When a research peptide is described as “99% pure,” what does that actually mean? The number usually comes from a single technique, high-performance liquid chromatography (HPLC), and it answers only part of the question. A second technique, mass spectrometry (MS), answers a different one.
Understanding what each method measures, and what it cannot, helps you read a Certificate of Analysis critically and judge whether a batch is suitable for your work.
The Short Version
| HPLC | Mass spectrometry | |
|---|---|---|
| Main question | How pure is the sample? | Is this the right molecule? |
| What it measures | Separation of components over time | Mass-to-charge ratio of ions |
| Typical output | Chromatogram and purity % | Mass spectrum and observed mass |
| Confirms identity? | No | Yes (molecular weight) |
| Quantifies purity? | Yes (relative peak area) | Not reliably on its own |
| Detects water and salts? | No | No |
Neither test is complete alone. Used together, they give a much more reliable picture.
How HPLC Works
HPLC pushes a dissolved sample through a column packed with fine particles, using a liquid mobile phase. Different molecules move through the column at different speeds, so they emerge at different times, called retention times.
For peptides, the standard approach is reversed-phase HPLC (RP-HPLC). The column is hydrophobic, and a gradient of increasing organic solvent, commonly acetonitrile with a small amount of acid, gradually elutes the peptides. More hydrophobic species leave the column later.
A detector, usually measuring UV absorbance at 214 to 220 nm where peptide bonds absorb, records a signal as each component exits. The result is a chromatogram: a plot of signal against time.
How Purity Is Calculated
The main peak should correspond to the target peptide. Purity is typically reported as:
Purity (%) = main peak area ÷ total peak area × 100
So a result of 98.5% means the main peak accounts for 98.5% of the detected UV-absorbing material.
What HPLC Does Well
- Separates the target peptide from many synthesis by-products, such as truncated and deletion sequences
- Provides a quantitative, reproducible purity value
- Shows the impurity profile visually, so you can see whether impurities are one large peak or many small ones
What HPLC Cannot Tell You
- Identity. A clean, single peak could be the wrong peptide. HPLC only shows that the sample is one dominant substance, not which substance.
- Co-eluting impurities. Two different species can emerge at nearly the same time and appear as one peak.
- Non-absorbing components. Water, salts, and many counterions do not absorb at 214 to 220 nm, so they are invisible to the detector.
- The effect of method choice. Column, gradient, wavelength, and integration settings can all shift the reported percentage.
How Mass Spectrometry Works
Mass spectrometry ionizes the sample and measures the mass-to-charge ratio (m/z) of the resulting ions. For peptides, two ionization approaches are common:
- Electrospray ionization (ESI): Often coupled directly to liquid chromatography (LC-MS). It produces multiply charged ions, so a single peptide appears at several m/z values.
- MALDI-TOF: A laser ionizes the sample from a solid matrix, typically producing singly charged ions and a simple spectrum.
Software then converts the observed signals into a molecular mass, which is compared with the theoretical mass calculated from the peptide’s sequence.
What Mass Spectrometry Does Well
- Confirms identity. Agreement between observed and theoretical mass supports that the correct sequence was made.
- Detects modifications. Oxidation, deamidation, or incomplete deprotection change the mass in predictable ways.
- Identifies impurities. When coupled with HPLC, MS can tell you what the minor peaks are.
What Mass Spectrometry Cannot Tell You
- Reliable purity. Signal intensity depends on how well each molecule ionizes, so peak heights in a spectrum do not reflect actual amounts.
- Sequence order. Two different sequences with the same composition have the same mass. Confirming sequence order requires tandem MS (MS/MS) or other methods.
- Water, salts, and counterions in the lyophilized powder.
Why You Need Both
Imagine two scenarios:
- High HPLC purity, no MS data. The sample is one dominant substance, but you do not know whether it is your peptide.
- Correct mass by MS, no HPLC data. The target peptide is present, but you do not know how much else is in the sample.
Only the combination answers both questions: is it the right molecule, and how much of the sample is that molecule? This is why a credible COA includes a chromatogram and a mass spectrum.
LC-MS: Both Techniques in One Run
Many laboratories use LC-MS, which connects an HPLC system to a mass spectrometer. As each peak elutes, the mass spectrometer records its mass. This lets you assign an identity to the main peak and to individual impurities in a single analysis.
Beyond HPLC and MS: Other Tests That Complete the Picture
Purity by HPLC is relative to UV-absorbing material only. For a fuller assessment, labs may also report:
- Net peptide content by amino acid analysis or nitrogen analysis, showing how much of the powder is actually peptide
- Water content by Karl Fischer titration
- Counterion content, such as TFA or acetate
- Endotoxin testing, for cell-based applications
- Amino acid analysis or MS/MS for sequence confirmation
How to Read the Data Critically
When reviewing HPLC and MS results on a COA:
- Check that a chromatogram is included, not just a purity percentage.
- Look at the baseline and minor peaks. Is the main peak dominant and symmetrical?
- Note the detection wavelength and method. Results at different wavelengths are not directly comparable.
- Compare observed and theoretical mass. They should agree within the instrument’s tolerance.
- Match the lot number on the report to your vial.
- Check who ran the test. Independent third-party analysis carries more weight than in-house results alone.
Common Misconceptions
- “99% purity means 99% peptide by weight.” Not necessarily. HPLC purity is relative peak area. The powder also contains water and counterions.
- “A single peak proves it’s the right peptide.” It proves it’s one dominant substance, not which one.
- “MS shows purity.” MS confirms identity, but peak intensities are not quantitative.
- “Higher purity is always better.” The right grade depends on the application. Screening work and quantitative assays have different requirements.
Final Thoughts
HPLC and mass spectrometry are complementary, not interchangeable. One tells you how clean the sample is, the other tells you what it is. When both are presented with raw data, a lot number, and a named lab, you can evaluate a batch with confidence.
You can review batch documentation in our COA Library, read our guide on how to read a peptide COA, or browse the research catalog.
All products are sold strictly for in vitro laboratory research use only. Not for human or veterinary use. Not FDA-approved.