Mass Spectrometry Identity Confirmation of Research Peptides: ESI, MALDI and Observed vs Calculated Mass

Mass-spectrometric identity confirmation of a peptide compares the mass observed for its ions with the mass calculated from its molecular formula. Electrospray ionization (ESI) produces multiply protonated ions, so the instrument reports mass-to-charge ratios (m/z) that are deconvoluted to one neutral mass; MALDI-TOF usually reports a singly charged ion. Agreement within the stated tolerance confirms elemental composition; it does not measure purity, sequence order or quantity.

What does a mass spectrometer measure?

A mass spectrometer measures the mass-to-charge ratio (m/z) of ions, not the mass of neutral molecules. The peptide is first converted into gas-phase ions, for peptides almost always by adding protons in positive-ion mode; an analyser then separates the ions by m/z and a detector counts them. Two ionization techniques cover nearly all peptide identity work:

• ESI (electrospray ionization) sprays a solution of the peptide from a charged capillary. One molecule picks up several protons, so a single peptide appears as a series of peaks, one per charge state. ESI is usually coupled to liquid chromatography (LC-MS), which lets the laboratory take the mass of the main HPLC peak itself rather than of the whole sample.
• MALDI-TOF (matrix-assisted laser desorption/ionization with a time-of-flight analyser) co-crystallizes the peptide with a UV-absorbing matrix and desorbs it with a laser pulse. It mostly forms singly protonated [M+H]⁺ ions, so the main peak sits one proton mass above the neutral mass.

Why does ESI show several peaks for one peptide?

ESI shows several peaks because molecules of the same peptide carry different numbers of extra protons, and each charge state lands at its own m/z. For an ion carrying z protons, m/z = (M + z × 1.007276) / z, where M is the neutral mass and 1.007276 Da is the proton mass used on the site's expected mass table. The number of charges follows the number of basic sites. BPC-157 (GEPPPGKPADDAGLV) has two, the N-terminal amine and one lysine, so its ESI spectrum is expected mainly at [M+H]⁺ and [M+2H]²⁺. TB-500 (43 residues, N-terminally acetylated, so no free N-terminal amine) has nine lysines and appears as an envelope of higher charge states.

Worked from the TB-500 listing value of 4963.44 Da (Pepta Labs compound record; PubChem CID 16132341 describes the same 43-residue peptide):

• [M+3H]³⁺ = (4963.44 + 3 × 1.007276) / 3 = m/z 1655.49
• [M+4H]⁴⁺ = (4963.44 + 4 × 1.007276) / 4 = m/z 1241.87
• [M+5H]⁵⁺ = (4963.44 + 5 × 1.007276) / 5 = m/z 993.70
• [M+6H]⁶⁺ = (4963.44 + 6 × 1.007276) / 6 = m/z 828.25

The same calculation for BPC-157 at its listing value of 1419.53 Da gives [M+H]⁺ at m/z 1420.54 and [M+2H]²⁺ at m/z 710.77. The expected mass table computes its m/z columns from the calculated average mass (1419.56 Da for BPC-157, 4963.51 Da for TB-500), so its values run about 0.02 higher; both are well inside a low-resolution tolerance.

What is deconvolution?

Deconvolution is the software step that converts a series of charge-state peaks back into one neutral mass. Two adjacent peaks are enough to do it by hand. If m1 is the peak at charge z and m2 the next peak at charge z + 1, then z = (m2 − 1.007276) / (m1 − m2) and M = z × (m1 − 1.007276). With the TB-500 pair above, z = (993.70 − 1.007276) / (1241.87 − 993.70) = 4.00, and M = 4 × (1241.87 − 1.007276) = 4963.45 Da, equal to the listing value of 4963.44 Da within rounding. The deconvoluted mass is the figure a report compares with the calculated mass, and the report should state whether that figure is monoisotopic or average.

Monoisotopic or average: which mass should the report match?

The monoisotopic mass is the mass of the molecule built only from the most abundant isotope of each element (¹²C, ¹H, ¹⁴N, ¹⁶O, ³²S); the average mass uses natural-abundance atomic weights and is the molecular weight printed on most listings. A low-resolution instrument sees the isotope cluster as one unresolved peak and reports its centroid, which corresponds to the average mass. A high-resolution instrument resolves the individual isotope peaks and can report the monoisotopic mass. The gap between the two grows with size, because a larger molecule has more carbon atoms that can be ¹³C:

• BPC-157, C62H98N16O22 (PubChem CID 9941957, CAS 137525-51-0): 1418.70 Da monoisotopic and 1419.56 Da average, computed from the formula with the atomic masses used on the expected mass table. The listing value is 1419.53 Da; the 0.03 Da difference comes from rounding and the choice of atomic-weight table.
• TB-500, C212H350N56O78S (PubChem CID 16132341, CAS 77591-33-4): 4960.49 Da monoisotopic and 4963.51 Da average, computed from the formula the same way. The listing value is 4963.44 Da.

Both conventions are correct; mixing them is not. A report that sets a monoisotopic observation against an average expectation builds in a 0.85 Da error for BPC-157 and a 3.02 Da error for TB-500. For peptides above roughly 3 kDa the monoisotopic peak is a small part of the isotope cluster, so high-resolution software usually fits the whole isotope pattern and reports a computed monoisotopic or most-abundant mass; the report should name which.

What tolerance counts as a match?

A match is an observed mass that falls within the laboratory's stated tolerance of the calculated mass. For low-resolution ESI the window used on the expected mass table is about ±0.1%, which is ±1.42 Da for BPC-157 at 1419.53 Da and ±4.96 Da for TB-500 at 4963.44 Da. High-resolution instruments state agreement in parts per million (ppm): 5 ppm of 1418.70 Da is 0.0071 Da. The tolerance decides what the test can see. A missing glycine (−57.02 Da) falls far outside a ±5 Da window, but a deamidation (+0.98 Da) on TB-500 falls inside it, so a low-resolution result cannot tell a deamidated TB-500 from an intact one.

Which mass differences point to a different molecule?

A difference larger than the tolerance means the main ion is not the expected molecule, and the size of the difference usually identifies the change. The residue masses below are standard monoisotopic residue masses, computed from the residue formula (for example glycine, C2H3NO, 57.02 Da) with the monoisotopic atomic masses used on the expected mass table.

Two rows show the limit of mass alone. A missing lysine (−128.09) and a missing glutamine (−128.06) differ by 0.04 Da, which only a high-resolution instrument separates, and leucine and isoleucine have the same residue formula, so no mass measurement tells them apart. On the m/z scale every difference is divided by the charge: a sodium adduct of an [M+2H]²⁺ ion appears 10.99 m/z units above it, not 21.98.

Are sodium and potassium adducts errors?

No. A sodium or potassium adduct is an ion in which a metal cation takes the place of a proton, and it is not a sequence error. Relative to the protonated ion, [M+Na]⁺ sits 21.98 Da higher and [M+K]⁺ 37.96 Da higher, computed from the monoisotopic masses of Na, K and H. A well-written report assigns identity from the protonated series and labels adduct peaks as adducts rather than listing them as unexplained masses.

Does the salt form change the observed mass?

No. Trifluoroacetate and acetate counter-ions dissociate from the peptide on ionization, so the observed ion mass is that of the peptide whatever the salt form, and the expected mass on a report is the free-peptide mass. BPC-157 is typically supplied as an acetate or trifluoroacetate salt and TB-500 typically as an acetate salt (Pepta Labs compound records); neither counter-ion is part of the calculated mass. Salt form changes net peptide content, not the identity result; see TFA vs acetate salts.

Can peptide sequencing by mass spectrometry confirm residue order?

Peptide sequencing by mass spectrometry needs tandem mass spectrometry (MS/MS); a single-stage mass measurement cannot read a sequence. In MS/MS a selected ion is broken into fragment ions along the peptide backbone, and the mass steps between fragments correspond to individual residues, which gives residue order. An identity check that matches only the intact mass confirms composition: two peptides containing the same residues in a different order have identical formulas and identical masses. A report shows MS/MS sequence coverage only when that experiment was run, and the Pepta Labs listings represent identity by mass spectrometry, not sequencing.

What should the mass-spectrometry section of a report state?

USP General Chapter <736>, Mass Spectrometry, is the United States Pharmacopeia chapter on the technique. Whatever standard a laboratory follows, a mass-spectrometry identity result can be checked only when the report states these six things:

1. Technique: ESI or MALDI, and whether it was coupled to liquid chromatography.
2. Analyser: for example quadrupole, ion trap or time-of-flight.
3. Calibration: the calibrant or reference compound and when the instrument was calibrated.
4. Resolution and mass accuracy: these show whether the result is an average or a monoisotopic mass and which tolerance applies.
5. Expected mass: the value, labelled monoisotopic or average, and the formula it was calculated from.
6. Observed mass: the deconvoluted mass or the m/z values with their charge states, and the difference in Da, percent or ppm.

How does the identity result appear on a Pepta Labs listing?

Every Pepta Labs product page carries an Identity row reading "Confirmed by mass spectrometry", beside the Purity row with the reported RP-HPLC purity (main-peak area percent). The testing behind both rows is stated as: "Independent third-party HPLC purity analysis and mass-spectrometry identity confirmation, commissioned through the purchasing group on a sampled production run from the source. Pepta Labs does not test in-house." In other words, independent confirmation of sample identity on a Pepta Labs listing means a third-party laboratory measured the mass of the main ion from a sampled production run and matched it to the calculated mass; it is not a test of each vial. The report is "Sent by email on request, before you order if you want to see it first."

Four limits apply. "Vial labels identify the compound, quantity and research-use designation. They do not carry lot or batch numbers." The report therefore covers the sampled production run, not an individual vial. "Because testing is commissioned by the purchasing group, the client field may name the group or source rather than Pepta Labs, and supplier-identifying fields are redacted." "Sterility, endotoxin or LAL, potency, pharmaceutical-grade, cGMP, contaminant-panel and vial fill-quantity testing are not claimed." Net peptide content is not reported per listing. Request a report through the quality page, then check its mass line against the expected mass table.

Frequently asked questions

Why can a report show m/z 993.7 for a 4963 Da peptide?

Because that peak would be the [M+5H]⁵⁺ ion: (4963.44 + 5 × 1.007276) / 5 = 993.70. The [M+4H]⁴⁺ ion of the same peptide sits at m/z 1241.87, and the deconvoluted mass on a report is the neutral value of about 4963 Da.

Which is correct, the monoisotopic or the average mass?

Both are correct for different purposes: the monoisotopic mass is what a high-resolution instrument resolves, the average mass is what a low-resolution instrument centroids and what most listings print, and the report should state which one it compared. For BPC-157 the two are 1418.70 Da and 1419.56 Da, computed from C62H98N16O22.

Does a matching mass prove the sequence?

No. A mass match confirms elemental composition within the tolerance, and two peptides with the same residues in a different order have the same formula and the same mass. Residue order needs tandem mass spectrometry (MS/MS).

Does the counter-ion appear in the mass spectrum?

No. Trifluoroacetate and acetate dissociate on ionization, so the observed ion mass is that of the peptide regardless of salt form, and the expected mass is the free-peptide mass.

What tolerance should a low-resolution ESI result meet?

About ±0.1% of the calculated mass, which is roughly ±1.4 Da at 1,420 Da and ±5 Da at 5,000 Da; high-resolution instruments report agreement in parts per million, where 5 ppm at 1,420 Da is about 0.007 Da.

Who performs the independent identity confirmation on Pepta Labs listings?

An independent third-party laboratory, commissioned through the purchasing group on a sampled production run from the source; Pepta Labs does not test in-house. The report covers that production run, not an individual vial, because vial labels carry no lot or batch numbers, and it is sent by email on request.

Sources

• USP General Chapter <736> Mass Spectrometry, United States Pharmacopeia–National Formulary (cited by chapter number and title).
• PubChem CID 16132341: thymosin beta-4, 43 residues, N-terminally acetylated, C212H350N56O78S (the TB-500 listing).
• PubChem CID 9941957: BPC-157, GEPPPGKPADDAGLV, C62H98N16O22.
• FDA Global Substance Registration System, UNII 2D5MRE3SSY (timbetasin, the TB-500 listing).
• Computed values on this page: formulas from the Pepta Labs compound records; atomic weights, monoisotopic atomic masses and the proton mass (1.007276 Da) as used on the expected mass table.

All products are supplied for laboratory research use only and are not for human or animal use.