(Lecture 9.) Chromatography pulls a mixture apart. Mass spectrometry is what most often sits at the end of that separation, weighing each component as it comes off the column — together, chromatography and MS answer how much and what, which is most of what a specification actually asks.
The one idea
A mass spectrometer measures mass-to-charge, nothing more. Everything useful — a formula, a structure, a concentration at parts-per-billion — is inference built on that one measurement.
Mass spectrometry — the pieces
| Stage | Options | What to know |
|---|---|---|
| Ionization | ESI, APCI, APPI (LC); EI, CI (GC); MALDI | ESI is the default for pharma LC–MS; “soft” (molecular ion survives) vs. EI “hard” (reproducible fragmentation, library-searchable) |
| Mass analyzer | Quadrupole, triple quadrupole (QqQ), ion trap, TOF, Q-TOF, Orbitrap, FT-ICR | Trades among resolution, mass accuracy, speed, dynamic range, cost |
Low-resolution (QqQ) excels at targeted quantitation (SRM/MRM) — very selective, very sensitive. High-resolution (HRMS, Q-TOF/Orbitrap) measures accurate mass to a few ppm, which gives an elemental formula — the starting point for identifying an unknown impurity or degradant.
What MS is used for, on a small molecule
| Use | Approach | Ties to |
|---|---|---|
| Impurity / degradant identification | LC–HRMS: accurate mass → formula → structure from fragmentation, confirmed against a standard where possible | Q3, Q1 |
| Trace mutagenic-impurity quantitation | LC–MS/MS (SRM) at ppb — nitrosamines, alkyl halides, hydrazine | Q9, ICH M7 |
| Extractables & leachables | LC–HRMS + GC–MS screening against databases | Container closure |
| Residual solvents | Headspace GC–MS/FID | Q3C |
The quantitation problem — ion suppression
ESI response is not a fixed property of an analyte. Co-eluting matrix components compete for charge and change the analyte’s signal, often suppressing it by more than half, and the effect drifts across a batch. The standard fix: a stable-isotope-labeled internal standard (SIL-IS) — chemically identical, co-elutes exactly, experiences the same suppression, so the analyte/IS ratio is preserved — plus matrix-matched calibration and post-column infusion experiments to map where suppression occurs.
Applied case — mass spec for biologics and advanced therapies
The same ionization and mass-analyzer fundamentals above do heavier lifting once the molecule is a protein, a capsid, or a strand of RNA:
- Peptide mapping — digest the protein, run the peptides by LC–MS/MS, and identify sequence variants, oxidations, deamidations, and glycoforms from the resulting map. This is the technique behind the charge-variant worked case from Week 8’s applied case: a mAb process moves to a larger bioreactor, post-change lots show acidic charge variants up from 18% to 26% by icIEF, and peptide mapping localises the extra acidic species to increased deamidation at a known site — resolved as comparable on function once HDX-MS and an FcRn binding assay confirm no effect on binding.
- Intact and subunit mass — confirm the whole molecule (or a reduced/deglycosylated subunit) matches the expected mass, catching mis-incorporations or clips that peptide mapping alone might miss.
- Native MS and charge-detection MS — ionize the protein without denaturing it, to weigh whole assemblies, including viral capsids for gene-therapy products.
- The multi-attribute method (MAM) — a single LC–HRMS peptide map monitoring a predefined list of quality attributes can replace several conventional assays run separately. Its second half is new peak detection (NPD): flagging any peak that’s new or changed as a safety net the targeted list would otherwise miss — too sensitive and every run throws false positives, too lax and it stops being a safety net.
- LC–MS for oligonucleotides — identity and sequence-related impurities (n−1, n+1, depurination) on the same ion-suppression and calibration principles taught above, applied to a synthetic nucleic acid rather than a small molecule or a protein.
Risk-assessment assignment (Risk Homework, checkpoint 3 of 3)
Build the method FMEA (Week 2) for an LC–MS/MS nitrosamine method at a 30 ng/day acceptable-intake limit. Give particular weight to the MS-specific failure modes: ion-suppression drift, a SIL-IS with isotopic impurity, in-source fragmentation creating an interfering ion, mass-calibration drift, and carryover. Score the detectability of each — which would the run’s own system-suitability and QC samples actually catch? This closes the risk-homework thread that began with atomic spectroscopy and continued through molecular spectroscopy.
Where the analyst sits
Deciding when an MS identification is confirmed rather than merely consistent is analytical judgment, not something the software’s library-match score settles for you — the STEAM “A”. The refrain: science → evidence → reduced uncertainty → control → regulatory confidence → patient trust.
On the job
- If your lab has LC–MS, you’ll likely start as the person running samples and flagging anything the automated software calls “possible new peak,” not the person doing structure elucidation — that judgment call comes later.
- If you’re asked to review an MAM/NPD run, your job is usually to triage the flagged new peaks, not to identify them yourself — know which ones get escalated and to whom.
For discussion
- HRMS gives you an unknown degradant’s formula to 2 ppm. Walk through what you do next to get to a structure, and where you would stop and call it “sufficiently identified.”
- Your LC–MS/MS assay for a drug in plasma reads 15% low on incurred samples versus spiked standards, even with a SIL-IS. What could still cause that?
- One MAM assay replaces icIEF, released glycans, and part of the peptide map. What is lost, if anything, by consolidating?
Source note. MS fundamentals follow standard texts (Gross, Mass Spectrometry; de Hoffmann & Stroobant). Bioanalytical validation: ICH M10; impurity work connects to ICH M7 and Q3. The biologics/ATMP applications follow ICH Q5E, Q6B, and the published MAM-consortium literature.