On a Certificate of Analysis, the purity figure describes the target compound. “Related substances” is the term for everything else that is chemically related to it – the smaller peaks on a chromatogram that represent impurities carried over from synthesis or formed as the material ages. This entry explains, in neutral terms, what related substances are, where they come from, and how an analytical laboratory measures and reports them.
Definition
Related substances are impurities that are structurally related to the compound being analysed – fragments, variants, or altered forms of the target molecule, rather than unrelated foreign matter. The phrase is standard analytical and pharmacopoeial language: a “related substances” test is the section of an analytical method that quantifies these compound-specific impurities, as distinct from residual solvents, water, or counter-ions, which are measured separately.
For a peptide, related substances are the by-products and breakdown products that share the peptide’s chemistry – a chain missing one amino acid, a chain that has been chemically modified, or two chains stuck together. They are reported as a proportion of the sample, usually by chromatographic peak area.
Related substances, purity, and net content are three different things
These three figures are often confused because they all appear near each other on a COA, but each answers a different question:
| Purity (area %) | The share of the chromatogram attributable to the target compound |
|---|---|
| Related substances | The compound-related impurities – the remaining peaks – individually and in total |
| Net peptide content | How much actual peptide, by mass, is in the vial (excludes salts and moisture) |
Purity and total related substances are two sides of the same chromatogram: if the main peak is 99.0% of total peak area, the related substances account for the remaining 1.0%. Net content is a separate, mass-based measurement entirely – a sample can be high purity yet report a lower net content because of salt and water in the fill. A thorough certificate reports all three.
Where related substances come from
Compound-related impurities fall into two broad origins, and a laboratory often reports them without distinguishing the cause, since both show up as additional chromatographic peaks.
Process-related impurities
These are introduced during synthesis and purification. In solid-phase peptide synthesis the common examples are deletion sequences (a residue that failed to couple, leaving the chain one amino acid short), truncated sequences (synthesis that stopped early), incompletely removed protecting-group fragments, and closely related structural variants. These are present from the moment the batch is made and are what a purification step is designed to reduce.
Degradation-related impurities
These form after synthesis, as the material is stored and handled. Typical peptide degradation pathways include oxidation (often at methionine or cysteine residues), deamidation (asparagine or glutamine converting over time), hydrolysis of the peptide bond, and aggregation, where individual chains associate into larger species. Because these accumulate with time, temperature, and moisture, the related-substances profile of a lot can change between the date of manufacture and the date of a later test – which is one reason the test date on a COA matters.
How a laboratory measures them
Related substances are quantified from the same HPLC analysis that produces the purity figure. On the chromatogram, the tall main peak is the target compound; every smaller peak that is not the solvent front or a known non-related component is counted as a related substance, and its area is expressed as a percentage of the total peak area. Detection is usually by UV absorbance at a wavelength suited to peptide bonds (commonly 214 nm).
Chromatography shows that an impurity is present and how large it is, but not what it is. To put a name to a specific peak – to confirm it is, say, an oxidised variant rather than a deletion sequence – a laboratory uses mass spectrometry, which measures the mass of the species under each peak. Most certificates report related substances quantitatively (by area) without identifying every peak; identification is done when a specific impurity needs to be characterised.
How they are reported on a COA
A related-substances result on a certificate is typically expressed in one or more of these ways:
- Total related substances – the summed area percentage of all compound-related impurity peaks.
- Largest single impurity – the area percentage of the biggest individual impurity peak, sometimes with its retention time.
- Individual named impurities – where a specific related substance has been identified, it may be listed by name or by relative retention time.
Reading these alongside the purity figure and the chromatogram gives a fuller picture than the headline number alone. A single large impurity peak and a scatter of tiny ones describe very different materials even at the same overall purity, and only the trace and the related-substances breakdown reveal which is which.
Why the figure is worth reading
For anyone evaluating a research material analytically, the related-substances profile is where a purity figure becomes interpretable. It indicates how cleanly a batch was synthesised and purified, and – when compared against the manufacture date – whether the material may have changed in storage. A certificate that reports the purity figure, the total and largest related substances, and shows the chromatogram behind them is describing its impurity profile transparently rather than asserting a single number.
Key takeaways
- Related substances are impurities chemically related to the target compound – the smaller peaks on a chromatogram, not foreign contaminants.
- They arise both from synthesis (deletion and truncated sequences, protecting-group fragments) and from degradation in storage (oxidation, deamidation, hydrolysis, aggregation).
- Purity and total related substances come from the same chromatogram; net peptide content is a separate mass measurement.
- HPLC quantifies how much impurity is present; mass spectrometry identifies what a given peak is.
- A COA that shows the chromatogram and breaks down total and largest impurities is more informative than a bare purity percentage.
Frequently asked questions
What are related substances?
Related substances are impurities that are structurally related to the compound being analysed – fragments, variants, or altered forms of the target molecule. On a peptide Certificate of Analysis they appear as the smaller chromatographic peaks alongside the main peak, and are reported as a proportion of the sample by peak area.
How are related substances different from purity?
They are two sides of the same chromatogram. Purity is the main peak’s area as a share of total peak area; total related substances are the remaining peaks. If a sample is 99.0% pure by area, related substances account for the other 1.0%.
How are related substances different from net peptide content?
Related substances are a chromatographic area measurement of compound-related impurities. Net peptide content is a separate, mass-based measurement of how much actual peptide is in the vial, excluding salts and moisture. A sample can be high purity yet report a lower net content.
Where do related substances come from?
From two sources. Process-related impurities are introduced during synthesis and purification – for example deletion sequences, truncated chains, and protecting-group fragments. Degradation-related impurities form afterwards during storage, such as oxidation, deamidation, hydrolysis, and aggregation.
How does a laboratory measure related substances?
They are quantified from the same HPLC analysis that produces the purity figure. Each impurity peak’s area is expressed as a percentage of the total peak area, with detection usually by UV absorbance at a wavelength such as 214 nm. Mass spectrometry can be used to identify what a specific peak is.
What is a deletion sequence?
A deletion sequence is a peptide chain that is missing one amino acid because a residue failed to couple during synthesis. It is a common process-related impurity in solid-phase peptide synthesis and appears as a related-substances peak on the chromatogram.
How are related substances reported on a COA?
Typically as total related substances (the summed area percentage of all impurity peaks), the largest single impurity (the biggest individual peak), and sometimes individually named impurities where a specific one has been identified. These are read alongside the purity figure and the chromatogram.
Why does the test date matter for related substances?
Degradation-related impurities accumulate with time, temperature, and moisture, so a lot’s impurity profile can change between manufacture and a later test. A recent test date describes the material’s current profile more accurately than an older one.
