How Peptide Purity Is Measured and Reported

Purity is the number buyers anchor on, and it is the number most often misread. A certificate stating 99 per cent is making a specific analytical claim, about a specific batch, measured by a specific method. It is not a general statement about everything in the box, and on its own it does not tell you how much peptide you have bought.

What purity means on a certificate of analysis

In almost every case, the headline purity figure is a chromatographic purity: the proportion of the detected material that is attributable to the main peak. The remainder is made up of related substances, which for a synthetic peptide typically means truncated and deletion sequences, incompletely deprotected material, oxidised forms, dimers and other aggregates, and stereoisomers.

The consequence is easy to miss. Anything the method does not detect does not appear in the number. Salts, water and residual solvents are not chromatographic impurities in the sense the figure describes, so a batch can be reported at high purity and still contain a substantial mass of material that is not peptide.

HPLC and what the figure represents

Reversed phase high performance liquid chromatography is the standard method. The sample is dissolved, loaded onto a column, and separated using a gradient of water and an organic solvent, usually acetonitrile, with an acidic modifier. Components elute at different times and pass a detector. UV detection at around 214 to 220 nanometres is common because the peptide bond absorbs in that region. Detection at 280 nanometres is sometimes reported as well, where aromatic residues are present.

The purity figure is normally an area percentage: the area of the main peak divided by the total area of all integrated peaks. That makes the result method dependent. A shallower gradient, a longer run, a different column chemistry or a different sample load can resolve peaks that a faster method would hide under the main one. Two competent laboratories analysing the same batch by different methods can report genuinely different numbers, and neither is wrong.

This is why the method conditions matter as much as the result. Ask for the column, the gradient, the run time, the detection wavelength and the sample concentration, and ask for the chromatogram rather than the summary line. A peak table with retention times tells you more about a batch than a single figure ever will.

Identity is a separate question from purity

Purity says how much of the detected material is the main component. It does not say what that component is. Identity confirmation is normally by mass spectrometry, using electrospray ionisation or MALDI time of flight, comparing the observed mass against the theoretical mass calculated from the sequence. A certificate should report both the expected and the found mass.

What should never be missing is the pairing. A purity figure without an identity confirmation for the same lot is an incomplete record, and a supplier that provides one but not the other should be asked why.

The tests that tell you how much peptide you have

This is where volume buyers lose money, and the mechanism is entirely predictable. A vial holds a gross mass of lyophilised solid. That solid contains the peptide, plus bound and residual water, plus counter-ions left from purification, commonly trifluoroacetate or acetate, plus any residual salts.

Several analyses address this directly. Karl Fischer titration measures water content. Ion chromatography or an equivalent method quantifies the counter-ion. Amino acid analysis, nitrogen determination or quantitative UV can be used to establish net peptide content, meaning the actual mass of peptide in the material as supplied.

The practical effect is that a vial filled to a gross mass, from a batch reported at high chromatographic purity, can still contain noticeably less peptide than the label mass suggests. That is not a defect, provided it is disclosed. What causes disputes is a quotation priced on gross mass being compared against one priced on net peptide content. Establish which basis you are buying on before you compare prices, and put the basis in the purchase order.

Reading the paperwork rather than the headline

A certificate worth relying on is batch specific, carries the lot number and a date, names the methods used, and is signed or otherwise attributable. The chromatogram should be attached, uncropped, with the baseline visible and the peak integration shown. Where a specification is stated, the result should be reported against it rather than in isolation.

Several things should prompt further questions. A certificate with no lot number. The same document appearing for successive batches with only the date changed. A purity figure quoted with no method attached. A chromatogram that has been trimmed so the early or late eluting region is not visible. A precision the method cannot support, such as purity quoted to two decimal places.

Independent testing and what to write into the order

For material that matters, arrange your own verification. Send a sample from the received lot to an independent laboratory and compare against the supplier's certificate. Do it on the first order from any new supplier, and periodically after that. The exercise tells you as much about the supplier's reporting culture as about the batch.

Then put the expectations in writing before the material ships. State the acceptance specification, including purity by a named method, identity confirmation, and where relevant water content and net peptide content. State that a batch specific certificate must accompany the goods. State your right to retest, the window in which you will do it, and what happens if the retest disagrees with the certificate. Suppliers who are confident in their analysis will agree to all of it without argument.