box, with no mixing of remnants. The record: an incoming log that links each container to the paperwork.
Incoming log fields
A workable log has these columns: date received; compound; lot alias from the inner carton; quote or order reference; format and quantity, for example 1 kit (10 vials) or bulk powder from 5 g, shipped cold-chain; COA received and its HPLC value; carton and seal condition; cold-pack state; storage location assigned; the person; and the photograph file reference. Add a line for each opening: date, mass removed, person.
The chain runs from the quote, which names the lot alias allocated to the order, to the COA for that alias, to the carton label, to the freezer log entry, to the label on each aliquot. If any link breaks, the certificate no longer describes that container with confidence.
Vials and bulk powder behave differently
Opening one vial does not expose the others. Bulk powder is one container, and every opening exposes the entire lot. Aliquot it once, soon after arrival; label each aliquot with the lot alias, an aliquot number and the date; return the primary container to the long-term condition as the reserve. For private-label orders of 50 kits (500 vials), keep at least one sealed vial per lot in reserve so a re-test stays possible.
| Point | Lyophilized vials | Bulk powder |
|---|---|---|
| Unit of exposure | One vial; the rest stay sealed | The whole lot, at every opening |
| Aliquoting | Not needed | Once, soon after arrival, under dry conditions |
| Retained sample | Sealed vials from the reserve set | A sealed aliquot set aside when aliquoting |
| Weighing | Not usually | Quickly, closed vessel, driest environment |
| Storage split | Reserve long-term, working set short-term | Primary long-term, current aliquot short-term |
Equilibrating, opening, weighing and resealing
Condensation is the commonest avoidable way to wet a lot. A container taken straight from −20 °C into a room-temperature laboratory is below the dew point of the room air, and water condenses on the glass at once, and on the cake the moment the seal is broken.
Move the container to the bench in its secondary bag with the desiccant still inside, and leave it closed until the outer surface is at room temperature: no fog on the glass, no chill to the touch. Only then take it out, wipe it and open it.
Weighing hygroscopic powder
Once a bulk container is open, the cake takes up water from the air and its mass rises. Weigh quickly, into a tared closed vessel, in the driest environment available, a glove box or dry cabinet if you have one. Do not return powder to the primary container, and close it before you write down the figure. The balance reading includes residual water and counter-ion, so the certificate’s content figure remains the reference for how much peptide a container holds.
Closing and resealing
Replace the stopper or cap at once, renew the desiccant if the indicator has changed, wrap the closure with sealing film, return the container to its bag and log the opening. A container that goes back to the freezer loosely capped collects frost inside.
When to re-test and what to ask the laboratory for
There are two reasons to re-test a sealed lot: the retest date is approaching, or something has happened that the certificate did not anticipate, such as fully thawed cold packs in transit, a freezer failure, a broken seal or a visible change in the cake.
What to ask the laboratory for
Ask for RP-HPLC purity as area percent by a gradient reversed-phase method, with the chromatogram, not only the number. Ask for MS identity with the observed mass compared against the theoretical mass; for semaglutide that is the PubChem formula C187H291N45O59 at about 4114 g/mol. If sample mass allows, ask for water by Karl Fischer titration.
The Janoshik Analytical report #210105 shows what an independent report looks like for this family: a retatrutide 20 mg sample, 19.52 mg content and 99.837% purity, analyzed 2026-10-07. It is sample-specific, but its format, content and purity on one page with a verification link, is what to ask for.
Comparing the result
Compare the re-test against the lot’s original certificate value and against the published range for the compound. For semaglutide the public records run 98.7%–99.6%; for retatrutide, 98.72%–99.55%. A re-test inside that range and close to the lot’s own value is the normal outcome.
Do not overread small differences. Two laboratories with different columns, gradients and integration settings will not report identical area percents, and a small shift is more likely method than material. What matters is a new peak absent from the original chromatogram, a +16 Da or +1 Da species on the mass spectrum, or a value well below the published range. Those are reasons to quarantine the lot and contact Pepti with the lot alias, your chromatogram and your storage log. The quality page explains what the original batch record contains.
Storage checklist by stage
The table collects the actions above by stage. Adapt the record column to your own QA system.
| Stage | Do | Record |
|---|---|---|
| On arrival | Inspect carton, box seal and cold-pack state; match the lot alias to the quote and the COA; check seals and count; store at the condition stated on the COA. | Incoming log entry; arrival photographs; COA filed under the lot alias. |
| First week | Split the kit into reserve and working sets; aliquot bulk powder once under dry conditions; label every aliquot with lot alias, number and date; confirm the freezer thermometer logs. | Aliquot list; freezer location; thermometer log started. |
| Monthly | Check indicator silica and desiccant; confirm no excursion in the freezer log; confirm every container carries a logged lot alias; note remaining quantity. | Monthly line in the lot log; any excursion with date and duration. |
| Before the retest date | Decide re-test or discard well ahead; send a sealed vial or aliquot for RP-HPLC purity, MS identity and water; compare with the COA value and the published range; set the next internal retest point or quarantine. | Laboratory report filed with the COA; decision and new retest point in the log. |
When a result on this lot is questioned later, the certificate, the chain of labels and the storage history should go on the table together. The public COA library on peptioem.com held 76 entries when checked on 2026-10-19; each record shows the same three dates, and the habits above keep them meaningful for the container in your freezer.
Pepti does not present itself as a cGMP drug-substance (API) manufacturer or CDMO and does not publish a GMP certificate or DMF. The storage conditions above are commonly used laboratory conditions; the one that applies to a lot is stated on its batch COA. For research use only. Not for human or veterinary use.
Sources and verification
- Pepti public COA library — 76 entries when checked on 2026-10-19; the three dates and the HPLC and MS fields on each record
- Batch record PEP260718-SM20, semaglutide 20 mg — HPLC 99.6%, manufactured 2026-07-18, tested 2026-07-28; worked example for the three dates
- Batch record PEP260819-RT20, retatrutide 20 mg — HPLC 99.26%, manufactured 2026-08-19, tested 2026-08-22
- PubChem CID 56843331, semaglutide — Molecular formula and computed molecular weight used for the MS identity comparison
- Janoshik Analytical report #210105 — Independent report on a retatrutide 20 mg sample analyzed 2026-10-07; sample-specific, not a claim about other lots