Custom synthesis makes a sequence to your specification, at your scale and purity, with a lead time measured in weeks and a quality package you define. A catalog peptide is a stocked sequence from a lot already made and tested, available now at a fixed specification. The choice turns on whether the sequence, the modification, or the specification is standard.
- Custom and catalog peptides are made by the same solid-phase chemistry and are chemically equivalent when done well; the differences are control, timing, documentation and price.
- Custom synthesis gives control of sequence, modification, scale, purity grade and analytical package, at a lead time in weeks and per-residue pricing.
- A catalog lot already exists and has been tested, so its certificate, lot identifier and verification route can be read before purchase.
- Repeatability across lots is arranged in advance, by a reserved custom batch or by catalog lot control; it is never automatic.
- The documentation standard is identical for both: identity, purity and quantity by named methods, on a documented sample, from a verifiable laboratory.
Two ways to obtain the same molecule
A laboratory that needs a peptide can have it made or buy it off a shelf, and the two routes produce chemically identical material by the same chemistry. What differs is who decided the specification, when the material was made, what documentation came with it, and what it cost per milligram. Those four differences decide the choice, and they are worth taking one at a time because the marketing on both sides obscures them.
Custom synthesis means a synthesis house takes a sequence you supply, with the modifications, scale and purity you specify, and makes it to order. Catalog supply means a supplier has already made or bought a lot of a stocked sequence, tested it, and holds it in vials for immediate dispatch at a specification the supplier set. A research laboratory buying a common sequence usually wants the second. A laboratory needing something the catalog does not hold has no choice but the first. The interesting cases are in between, and the sections below are meant to settle them.
How both are made
Nearly every research peptide, custom or catalog, is made by solid-phase peptide synthesis, the method for which Bruce Merrifield received the 1984 Nobel Prize in Chemistry. The growing chain is anchored to an insoluble resin bead, and residues are added one at a time from the C-terminus toward the N-terminus, each cycle coupling a protected amino acid and then removing the protecting group to expose the next coupling site. Because the chain stays on the resin, excess reagents wash away between steps, and the cycle can be automated. At the end the chain is cleaved from the resin, side-chain protecting groups are removed, and the crude product is purified by preparative reversed-phase HPLC and lyophilized.
That description is the same for a custom order and for a catalog lot, and it is why the two are chemically equivalent when both are done well. The crude product from either contains the same families of impurity: sequences missing a residue where a coupling failed, sequences with a protecting group still attached, sequences truncated where synthesis was capped, and variants where a sensitive residue reacted during cleavage. Purification removes most of them; the purity figure on the certificate says how much remained. The purity versus identity guide covers what that figure can and cannot see.
Where the routes diverge is in who chose the targets for that purification. A custom house purifies to the grade you ordered, because that is the contract. A catalog supplier purified to its own specification, once, for the whole lot, and every buyer of that lot receives the same grade.
What custom synthesis gives you
Control of the specification is the whole product. A custom order lets a laboratory set the sequence exactly, including residues that no catalog carries: unnatural amino acids, D-residues at chosen positions, isotopically labelled residues for mass-spectrometry standards, fluorescent or biotin tags, cyclisation, lipidation, or a specific salt form. It lets the laboratory set the scale, from a few milligrams for a screening study to grams for a method-development programme. And it lets the laboratory set the purity grade, which synthesis houses tier from crude material through several purified grades, with price rising steeply at the top.
Control extends to the documentation. A custom quotation states what analytical package accompanies the material, and a laboratory can specify more: mass spectrometry and HPLC are standard, and amino acid analysis for net content, water by Karl Fischer, counterion analysis or endotoxin testing can be added at cost. Because the sample tested is the batch made for you, the sample chain that the independent testing guide worries about is short and documented by default.
The costs are time and price. Custom synthesis is quoted in weeks rather than days, longer for difficult sequences, and priced per residue with multipliers for scale, purity and modifications, so a modified sequence at high purity is expensive per milligram in a way that a stocked common sequence is not. There is also sequence risk: some sequences aggregate on the resin, some racemise at sensitive residues, some simply fail to reach the requested purity, and a custom house will say so in the quotation or discover it in the attempt. A catalog lot has already survived that risk.
Purity grades, in plain terms
Synthesis houses sell purity in tiers, and the tiers are easy to misread. Crude material is the product as cleaved from the resin and desalted, with no preparative purification; it is the cheapest grade and is sold for work where the sequence's presence matters more than its proportion. Above that sit purified grades, commonly tiered by HPLC area percentage with thresholds in the region of seventy, eighty-five, ninety-five and ninety-eight percent, each step costing more because each removes impurities closer in behaviour to the target.
Two things about the tiers deserve saying. The percentage is an area figure by the house's own HPLC method, with every blind spot the purity versus identity guide describes: it does not see water or counterion, and it does not establish identity. And a catalog supplier's stated purity is the same kind of figure by the same kind of method, so the two routes are compared on equal terms only when both certificates name their conditions. A ninety-eight from a shallow gradient at 214 nanometres and a ninety-eight from a fast gradient at 280 are not the same ninety-eight.
What a catalog peptide gives you
Immediacy is the obvious advantage. A stocked lot ships on order, and a laboratory that needs material this week has no other route. Price per milligram for a common sequence is usually lower, because the supplier made or bought at a scale a single laboratory's order would not justify and spread the synthesis and testing cost across every vial.
The less obvious advantage is that the lot already exists and has already been tested, so its documentation can be read before money moves. A custom quotation promises a specification; a catalog record, if it is a good one, shows a certificate for the actual lot, the lot identifier that will be on the vial, and the issuing laboratory's verification route. The certificate overview and the field-by-field guide cover how to read that evidence. A buyer can also compare lots across time: a supplier with several lots of the same sequence behind it has a history, and a consistent history is worth more than any single certificate.
The costs are the mirror of custom's advantages. The specification is the supplier's, not yours; if the catalog grade is lower than the study needs, or the salt form is wrong for the buffer, there is no lever to pull. The sequence is whatever the catalog holds, without modification. And repeatability across lots is the supplier's to manage: the next lot may come from a different synthesis or a different manufacturer, and a laboratory that needs the same material for a year should ask how the supplier controls that, and what the lot records show about it.
Deciding between them
| The need | Route | Why |
|---|---|---|
| A standard sequence, catalog grade sufficient, needed soon | Catalog | Already made, already tested, documentation readable before purchase |
| A modified sequence: unnatural residue, label, tag, cyclisation | Custom | No catalog carries it; the modification is the specification |
| A standard sequence at a purity grade above the catalog's | Custom, or a catalog supplier that stocks a higher grade | Purity is set at purification and cannot be raised after the fact |
| A specific salt form or counterion | Custom, unless the catalog states its form and it matches | Counterion is decided by the final purification buffer |
| Gram-scale quantity | Custom, or a catalog supplier with a bulk programme | Catalog vials are sized for bench use; scale changes the economics |
| Identical material across many lots over a long study | Custom with a reserved batch, or catalog with lot control | Repeatability has to be arranged, not assumed |
| An isotopically labelled internal standard | Custom | Labelled residues are made to order |
Two of those rows deserve a word. Repeatability is the one most often assumed and least often arranged. A laboratory that will use one sequence for a year has two honest options: order a single custom batch large enough for the whole programme and hold it, or buy from a catalog supplier whose lot records let the laboratory see when the lot changes and compare the certificates. Either works; buying vial by vial from a catalog without reading the lot identifier does not, because the material may change under the same label.
Scale is the other. Catalog vials are sized for bench work, and a laboratory buying dozens of them for one study is paying vial-by-vial for what a bulk order or a custom batch would supply at a fraction per milligram. The bulk supply guide covers the arithmetic on the catalog side.
The documentation is the same standard either way
Whichever route a laboratory takes, the material arrives with documentation, and the standard for reading it does not change. The ICH specification guidance, Q6B, treats identity, purity and quantity as separate tests with separate methods; a custom certificate and a catalog certificate both need all three to say what the material is, how clean, and how much. The FDA's guidance on evaluating whether a synthetic peptide is the same active ingredient as a reference, written for a regulatory context that does not apply to research supply, is nonetheless a clear statement of what characterisation of a synthetic peptide looks like: sequence confirmed, impurity profile characterised, and each impurity above a threshold identified.
Custom synthesis makes that standard easier to reach because the buyer writes it into the order. Catalog supply makes it easier to verify because the evidence already exists and can be checked with the issuing laboratory before purchase. Neither route produces trustworthy material without it, and a laboratory choosing between a custom house that offers a bare HPLC trace and a catalog supplier that publishes full certificates with a verification route has an easy decision that has nothing to do with the route.
The ISO/IEC 17025 accreditation scope of the testing laboratory matters equally for both. A custom house's in-house quality control is release testing by an interested party, exactly as a catalog supplier's is, and a laboratory that wants independent confirmation of either sends a sample of what it received to a separate accredited laboratory. That is not a criticism of custom houses or catalog suppliers. It is the same rule applied evenly.
Questions to ask before ordering either
- For custom: what analytical package is included, and what is the sample it will be run on? Mass spectrometry and HPLC should be standard. Ask for net content by amino acid analysis if quantity matters, and ask whether the certificate will name the instrument and conditions.
- For custom: what happens if the sequence fails to reach the requested purity? A good house tells you the risk in the quotation and the terms if it materialises.
- For catalog: which lot will ship, and can I read its certificate first? A record that names the lot and links the report answers this without an email.
- For catalog: what is the salt form and the labelled-mass convention? Both change the arithmetic on the bench, and the net content guide explains how.
- For either: which laboratory tested it, and how do I verify the report? The Verify a COA page describes the check.
- For either: what will the next lot be, and how will I know it changed? Repeatability is arranged in advance or not at all.
A supplier or synthesis house that answers all six readily is one whose material can be bought on evidence. One that treats the questions as unusual has told you what its other customers accept.
Where LabFirst sits
LabFirst is a catalog supplier. The families listed are stocked sequences from lots that already exist, each product record names the lot currently shipping and links the issuing laboratory's certificate where it has landed, and the salt form and labelled-mass convention are stated on the record so the comparison this page describes can be made before ordering. We do not offer custom synthesis, and a laboratory that needs a modified sequence, a labelled standard or a grade above the catalog's should take that need to a synthesis house and hold it to the same documentation standard. The compliance page states what LabFirst is and is not.
Custom or catalog describes how a substance was obtained. It describes nothing about what the substance is for. FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. Both routes supply research materials to laboratories, and the documentation on either exists so a laboratory can confirm what it received, and for no other purpose.
Common questions
Is a custom-synthesised peptide chemically different from a catalog one?
When is custom peptide synthesis the only option?
Why is a catalog peptide usually cheaper per milligram?
How do I get the same peptide lot for a long study?
Does the documentation standard differ between custom and catalog peptides?
Sources
- The Nobel Prize in Chemistry 1984, Bruce Merrifield: summary. The origin of solid-phase peptide synthesis, the method by which nearly all research peptides, custom or catalog, are made.
- FDA / ICH Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. Identity, purity and quantity as separate specification tests, which both a custom and a catalog certificate need to carry.
- FDA, Sameness Evaluations in an ANDA — Active Ingredients, Guidance for Industry. The agency's account of how a synthetic peptide is characterised and compared, cited only as a clear statement of what characterisation looks like; the regulatory context does not apply to research supply.
- FDA / ICH Q7, Good Manufacturing Practice Guidance for Active Pharmaceutical Ingredients. Background on batch definition, purification and release in pharmaceutical synthesis; the model the lot-control discussion borrows from.
- ISO/IEC 17025:2017, General requirements for the competence of testing and calibration laboratories. The accreditation standard that applies equally to the laboratory testing a custom batch or a catalog lot.
- FDA / ICH Q2(R1), Validation of Analytical Procedures: Text and Methodology. What the analytical package accompanying either route has to demonstrate for its results to mean anything.