What a peptide test actually measures

Is the molecule in the vial the molecule on the label?

The sample is separated by liquid chromatography and then ionised and weighed by a mass spectrometer. Every molecule has a calculable exact mass from its formula, so the measured mass is compared against the theoretical mass for the declared compound. A blend is resolved component by component: the spectrometer sees each analyte separately, so each declared component gets its own theoretical mass, its own observed mass, and its own verdict.

Look for three numbers, not one: theoretical mass, observed mass, and the error. Error is reported in daltons and in ppm. The ppm figure is the one that travels: a 1.0 Da error is enormous on a 161 Da small molecule and unremarkable on a 4813 Da peptide, and only the ppm number tells you which situation you are in. Also check the mass basis. Average mass and monoisotopic mass are two different numbers for the same molecule, and comparing an observed monoisotopic reading against an average theoretical value produces an error that is an artefact of the reference, not of the vial.

• Mass error vs theoretical: ≤ ±1.0 Da

Of everything the detector could see, what proportion is the main component?

Reversed-phase HPLC separates the sample over time, and a UV detector at 214 nm records the response. 214 nm is chosen because that is where the peptide bond absorbs. The result is area percent: the area of the main peak divided by the total area of all peaks.

A purity figure is a ratio, and a ratio has a denominator. The denominator here is everything that absorbs at 214 nm and eluted during the run. That is the correct denominator for related peptide impurities, which is what the number is for. Look for the wavelength and the method reference beside the percentage. A purity number with no method statement is not interpretable.

• Chromatographic purity: ≥ spec (default ≥ 98.0) %

How many milligrams of the actual target compound are in the vial?

Quantitative HPLC against a qualified reference standard of known concentration. The sample response is compared against the standard's response, which converts an area into a mass. Residual moisture, salts and counter-ions are excluded, so the number is the peptide rather than the powder.

Compare the reported milligrams against the label claim, and look at the percentage variance. Both directions matter: significantly over the claim is a manufacturing control problem in the same way that under is. Where more than one vial from a lot was tested, the report should show each vial, the mean, and the spread.

• Net peptide vs label: within ±10 %

Did a rapid screen detect viable microorganisms in the sample?

A rapid molecular microarray screen looks for microbial genetic material and reports a colony forming unit figure against a limit. It returns in days rather than the fortnight a compendial sterility test needs.

Read the method label first, before the result. A rapid screen and a compendial sterility test are different tests answering questions of different strength, and the difference is not a detail. A compendial USP 71 sterility test is a growth or no-growth verdict on incubated media with positive and negative controls that must both behave correctly for the result to mean anything, and it takes a minimum of two weeks. A rapid screen is informative and fast and is not a substitute for it.

• Rapid screen (CFU): ≤ 10 CFU
• USP <71> outcome: No growth in all containers; controls valid

How much bacterial endotoxin is present, in endotoxin units per milligram?

Kinetic chromogenic LAL, the compendial method under USP 85. It measures the biological activity of lipopolysaccharide rather than counting organisms.

The result is a number against a stated limit, in EU per mg or EU per mL. Both the number and the limit have to be present for the result to mean anything, and the limit depends on the product and the route.

• Bacterial endotoxin: ≤ 0.5 EU/mg

Which specified elements are present, and at what levels?

ICP-MS against the elemental impurity limits in USP 232 and 233. Each element gets its own result, unit, quantitation limit and specification source.

Look for a per-element table rather than a single pass mark, and check that the limit beside each element is stated with its source. A result that reports only that the sample passed does not tell you which elements were screened.

• Each heavy metal: ≤ its element limit ppb

What happens to the solution after it is reconstituted?

pH is measured and the reconstituted solution is held and re-analysed over time, so degradation is measured as a rate rather than assumed.

Look at both the pH against its target range and the degradation percentage over the stated interval.

• pH: within target range
• 24 h degradation: < 5 %

Was fentanyl or a common analog detected in the sample?

A targeted LC-MS/MS screen against a defined list of fentanyl analogs, reported as detected or not detected against a method detection limit.

This is a targeted screen, so the answer is only as broad as the target list and only as sensitive as the stated detection limit. Both should be on the certificate.

• Fentanyl / analogs: Not detected (≥ MDL)