LC-MS peptide identity testing: how a mass result confirms a sequence

The question the instrument answers

A liquid chromatography mass spectrometry run answers one question about a research peptide better than any other method a supplier is likely to use: does the main component of this sample have the mass that the labelled sequence predicts? That is the identity question. It is not the purity question, which the chromatogram alone answers, and it is not the quantity question, which neither answers. The purity versus identity guide covers why the questions are separate; this page covers how the mass measurement is made and how to read the result.

Two things make LC-MS the working standard for peptide identity. The chromatography in front of the mass spectrometer means the mass is measured on a separated component rather than on the whole mixture, so the main peak's mass is the main component's mass. And electrospray ionisation, the technique nearly always used for peptides, carries an intact molecule into the gas phase without breaking it, so the measured mass is the mass of the molecule and not of a fragment. What comes out is a spectrum, and the rest of this page is about reading it. Everything below is written for a reader with a certificate in one hand and no mass spectrometer in the other, which is the usual position of a laboratory buyer.

Charge states and what a spectrum shows

Electrospray puts charge on a peptide by attaching protons, and a peptide with several basic sites picks up several. The spectrometer measures mass-to-charge ratio, written m/z, not mass. A molecule of neutral mass M carrying n protons appears at m/z equal to (M + n × 1.00728) divided by n, where 1.00728 is the mass of a proton. The same molecule therefore appears at several m/z values at once, one for each charge state it took, and a raw peptide spectrum is a series of peaks whose spacing encodes the mass.

Take BPC-157, whose monoisotopic mass from the sequence is about 1418.7. With one proton it appears near m/z 1419.7; with two, near 710.4; with three, near 473.9. An analyst who sees peaks at those three positions has, in effect, measured the same mass three times, and the agreement between them is itself a check on the assignment. Software performs the arithmetic in reverse, a step called deconvolution, and reports a single neutral mass; a good report prints that neutral mass and, ideally, the charge states it was derived from.

Each charge-state peak is not one line but a small cluster, because the molecule exists as a mixture of isotopologues, most of them differing by one neutron. For a peptide of this size the lightest member of the cluster, the monoisotopic peak, is the tallest, and that is the one compared to the theoretical monoisotopic mass. For larger peptides the tallest peak shifts up the cluster, and the report should say which peak it is quoting. The formula and weight guide sets out the arithmetic of the two masses.

Accuracy, resolution and what a match means

Whether an observed mass matches a theoretical one depends on how accurately the instrument measures, and the certificate should say what instrument it was. Two broad classes are in use. A quadrupole instrument measures to roughly a tenth of a mass unit and cannot separate peaks closer than that; it establishes that the mass is right to within about one part in ten thousand, which excludes the great majority of wrong molecules but not all. A high-resolution instrument, a time-of-flight or an Orbitrap, measures to a few parts per million and resolves the isotope cluster cleanly; it can distinguish two molecules whose masses differ in the second decimal place.

The tolerances are worth putting in numbers, because certificates quote them in different units. Five parts per million on a mass of 1418.7 is a tolerance of about 0.007 units, which is what a high-resolution instrument can hold. A tenth of a unit on the same mass is about 70 parts per million, which is a quadrupole's territory. A certificate that claims a five parts-per-million match from a quadrupole has quoted a tolerance its instrument cannot deliver, and that is a reason to ask which instrument actually ran the sample.

The last row is the important one. A mass match is evidence of composition, not of structure. Leucine and isoleucine have identical mass; a D-residue has the same mass as the L-form; a scrambled sequence has the same mass as the right one. A certificate that reports a matching mass has shown the molecule has the right atoms. Showing they are in the right order needs more, which the section after next covers.

Adducts and the peaks that are not the peptide

Not every peak in a spectrum is the molecule. Electrospray attaches whatever cations are present, and a peptide sample carries sodium and potassium from glassware and buffers, so peaks appear 21.98 units above the protonated molecule for a sodium adduct and 37.96 above for potassium. Trifluoroacetate, present in nearly every research peptide as the counterion, forms an adduct 113.99 units above. Acetate does the same at 60.02. Each of these can be misread as a heavier molecule, or worse, a heavier adduct can be misread as the peptide itself when the analyst is looking for a mass and finds one.

A competent report handles this by listing the observed peaks, assigning each to the molecule or to a named adduct, and reporting the neutral mass from the protonated species. A report that prints one number and no spectrum has done that work invisibly, if at all, and a reader cannot tell whether the number came from the molecule or from its sodium adduct minus a guess. This is why the spectrum, or at least the list of assigned peaks, belongs on the certificate and not in a drawer.

The trifluoroacetate adduct has a second lesson in it. Its presence in the spectrum is direct evidence that the counterion is trifluoroacetate and not acetate, which the chromatogram cannot show and which changes the net peptide content arithmetic. A mass spectrum read carefully gives away more about the sample than the identity result alone.

The sample, the peak, and which spectrum was read

A mass result is only as good as the choice of what to take it from, and a certificate rarely says. Three things happen before the spectrometer sees the peptide, and each can change the answer.

The sample is dissolved, typically in water with a little acetonitrile and a trace of formic acid. The acid supplies protons for electrospray and the choice of acid matters: trifluoroacetic acid, the standard additive for HPLC purity work, suppresses electrospray signal badly, so a laboratory that runs both tests uses different solvent systems for each, and a report that names the mobile phase for the mass run has shown it knows this. Concentration matters too; a sample too dilute gives a weak spectrum in which adducts and noise compete with the molecule, and one too concentrated saturates the detector.

The liquid chromatography in front of the spectrometer separates the sample, and the analyst then chooses where in the run to read the spectrum. The honest choice is the main peak: the spectrum averaged across the largest chromatographic peak is the mass of the main component. A spectrum read from a minor peak, or averaged across the whole run, reports something else. Software can also extract a chromatogram for one m/z value, which shows when a given mass eluted and is the cleanest way to confirm that the mass of interest belongs to the main peak rather than to a shoulder on it.

Finally, the software reports a mass, and the word it uses matters. Some packages report the monoisotopic mass; some report the most abundant mass in the cluster; some report an average. For a small peptide the first two coincide and the third differs by about a unit. For a large one all three differ. A certificate that states which it reports can be read; one that prints a number can be read only by guessing, and the difference between a guess and a match is the whole test. The blank injection, the mobile phase, the peak the spectrum was read from and the kind of mass reported are the four details that turn a printed number back into a measurement.

Beyond intact mass: tandem MS and sequence

When a mass match is not enough, because the sequence has isobaric residues, or because a supplier wants to demonstrate order rather than composition, the next step is tandem mass spectrometry. The instrument isolates the peptide ion, fragments it by collision with gas, and measures the fragments. A peptide breaks preferentially at its backbone amide bonds, producing two series of fragment ions, conventionally called b ions from the N-terminal side and y ions from the C-terminal side, whose masses step through the sequence one residue at a time. Reading the steps reads the sequence.

Tandem mass spectrometry is standard in proteomics and available from any laboratory that runs a modern instrument, and for a research peptide it is the difference between a mass that fits the sequence and a sequence that has been confirmed. It still cannot distinguish leucine from isoleucine without specialised fragmentation, and it cannot distinguish D from L at all, since fragmentation does not read chirality. Chirality needs hydrolysis of the peptide and chiral analysis of the free amino acids, a separate method entirely.

For most research procurement an intact mass on a stated instrument, read against the correct theoretical value, with adducts assigned and the spectrum shown, is the reasonable standard. Tandem data is worth asking for when a lot's origin is unfamiliar, when the sequence contains isobaric residues at critical positions, or when a result downstream has gone unexpectedly wrong and the reagent needs to be ruled in or out.

What an LC-MS identity report has to show

The ICH validation guidance, Q2(R1), asks that an identity test be shown to be specific, and the FDA's guidance on analytical procedures asks that the procedure be documented well enough to be reproduced. Applied to a peptide certificate, those translate into a short list of fields without which the identity result is a number rather than evidence.

1. The instrument and ionisation mode. Electrospray, positive mode, and the instrument class or model. This sets the accuracy the match can claim.
2. The theoretical mass and how it was derived. Monoisotopic or average, computed from the stated sequence with its modifications. Without it the observed mass has nothing to be compared to.
3. The observed mass, and the charge states it came from. The deconvoluted neutral mass, with the m/z peaks and charge assignments listed or the spectrum shown.
4. The difference, in units or parts per million. And whether it sits inside the instrument's stated accuracy.
5. Adduct assignments. Which peaks were the molecule and which were sodium, potassium or counterion adducts.
6. The sample, lot and date. Tying the spectrum to the vial the buyer receives, which the certificate reading guide covers field by field.

A report with all six can be read, repeated and disputed. A report with the observed mass alone has asked the reader to assume everything else, and a buyer who has read this page knows how many places an assumption can hide.

How mass results appear on our records

Each LabFirst product record states the sequence and the theoretical mass a certificate's identity result should be read against, and links the issuing laboratory's certificate for the lot currently shipping where that certificate has landed. Where the laboratory reports the instrument, the observed mass and the difference, those appear as reported; where a field is missing from the report, the record says so rather than filling it in. The Verify a COA page explains how to confirm a mass result with the laboratory that produced it, and the independent testing guide covers what makes that laboratory's result worth confirming.

A mass result establishes what a molecule is. It establishes nothing about what it is for. FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION. Identity documentation exists so that a laboratory can confirm what it received, and for no other purpose.

What is BPC 157 studied for?

Published research on BPC 157 investigates the areas below — which is a different question from what BPC 157 will do for anyone, a claim about a living system that nothing on this site is sold for.

What it is. A man-made fifteen-amino-acid peptide, based on a protein found in stomach juice.

What the research looks at. It appears in animal and cell-culture research on tissue and blood-vessel biology. Almost everything published is animal or cell work.

How it is thought to work. Nobody has settled how it works. Cell and animal studies report effects on blood-vessel growth signals and on several growth-factor pathways, but no single receptor has been pinned down as its target and the studies do not agree on one model.

What is not established. No FDA-approved product contains BPC 157, and no USP or NF standard defines what an acceptable batch is. Human evidence is close to nonexistent. Any figure printed on a research vial is the supplier’s number, not an official one.

The full record, including the certificate for the lot in stock, is on the BPC 157 product page.

Common questions

Sources

• American Society for Mass Spectrometry, About Mass Spectrometry. What a mass spectrometer measures and how ions are produced and analysed; the background for charge states and m/z.
• NIST, Atomic Weights and Isotopic Compositions for All Elements. The isotopic masses behind monoisotopic mass, the isotope cluster, and the adduct mass differences quoted.
• FDA / ICH Q2(R1), Validation of Analytical Procedures: Text and Methodology. Specificity as the validation characteristic an identity test must demonstrate.
• FDA, Analytical Procedures and Methods Validation for Drugs and Biologics, Guidance for Industry. The documentation a procedure needs to be reproducible, which the report checklist follows.
• FDA / ICH Q6B, Specifications: Test Procedures and Acceptance Criteria for Biotechnological/Biological Products. Identity as a distinct specification test that must distinguish the product from other materials.
• PubChem compound record: BPC-157. The sequence and mass used for the worked charge-state example.