Ninety-nine percent by HPLC is the most quoted number in this field and the least examined. It is a real measurement with a precise meaning, and that meaning is narrower than most people assume.
What the number is
The sample is pushed through a column that separates its components, a detector records each one as it leaves, and the result is a chromatogram: a trace with peaks. The target peptide is the main peak. Purity is the area under that peak divided by the total area of all the peaks the detector saw, expressed as a percentage.
A figure of 99 percent says the target accounted for about 99 percent of what this method detected under these conditions. Every part of that sentence is doing work.
Area is not mass
This is the misreading that matters most. Area percent answers a question about the peaks. It does not answer what share of the powder in the vial is peptide.
Counterion from purification, absorbed water and any excipients are part of the weighed material and contribute no peak at the detection wavelength. A batch can be 99 percent pure by area and well under 99 percent peptide by mass, with no contradiction between the two figures. Net peptide content is the separate number that answers the second question, and a thorough certificate reports both.
Detected is doing work too
Peptide methods normally detect in the ultraviolet, and the wavelength decides what is visible. Around 214 nm the amide bond itself absorbs, so essentially any peptide registers. At 280 nm the signal comes mainly from tryptophan and tyrosine, so a sequence without aromatic residues is nearly invisible.
Anything with no chromophore at the chosen wavelength contributes nothing to the total, which means it cannot reduce the percentage. The same sample analysed at two wavelengths can return two different purity figures, both honest.
Under these conditions is doing work as well
The column, the gradient, the flow rate and the run time all change what separates from what. A shallow gradient resolves species that a steep one merges into a single peak, and two laboratories can report different figures for the same batch because one gave the separation more room. (Reference: Dong, 2006) (Reference: Snyder et al., 2009)
A percentage quoted without its conditions is a statement rather than a result. Guidance on validating analytical procedures exists because a number is inseparable from the procedure that produced it. (Reference: Swartz & Krull, 2012)
Why the acid in the mobile phase matters to the number
Peptide methods run at low pH with an ion-pairing additive, usually trifluoroacetic acid, which sharpens peaks and improves the separation of closely related sequences. The concentration of that additive changes retention and resolution, and the effect has been studied systematically. (Reference: Shibue et al., 2005)
Two consequences reach the certificate. A method with too little additive may fail to resolve an impurity that a better-tuned one separates, which raises the apparent purity. And for anything that is not a simple peptide, such as a metal complex, acidic conditions can shift the material toward its dissociated form, so the peak being measured may not be the species named on the label.
A percentage has a precision
Integration is not exact. Where the software places the start and end of a peak, how it draws the baseline under a drifting trace, and whether it groups a shoulder with the main peak or separates it all move the result. Two analysts integrating the same chromatogram can differ in the first decimal place.
The difference between 99.1 and 99.4 percent usually carries less information than it appears to, while the difference between 95 and 99 is real.
What hides inside the main peak
An impurity that leaves the column at the same time as the target is counted as target. The usual candidates are the ones synthesis produces: deletion sequences missing a single residue, or chains that kept a protecting group. They resemble the target closely enough that resolving them is exactly what the method has to do well.
A shoulder on the main peak, or a small peak sitting very close to it, is therefore more informative than a clean number. If the certificate includes the trace, the trace is worth reading. (Reference: Kazakevich & LoBrutto, 2006)
Purity says nothing about identity
A chromatogram reports that one component dominates. It has no opinion about what that component is. A sample could be 99 percent pure and be 99 percent of the wrong molecule, and the chromatogram would look the same.
Identity comes from mass spectrometry, which measures mass and compares it with the mass calculated from the intended sequence. Purity and identity are two questions, and a document answering one of them is half a document.
And nothing about contamination of other kinds
Endotoxin is the clearest example. It is not a peptide, it does not behave like one on the column, and a purity figure is not designed to see it. Where the intended work is sensitive to it, endotoxin content is measured by its own method and reported as its own line.
The same applies to residual solvents and to water content. Each is a separate determination, and none of them moves the purity percentage.
Reading a specification
A figure such as "≥99% by HPLC" is a specification, not a result: it states the threshold the batch was required to meet. A result is a measured number for a named batch on a named date. The two look similar on a page and mean different things, which is why the batch number is the thread that connects a claim to a measurement you can check. A specification applies to every batch; a result applies to one.
Check this against a real batch
Every compound in the catalog lists its formula, molecular weight and CAS number, and every batch we ship has a certificate you can look up by number.
Frequently asked questions
What does 99 percent by HPLC mean?
That the target peak accounted for about 99 percent of the total peak area the method detected, under the conditions used. It is a statement about the detected components, not about the weight of the powder.
Is 99 percent pure the same as 99 percent peptide?
No. Counterion, absorbed water and any excipients are part of the weighed material and produce no peak. A batch can be 99 percent pure by area and considerably less than that by mass. Net peptide content is the separate figure.
Can the same sample give two different purity figures?
Yes. The detection wavelength changes what is visible, and the gradient and column change what separates. A shallow gradient can resolve species that a steep one merges. Both results can be honest.
Can an impurity hide in the main peak?
Yes. Anything that leaves the column at the same time as the target is counted as target. Deletion sequences missing a single residue are the usual candidates, which is why a shoulder on the main peak is worth looking at.
Does a purity figure confirm the peptide is the right one?
No. A chromatogram shows that one component dominates without identifying it. Identity comes from mass spectrometry, comparing the measured mass with the mass calculated from the intended sequence.
What is the difference between a specification and a result?
A specification such as ≥99% states the threshold a batch was required to meet. A result is a measured value for a named batch on a named date. The batch number is what connects the two.
References
- Dong, M.W. (2006). Modern HPLC for Practicing Scientists. Wiley.
- Snyder, L.R., Kirkland, J.J., & Dolan, J.W. (2009). Introduction to Modern Liquid Chromatography (3rd ed.). Wiley.
- Kazakevich, Y.V., & LoBrutto, R. (Eds.). (2006). HPLC for Pharmaceutical Scientists. Wiley.
- Swartz, M.E., & Krull, I.S. (2012). Handbook of Analytical Validation. CRC Press.
- Shibue, M., Mant, C.T., & Hodges, R.S. (2005). Effect of anionic ion-pairing reagent concentration (1-60 mM) on reversed-phase liquid chromatography elution behaviour of peptides. Journal of Chromatography A, 1080(1), 58-67.