What does a shoulder on an HPLC peak mean?

A species that nearly co-elutes with the target — for peptides containing methionine, most often the oxidised form. Integration can absorb a shoulder into the main peak, which raises the reported purity while the impurity is still present.

Why does the retention time of the main peak matter?

A peak eluting very early barely interacted with the column, and separation is worst in that region. A high area percent measured there is less trustworthy than the same figure on a well-retained peak.

What should the baseline of a good chromatogram look like?

Flat and quiet. A drifting baseline moves the integrated areas and therefore the purity number, and noise comparable to the small peaks means real minor impurities cannot be distinguished from nothing.

Quality Control·3 min read

How to read the chromatogram on a peptide COA

The purity number is a summary of the trace. Reading the trace itself catches things the number hides — shoulders, baseline drift and a peak too early to trust.

A chromatogram plots detector response against time, and the purity figure on a Certificate of Analysis is one summary of it. Reading the trace directly catches what the summary hides: a shoulder that indicates an oxidised species, a drifting baseline that inflates the number, and a main peak eluting too early to have separated from anything.

By the Bench Grade Research Team · Last updated October 7, 2026

What the axes are

Time runs along the bottom and detector response up the side. Each peak is material leaving the column at a characteristic time, and the area under a peak is proportional to how much of that material was detected. Area percent for the main peak, against the total area of all peaks, is the purity figure quoted on the COA.

A shoulder is not a clean peak

A bump on the leading or trailing edge of the main peak, rather than a separate peak, means a species that nearly co-elutes with the target. For peptides carrying a methionine the classic case is the oxidised form, sixteen daltons heavier and only slightly different in retention.

Integration software can include a shoulder inside the main peak, which raises the reported purity while the impurity is still there. This is the single best reason to look at the trace rather than accept the number.

Where the peak sits matters

A main peak eluting very early, close to where unretained material comes off the column, means the compound barely interacted with the stationary phase. Separation is worst in that region, so a high area percent measured there is less trustworthy than the same number measured on a well-retained peak. Short, polar peptides are the usual case.

Baseline tells you about the method

A flat, quiet baseline means the run was clean and the integration limits are meaningful.
A drifting baseline can inflate or deflate areas depending on how integration was set, and the purity number moves with it.
Noise comparable to small peaks means genuine minor impurities cannot be distinguished from nothing at all.

What to check in order

Is a chromatogram present at all? Its absence is the most common red flag on a peptide COA.
Does the main peak sit clear of both the void volume and the wash?
Is it symmetrical, or does it carry a shoulder?
Is the baseline flat and quiet?
Does the stated area percent look consistent with what the trace shows?

References

Primary literature for the compounds discussed above. Links open the record on PubMed.

BPC 157 and Standard Angiogenic Growth Factors. Gastrointestinal Tract Healing, Lessons from Tendon, Ligament, Muscle and Bone Healing Sikiric P, et al. Curr Pharm Des. 2018. PMID 29998800 → · summary
The promoting effect of pentadecapeptide BPC 157 on tendon healing involves tendon outgrowth, cell survival, and cell migration Chang CH, et al. J Appl Physiol. 2011. PMID 21030672 → · summary
The mitochondrial-derived peptide MOTS-c is a regulator of plasma metabolites and enhances insulin sensitivity Kim SJ, et al. Physiol Rep. 2019. PMID 31293078 → · summary
The Mitochondrial-Derived Peptide MOTS-c Promotes Metabolic Homeostasis and Reduces Obesity and Insulin Resistance Lee C, et al. Cell Metab. 2015. PMID 25738459 → · summary
The Mitochondrial-Derived Peptide MOTS-c Promotes Metabolic Homeostasis and Reduces Obesity and Insulin Resistance Lee C, et al. Cell Metab. 2015. PMID 25738459 → · summary
Thymosin beta 4 improves dermal burn wound healing via downregulation of receptor of advanced glycation end products in db/db mice Xu TJ, et al. Biochim Biophys Acta. 2014. PMID 25230158 → · summary
Pentadecapeptide BPC 157 (PL 14736) improves ligament healing in the rat Cerovecki T, et al. J Orthop Res. 2010. PMID 20225319 → · summary
Gastric pentadecapeptide BPC 157 as an effective therapy for muscle crush injury in the rat Novinscak T, et al. Surg Today. 2008. PMID 18668315 → · summary

Primary sources

Frequently asked questions

What does a shoulder on an HPLC peak mean?: A species that nearly co-elutes with the target — for peptides containing methionine, most often the oxidised form. Integration can absorb a shoulder into the main peak, which raises the reported purity while the impurity is still present.
Why does the retention time of the main peak matter?: A peak eluting very early barely interacted with the column, and separation is worst in that region. A high area percent measured there is less trustworthy than the same figure on a well-retained peak.
What should the baseline of a good chromatogram look like?: Flat and quiet. A drifting baseline moves the integrated areas and therefore the purity number, and noise comparable to the small peaks means real minor impurities cannot be distinguished from nothing.

Compounds discussed in this guide

Who publishes this guide

Who sells it: Bench Grade Peptides LLC, a US supplier of lyophilized synthetic peptides for laboratory research use only. It works with US manufacturing partners rather than running a facility of its own. Is Bench Grade Peptides legit? →
Certificates of Analysis: 139 third-party Certificates of Analysis covering 39 of 44 live products, published per lot in a public library anyone can read before ordering. A very recent lot can appear a few days after it enters the catalogue. COA library →
Ships from: Florida, by UPS, to US addresses only (no international shipping). The registered address in Dover, Delaware is the legal address only. Shipping →
Use: For laboratory research use only — not for human or veterinary use. Not a drug, supplement or medical device. Compliance and research use →

All products discussed in this guide are supplied by Bench Grade Peptides for laboratory research use only. Not for human or veterinary use.