Peptide analytical techniques answer two basic questions about any sample: is this the right molecule? and how much of the sample is that molecule? Synthesis rarely produces a perfectly clean product, so laboratories rely on a toolkit of complementary methods. This overview explains the most widely used peptide analytical techniques, what each one measures, and the limits worth keeping in mind when you read the results.
Identity, Purity and Content: Three Different Questions
It helps to separate three ideas that are often confused:
- Identity: does the sample contain the intended sequence? Usually confirmed by mass spectrometry.
- Purity: what fraction of the peptide material is the target peptide rather than related impurities? Usually measured by HPLC.
- Peptide content: what fraction of the total powder weight is peptide at all, as opposed to water, salts and counter-ions? Usually measured by amino acid analysis or elemental analysis.
A sample can be highly pure by HPLC yet contain a significant proportion of non-peptide mass, so these numbers are not interchangeable.
Reversed-Phase HPLC
High-performance liquid chromatography is the standard method for assessing peptide purity (HPLC overview). In the reversed-phase form, the sample is pumped through a column packed with hydrophobic material, commonly silica bonded with C18 chains. A gradient that gradually increases the proportion of an organic solvent, typically acetonitrile in water, elutes peptides in order of increasing hydrophobicity.
How Detection Works
Most peptide HPLC uses ultraviolet detection at around 214 to 220 nm, where the peptide bond itself absorbs. Detection at 280 nm picks up the aromatic residues tryptophan and tyrosine. An ion-pairing additive such as trifluoroacetic acid (TFA) is usually added to the mobile phase to sharpen peaks.
Reading an HPLC Purity Result
Purity is normally reported as the area of the main peak as a percentage of the total peak area. Two cautions apply. First, an impurity that elutes at the same time as the main peak will be hidden inside it. Second, the percentage only covers material that absorbs at the chosen wavelength, so it says nothing about water or salts. That is why HPLC is paired with mass spectrometry and content measurements.
Mass Spectrometry
Mass spectrometry measures the mass-to-charge ratio (m/z) of ions and is the main tool for confirming identity (mass spectrometry overview). Two soft ionization techniques dominate peptide work:
- Electrospray ionization (ESI) sprays the sample from a charged needle and typically produces multiply charged ions, so a single peptide appears as a series of peaks that can be deconvoluted back to its mass. ESI couples naturally to liquid chromatography as LC-MS.
- Matrix-assisted laser desorption/ionization (MALDI) mixes the sample with a light-absorbing matrix and fires a laser at it, producing mostly singly charged ions. It is fast and tolerant of complex samples.
The measured mass is compared with the theoretical value calculated from the sequence. When comparing, note whether a report gives the monoisotopic mass (the most common isotopes only) or the average mass (weighted by natural isotope abundance). For longer peptides the two can differ by a dalton or more.
Tandem MS and Sequencing
In tandem mass spectrometry (MS/MS), selected ions are fragmented, most often along the backbone. The resulting fragment series, conventionally called b ions (containing the N-terminus) and y ions (containing the C-terminus), can be read to confirm the sequence residue by residue.
Amino Acid Analysis and Peptide Content
Amino acid analysis (AAA) breaks a peptide into its individual amino acids, classically by acid hydrolysis in 6 M hydrochloric acid at around 110 °C, then separates and quantifies them. Comparing the measured amounts with the known sequence confirms composition and gives an absolute measure of how much peptide is present. Hydrolysis has known limitations: tryptophan is largely destroyed, and asparagine and glutamine are converted to aspartic and glutamic acid.
AAA is the basis of the net peptide content figure. Freeze-dried peptides normally contain bound water and counter-ions left over from purification, such as trifluoroacetate or acetate, so the peptide itself may be well under 100% of the powder weight. Researchers who need exact concentrations take this into account.
Other Useful Techniques
| Technique | What it measures |
|---|---|
| Karl Fischer titration | Water content of the powder |
| Ion chromatography | Counter-ion content, for example TFA or acetate |
| Edman degradation | N-terminal sequence, read one residue at a time |
| Capillary electrophoresis | Purity by charge-to-size ratio, complementary to HPLC |
| Circular dichroism (CD) | Secondary structure content such as helix or sheet |
| Nuclear magnetic resonance (NMR) | Detailed structure and conformation in solution |
| Limulus amebocyte lysate (LAL) test | Bacterial endotoxin levels |
How to Read a Certificate of Analysis
A certificate of analysis (COA) summarizes test results for a specific batch. When reading one, check that:
- The batch or lot number matches the vial you are using.
- Identity is supported by a measured mass that agrees with the theoretical mass for the sequence.
- Purity is reported with the method used, ideally with the chromatogram attached.
- The test date and the laboratory that performed the analysis are stated.
- You understand whether a figure refers to purity or to peptide content.
Analytical results describe a sample at the time of testing. How the material is stored afterwards affects whether it stays that way, which is covered in our guide to peptide storage and stability.
Putting the Methods Together
No single technique tells the whole story. A typical quality picture combines HPLC for purity, mass spectrometry for identity, and content or water measurements for an accurate weight basis. For background on why sequence and structure affect all of these measurements, read Understanding Peptides: Structure, Properties and Their Role in Laboratory Research, or browse the compounds in our research peptide catalogue.
Further Reading
Educational content. This article is for general scientific education. It is not medical advice, and nothing here describes human or veterinary use. All Direct Peptides products are supplied for laboratory research use only.