ESI Mass Spectrometry Verification

End-to-end workflow

Six steps from sample aliquot to identity confirmed.

Mass spec is a confirmatory assay — it answers a question HPLC can't. The same lot that cleared the chromatographic purity floor goes through this path before release is signed.

01
Aliquot & dilution

20 µg of the same sample analysed by HPLC is diluted in 50:50 acetonitrile / water + 0.1% formic acid to a working concentration of 5 µg/mL — the sweet spot for stable electrospray without source overload.
≈ 0.2 h · 5 µg/mL prep
02
Calibration check

Instrument calibrated against a polypeptide reference (cesium iodide cluster ions or a sequenced standard), m/z accuracy verified at < 5 ppm across the full scan range. No sample is run on a drifted instrument.
pre-run · < 5 ppm drift
03
Direct infusion (or LC-MS)

Sample loaded into a glass syringe and infused at 5 µL/min through the ESI source. For complex samples, an in-line HPLC column is used in MS-compatible mobile phase (formic acid replaces TFA) for inline desalting.
≈ 0.3 h · 5 µL/min infusion
04
Full-scan acquisition

Positive electrospray, capillary 4.0 kV, source 150 °C, cone 40 V. Scan range m/z 100–3000 at 1 spectrum/sec, averaged over 60 seconds for signal stability and isotope envelope clarity.
≈ 1 min · scan averaged
05
Charge-state deconvolution

Multi-charged peaks (typically [M+4H]4+ through [M+12H]12+ for a 9 kDa peptide) are deconvoluted to a single neutral molecular weight by maximum-entropy algorithm. Output: one MW value with isotope-resolved precision.
automated · MaxEnt
06
Identity decision

Observed MW compared to the theoretical MW calculated from the sequence. ≤ ±1 Da deviation: identity confirmed, the lot moves to LAL endotoxin testing. Larger deviation: lot is held and the synthesis batch is investigated.
QC officer · signed

Anatomy of a mass spectrum

A multi-charge envelope, deconvoluted to one number.

Electrospray produces a series of multi-protonated ions of the same molecule. The pattern of the envelope is itself a confirmation: a clean, predictable distribution means a single defined species. Here is what to read in one.

ESI-MS · IGF-1 LR3 · Lot LR3-2025-A47 · positive mode Deconvoluted MW · 9111.2 Da

50010001500200025003000 m/z

Charge envelope

Eight protonation states from [M+12H]12+ down to [M+4H]4+, distributed in a clean Gaussian envelope. The shape itself confirms a single defined molecule; a chimera or contamination would distort it.

Deconvoluted MW

Maximum-entropy algorithm reduces the multi-charge envelope to one neutral mass: 9111.2 Da. Theoretical: 9111.5 Da. Δ = −0.3 Da — well inside the ±1 Da release window.

Baseline

Flat between charge states, no parasitic ions, no Na+/K+ adduct ladders, no early-eluting matrix interference. The signal-to-noise of the most abundant peak exceeds 200:1.

Method parameters

Source, analyser, scan, calibration.

Mass spectrometry without disclosed parameters is just a number with a stamp. Here is exactly what the lab runs against — reproducible on any modern Q-TOF or Orbitrap-class instrument.

Ionisation source

Mode	Electrospray, positive (ESI+)
Capillary voltage	4.0 kV
Cone voltage	40 V
Source temperature	150 °C
Desolvation gas	N₂, 800 L/h, 350 °C
Flow rate	5 µL/min (direct infusion)

Mass analyser

Type	Q-TOF, high resolution
Resolution	≥ 25 000 FWHM
Mass accuracy	< 5 ppm RMS
Scan range	m/z 100 – 3000
Scan rate	1 spectrum/sec
Acquisition	60 s, signal-averaged

Calibration & QC

Calibrant	Sodium formate cluster ions
Cadence	Daily lock-mass + per-batch verify
Drift rejection	> 5 ppm = no run
System suitability	Reference peptide, m/z and intensity
Replicates	2 acquisitions per lot
Reference standard	Independent, lot-traceable

Sample & processing

Diluent	50:50 ACN/H₂O + 0.1% formic acid
Concentration	5 µg/mL working solution
Charge states processed	+4 to +12 typical
Deconvolution	MaxEnt, Bayesian model
Output mass tolerance	± 0.1 Da resolved
Reporting	Δ vs. theoretical MW, in Da

Mass accuracy tiers

What an MW deviation actually means.

A peptide is its sequence; a sequence has a defined mass. A deviation between observed and theoretical MW is a signal — sometimes negligible, sometimes a synthesis failure. Here is how we read it.

< 0.5 Datypical IGF1 Shop release

Equivalent ppm (9 kDa)< 55 ppm

What it meansIdentity confirmed

Where most of our lots actually land. Within isotope envelope precision — the molecule matches the sequence atom-for-atom.

0.5 – 1.0 Darelease ceiling

Equivalent ppm (9 kDa)55 – 110 ppm

What it meansAcceptable, no flag

Within the ±1 Da release window. Likely calibration noise on a large peptide; sequence integrity is intact.

1 – 5 Dainvestigate

Equivalent ppm (9 kDa)110 – 550 ppm

What it meansRe-acquire, recalibrate

Outside the release window but inside common modification masses. Re-run on a freshly calibrated instrument; if confirmed, lot is held.

> 5 Dareject

Equivalent ppm (9 kDa)> 550 ppm

What it meansWrong molecule

Sequence error, missing residue, gross modification, or wrong vial labelled. Rejected immediately and the synthesis batch is investigated.

Reading the secondary peaks

What every mass-shift in the spectrum means.

Just like HPLC tells a story in retention shifts, ESI-MS tells one in mass shifts. The same secondary peaks recur across syntheses — and each one points at a specific failure mode.

Mass shiftModificationLikely causeAction

+ 16 Da Methionine / Trp oxidation Air exposure during purification Acceptable < 0.5%

+ 1 Da Asn / Gln deamidation Aged sample, basic conditions Investigate > 0.3%

+ 22 Da Sodium adduct [M+Na]+ Salt contamination, glassware Cosmetic < 1%

+ 38 Da Potassium adduct [M+K]+ Salt contamination Cosmetic < 1%

+ 114 Da TFA adduct Residual ion-pairing reagent from HPLC Cosmetic < 0.5%

− residue mass Truncation / deletion Coupling failure during synthesis Reject > 0.5%

+ residue mass Insertion / double coupling Side reaction, contaminated reagent Reject > 0.3%

≈ 2× MW Dimer (covalent or non-covalent) Disulfide bond, aggregation Reject > 1%

Hard fails

Four conditions that reject identity.

HPLC purity above 99% does not save a lot from MS rejection. These are the conditions under which the QC officer signs an MS rejection — independent of the chromatographic result.

F1

Deconvoluted MW deviation > ±1 Da

The release window. A 9 kDa peptide reading 9113 instead of 9111.5 is not "close enough" — it's a +1.5 Da shift consistent with deamidation, and the lot is held until the source of the shift is identified.

F2

Secondary species > 1% of main

An impurity peak in the deconvoluted spectrum at ≥ 1% of the main MW signal indicates a modification or truncation that HPLC could not separate. The lot is investigated regardless of HPLC purity.

F3

Charge envelope distortion

If the multi-charge distribution is not Gaussian — gaps in the expected charge states, asymmetric tail, multiple envelopes overlapping — the sample contains more than one species and identity cannot be reported.

F4

Calibration drift > 5 ppm

Calibration is verified against a sodium formate ladder before every batch. If mass accuracy across the scan range exceeds 5 ppm RMS, no client sample is acquired until the instrument is recalibrated.

FAQ

What researchers ask about MS verification.

Why ESI and not MALDI?

MALDI (matrix-assisted laser desorption / ionisation) is faster and tolerates dirtier samples, but produces predominantly singly-charged ions — which limits the usable mass range and makes deconvolution noisier. ESI generates a multi-charge envelope that supports MaxEnt deconvolution to single-Da precision on peptides up to ~30 kDa. For our catalogue, ESI is the better trade-off.

Why ±1 Da and not parts-per-million accuracy?

PPM accuracy describes the instrument; ±1 Da is the *biological* threshold. A 1 Da shift discriminates between intact peptide and deamidated isoforms — exactly the kind of error that matters for identity. We report mass accuracy in ppm internally (target < 5 ppm), and translate it to ±1 Da on the COA so the number stays interpretable.

What about isotope envelope and monoisotopic mass?

For peptides under 5 kDa we report the monoisotopic mass (lowest-mass isotopologue); for peptides above 5 kDa where isotopes are unresolved, we report the average mass calculated from natural abundance. The COA always specifies which convention is used so the comparison to theoretical is unambiguous.

Does MS confirm the sequence, or just the molecular weight?

A standard ESI-MS scan confirms MW only. For sequence-level confirmation we run MS/MS (collision-induced dissociation, fragmenting the peptide into b- and y-ions and reading the residue ladder) on demand — included by request for institutional buyers, or if a release sample triggers an investigation.

Can MS detect glycation, PEGylation or lipidation?

Yes — these all produce defined mass shifts (162 Da per hexose, 44 Da per ethylene oxide for PEG, etc.) that show up as either a shifted main peak or a secondary species. We flag any unexpected mass shift consistent with a known modification on the COA's "observations" line.

Why not just trust the synthesis facility's in-house MS?

Same reason we don't trust their HPLC: a lab that signs off its own product can't catch its own systematic errors. The independent contract lab uses different instruments, different calibration history, different operators — and that independence is the whole point of the COA.

ESI-MS confirms the molecule is what the synthesis claims.