Testing bulk peptide powder before you buy a batch: sampling, tests and retained samples

Peptide raw material testing: what to check before you pay

Bulk peptide powder is bought by the gram or kilogram, often from manufacturers in China, then filled into vials or sold on. A problem found after filling costs far more than one found before payment. Peptide raw material testing answers three questions about the lot:

1. Is it the right peptide, and how pure is it?
2. How much of each gram is actually peptide?
3. Is the whole lot the same?

The EMA guideline on synthetic peptides lists what a full specification for a peptide substance covers. It includes appearance, identification, purity, assay (content), counter-ion identity and content, residual TFA, water, residual solvents, bacterial endotoxins and microbiological purity. A buyer does not need every test on every lot, but the list shows what the factory should be controlling.

Test	The question it answers	Why it matters for powder
Identity (LC-MS)	Is the main peptide the one on the label?	A wrong peptide is the costliest error in a bulk lot
Purity (HPLC)	What share of UV peak area is the target peptide?	Shows related impurities from synthesis
Content	How many mg of peptide, as free base, against a reference standard?	Sets how much peptide you are paying for
Counter-ion (TFA, acetate)	What share of the powder is salt?	Salt adds weight but no peptide
Water content (Karl Fischer)	What share of the powder is water?	Lyophilised peptides take up water from air
Residual solvents	Which solvents are left from synthesis or purification?	Solvents are process residues with set limits

Add endotoxin and sterility if the powder will be filled into vials sold as sterile, and test the finished vials as well as the powder. Testing those vials before the batch ships is covered in pre-shipment testing, and the wider checklist for a new brand in what to test before your first batch ships.

Sampling bulk peptides from China: a representative sample

A result describes only the sample tested. If the sample is not representative, the result is not about the lot.

The WHO guidelines for sampling pharmaceutical products give three plans, based on N, the number of containers in the consignment:

Plan	Formula	When WHO suggests it
n plan	n = 1 + √N containers, combined after identity checks	Only when the material is uniform and comes from a recognised source; “with great caution”
p plan	p = 0.4 √N final samples, from every container	Uniform material from a recognised source, mainly to check identity
r plan	r = 1.5 √N samples tested individually, from every container	Material that may not be uniform, or from a source that is not well known

WHO’s own example uses 40 containers. Under the n plan, 7 containers are sampled and combined into one composite. Under the r plan, every container is sampled and checked for identity, then 10 samples chosen at random are tested in full, one by one.

For a first order from a new supplier, the r plan is the cautious choice. In practice:

• Sample the lot you will receive. A pre-shipment sample from another batch tells you nothing about this one.
• Choose containers at random. Do not let the seller pick them.
• Seal and label each portion. Write the container number, lot and date on each, and photograph it.
• Split each portion. One part for testing, one part for the retained sample.
• Protect the powder from air. For water testing, the USP authors opened peptide vials under dry nitrogen to minimise moisture uptake. At minimum, seal portions quickly in airtight glass vials.

Identity, purity and content

Purity and content are different numbers, and bulk buyers need both. Purity is the main peak’s share of UV peak area. Content is the mass of peptide against a reference standard of known content.

The EMA guideline expresses assay on a counter-ion-free, anhydrous basis, and defines strength as the mass of peptide base. Bachem’s worked example shows why this matters: 5 mg of powder with 85% net peptide content and 98.7% purity holds 4.195 mg of the target peptide. A lot can show 99% purity and still contain much less peptide per gram than the invoice implies. For more, see net peptide content.

For GLP-1 powder such as tirzepatide, a blind GLP-1 identification compares the measured masses with each candidate peptide, so the label does not steer the result.

Counter-ion, water and residual solvents

These three tests explain the gap between powder weight and peptide content.

• Counter-ion. Peptides are usually isolated as salts, and TFA is a common leftover from synthesis and purification. The EMA guideline notes that acetate is usual and that TFA and chloride also occur, and it says the counter-ion must be defined and controlled with an upper limit. See TFA in peptides.
• Water. Karl Fischer titration measures water as % of the powder. See water content in freeze-dried peptides.
• Residual solvents. ICH Q3C sets limits for solvents used in peptide synthesis. Class 2 examples: acetonitrile 410 ppm, dichloromethane 600 ppm, NMP 530 ppm, DMF 880 ppm and methanol 3000 ppm. Class 3 solvents such as acetic acid and ethanol are limited to 5000 ppm. TFA has no permitted daily exposure in Q3C, because adequate toxicology data are lacking. See residual solvents in peptides.

The USP authors show how the pieces add up. In their worked mass balance, 99% chromatographic purity multiplied by the 94% of the powder left after acetate, TFA, solvents and residue on ignition gives 0.93 mg of peptide per mg of powder, on a dry basis.

Homogeneity: is the whole lot the same

A composite sample gives one average. It cannot show that one drum differs from another. To check homogeneity, test several portions separately and compare identity, purity and content. If they agree, the lot behaves as one material. If one portion differs, treat the lot as non-uniform and sample more widely before you accept it.

The same logic applies after filling. Vials filled from one lot can still differ in fill, so test several filled vials for vial-to-vial consistency before release.

Retained samples

The WHO guideline defines a retention sample as one kept back from the original sampling for future testing, large enough for at least two confirmatory analyses. For a bulk purchase:

• Keep one sealed set yourself, and agree that the supplier keeps one.
• Agree in the contract which lab’s result decides a dispute.
• Store retained portions as the product label says, and record where they are.

A retained sample lets either side check a disputed result on the same material, months later.

How HK Analytical helps bulk buyers

Our lab is in Hong Kong, so portions sampled at a factory in mainland China reach it by cross-border courier before the lot ships further. One order can cover identity (LC-MS), purity (HPLC), content per vial, counter-ion (TFA, acetate), water content (Karl Fischer), residual solvents, endotoxin (LAL) and Sterility (USP <71>, 14 days), in any mix per sample. Results within days of receipt. Sterility (USP <71>, 14 days) is the one test that takes 14 days, because the method incubates for 14 days. Every number comes with its raw data, in a signed record you can check on /verify. Brands can arrange batch testing through our vendor page, or request a quote with the lot details.