Mass spectrometry tells you very precisely what is in a sample, but on its own it is poor at telling you how much. A synthetic peptide of known sequence and known amount is what converts a spectrum into a number. This hub covers the main roles synthetic peptides play in proteomics and peptide mapping work, how to specify them so they do the job, and why the amount in the vial deserves more attention than the purity figure.
A heavy-isotope version of a target peptide, identical in chemistry but shifted in mass. Spiked into the sample, it travels through digestion, cleanup and chromatography alongside its natural counterpart, so the ratio of the two signals gives a concentration that is largely insensitive to sample losses.
An unlabelled peptide of the target sequence, used to establish retention time, choose precursor and fragment transitions, and tune the instrument before touching real samples. Far cheaper than a labelled standard and usually the first thing to order.
Matched pairs, modified and unmodified, that establish how a phosphorylated, acetylated or otherwise modified peptide behaves, and allow positional isomers to be told apart by retention time when their masses are identical.
A defined mixture injected regularly to confirm the chromatography and the instrument response have not drifted, which protects a long study from a slow change nobody notices.
For a targeted assay the choice of which peptide represents the protein usually matters more than any instrument setting. A peptide that is unique to the protein, generated reliably by the chosen protease, and chemically well behaved will give a workable assay; a poor choice cannot be rescued downstream.
| Decision | What to consider |
|---|---|
| Which residue to label | A C-terminal labelled lysine or arginine is the common choice for tryptic peptides, since every tryptic peptide ends in one. Other residues can be labelled if the sequence needs it. |
| Mass shift | Large enough that the heavy and light isotope envelopes do not overlap. See the shifts available on our stable isotope labelled peptides page. |
| Purity | Higher purity than routine work, because a closely related impurity can contribute to the signal you are measuring. |
| Isotopic enrichment | Incomplete enrichment leaves a residual signal at the light mass, which matters most when the natural peptide is scarce. |
| Quantity and format | Small aliquots rather than one large vial, since repeated freeze-thaw and weighing of tiny amounts are both sources of error. See how to aliquot peptides. |
The purity figure is not the amount
This is the single most common source of error in peptide-based quantification. HPLC purity describes the share of peptide material that is the target sequence. It says nothing about how much of the powder in the vial is peptide at all: counter-ions and bound water make up a substantial and variable fraction. Calibrating against gross weight can therefore be wrong by a wide margin. For quantitative work, order a content measurement, or calibrate your stock against an independent method. See net peptide content, calculating peptide concentration and our quality documentation.
Peptides purified by reverse-phase HPLC are normally supplied as TFA salts. Residual TFA can suppress ionization in electrospray and raises background in the mass spectrometer, so for sensitive mass spectrometry work it is worth considering a salt exchange, usually to acetate or hydrochloride. We offer TFA removal and salt conversion, TFA content analysis when the level needs to be documented, and practical guidance in how to remove TFA salt.
Dissolve and store standards with the same care as the samples they calibrate. Prepare a concentrated stock in a solvent the peptide genuinely dissolves in, verify there is no visible undissolved material, aliquot, and avoid repeated freeze-thaw cycles. Practical steps are in how to dissolve peptides and storage and handling.
Peptide mapping in the sense of characterizing a protein, by digesting it and identifying the resulting fragments, uses synthetic peptides differently: not as quantitative standards but as identity references. When a fragment in the map is ambiguous, or when two candidate assignments have the same mass, synthesizing the candidate and comparing retention time and fragmentation settles it. The same approach confirms the position of a modification within a peptide, where mass alone cannot distinguish one site from another.
Related impurities and sequence variants serve the same role in analytical method development: a defined, individually synthesized variant is what lets a method be shown to resolve it. For sets of defined sequence-related variants, see peptide library synthesis, and for mass shifts of individual modifications, our modification mass table.
Tell us the sequences, whether you need labelled or unlabelled peptides, the purity and quantity, and whether the material will be used for quantification. If quantification is the aim, say so, because it changes the purity, the aliquoting and the analysis we recommend. Email sales@lifetein.com or use the quotation form.
Mass spectrometry identifies what is present far better than it measures how much. A synthetic peptide of known sequence and known amount provides the reference that turns a signal into a concentration, and provides the retention time and fragmentation behaviour needed to build a method.
A synthetic version of a target peptide made with stable isotopes such as carbon-13 and nitrogen-15, so it is chemically identical but shifted in mass. Spiked into the sample, it experiences the same losses as the natural peptide, and the ratio of the two signals gives a robust concentration.
One that is unique to the protein, released cleanly by the chosen protease, chemically stable, and roughly seven to twenty residues long. Avoid methionine, tryptophan and asparagine-glycine where possible, and use two independent peptides per protein when the result matters.
No. Purity is the share of peptide material that is the target sequence, not how much of the vial contents is peptide. Counter-ions and water make up a variable fraction, so a content measurement or independent calibration is needed for quantification.
Often yes. Residual TFA from reverse-phase purification can suppress electrospray ionization and raise background, so exchanging to acetate or hydrochloride is worth considering for sensitive work.
Research use only. LifeTein peptides and proteins are supplied for laboratory research use. They are not for human or veterinary use, and not for diagnostic or therapeutic use. See our research use policy.