What Does 99% Peptide Purity Actually Mean?

Where does the percentage come from?

High-performance liquid chromatography (HPLC) separates components as a sample travels through a column. A detector records a signal as those components emerge. The resulting graph, called a chromatogram, plots signal against time. With ultraviolet (UV) detection, that signal reflects light absorption at the selected wavelength. [1]

For an uncorrected area-percentage result, the calculation is: selected peak area ÷ total included peak area × 100. Area measures the signal across the width of a peak, rather than just its height. Check the report’s calculation basis: adjusted areas and percentages relative to the main peak use different calculations. [1] [2]

A worked example: 99% of what?

Imagine an analysis with three separately integrated peaks. Integration is the calculation of the area under each peak. The numbers below are invented, use arbitrary area units, and do not represent a Freedom Peptides batch or laboratory result.

The remaining 1% is also an area measurement

In this example, 9,900 ÷ 10,000 × 100 = 99%. The two other peaks contribute the remaining 1% of the included area. That arithmetic does not identify the substances behind those peaks or establish their percentage by mass.

Laboratory software can apply response corrections when calculating impurity results. A response correction accounts for differences in how strongly components register at the detector. Read “area %,” “adjusted area %” and “% against main component” as distinct reporting terms, rather than interchangeable labels. [2]

Can one peak contain more than one substance?

Yes. Components can emerge from a column together, a situation called co-elution. Their signals may overlap within a peak that looks like a single component. A clean-looking graph alone cannot establish that every substance has been separated. [3]

A Waters application note analyzed synthetic eledoisin, an 11-residue peptide, using chromatography and mass spectrometry. Mass spectrometry (MS) measures mass-to-charge signals, adding evidence about the components present. The analysis found peptide-related impurities co-eluting with the main peptide that were not separately resolved in the optical result. This was an analytical study of a particular sample and method, not a finding about all peptides or any Freedom Peptides material. [3]

You may also see “peak purity” in software reports. This can refer to comparing UV spectra across a peak to look for overlapping components, rather than calculating its percentage of total area. Agilent’s documentation describes this as a separate check with limitations related to noise, instrument conditions and data processing. A passing spectral check is not a universal certificate of purity. [4]

Why 99% purity does not establish the amount in a vial

Peptide mass purity asks how much of the material’s mass is peptide. Chromatographic area purity asks how much of the included detector signal belongs to the assigned peak. An archived National Institute of Standards and Technology (NIST) peptide reference report defines and reports these separately. Its reference values expired in 2015; it is useful here for the measurement distinction, not as a current reference standard. [1]

Consequently, multiplying a 10 mg label by 99% area purity does not demonstrate that the vial contains 9.9 mg of peptide. The label and area percentage alone leave the measured peptide quantity unresolved.

HPLC can also be used to measure peptide quantity when a suitable calibrated assay is used. A 2019 multi-laboratory study compared an HPLC assay, quantitative nuclear magnetic resonance spectroscopy and amino acid analysis for oxytocin quantification. That is a different analytical task from expressing a peak as a fraction of total area. Look for the quantification method, reference standard and reported units alongside a content result. [5]

How to compare two reported purity results

Before treating 99.5% as meaningfully different from 99.0%, check whether the reports use comparable methods and calculations. Identify the detector and wavelength, which peaks were included, whether response corrections were applied, and what evidence supports the main peak’s identity. If the difference matters to a research decision, ask the laboratory about measurement uncertainty and method precision. [1] [2] [3]

A purity result is useful evidence for the property actually measured. It does not, by itself, establish sterility, biological activity or suitability for human use. Read it alongside the other results on the Certificate of Analysis (COA), keeping each measurement attached to its sample and method.

Read the complete guide to a peptide Certificate of Analysis →