Interpret Mass Spectrum

Analyze MS → mol ion, formula, fragmentation pathways, structural features.

Use When

• MW + formula of unknown
• Confirm synthetic product (mol ion + fragmentation)
• ID impurities / degradation products
• Propose structural features from characteristic frag losses
• Isotope patterns → halogens, S, metals

In

• Req: MS data (m/z + rel int, min full scan)
• Req: Ionization method (EI, ESI, MALDI, CI, APCI, APPI)
• Opt: HRMS exact mass (measured vs calc)
• Opt: Mol formula from other (EA, NMR)
• Opt: MS/MS data (frag of selected precursor)
• Opt: Chrom ctx (LC-MS / GC-MS tR, purity)

Do

Step 1: Ionization Method + Expected Ion Types

Determine what species present before peak assignment:

1. Classify ionization:

2. Polarity mode: +ve → cations; -ve → anions. ESI uses both commonly.
3. Adducts + clusters: Soft ionization → [M+Na]+ (M+23), [M+K]+ (M+39), [2M+H]+, [2M+Na]+ besides [M+H]+. ID these before mol ion.
4. Multiply charged: ESI → m/z = (M + nH) / n. Look for fractional m/z spacing (0.5 Da = z=2).

→ Method documented, expected ion types listed, adducts/clusters ID'd → true mol ion determinable.

If err: Method unknown → examine spectrum for clues: extensive frag → EI; adduct patterns → ESI; matrix peaks → MALDI. Check instrument log.

Step 2: Mol Ion + Mol Formula

ID mol ion peak + derive formula:

1. Locate mol ion (M): EI → M+. highest m/z w/ reasonable isotope pattern (may be weak/absent for labile compounds). Soft → ID [M+H]+ / [M+Na]+ + subtract adduct → M.
2. N rule: Odd MW → odd # N. Even MW → 0 or even # N.
3. DBE: DBE = (2C + 2 + N - H - X) / 2, X = halogens. Ring / π bond = 1 DBE. Benzene = 4, carbonyl = 1.
4. HRMS: Exact mass avail → calc formula using mass defect. Compare measured vs candidate formulas in accuracy window (typ < 5 ppm modern instruments).
5. Cross-check isotope pattern: Observed must match proposed formula (Step 3).

→ Mol ion ID'd, MW determined, N rule applied, formula proposed (confirmed by HRMS if avail).

If err: No mol ion in EI (common thermally labile / highly branched) → try softer ionization. Ambiguous mol ion → check loss of common small frags from highest m/z (M-1, M-15, M-18 → help ID M).

Step 3: Isotope Patterns

Use isotopic signatures → detect elements:

1. Monoisotopic elements: H, C, N, O, F, P, I have characteristic abundances. CHNO only → M+1 ≈ 1.1% per C.
2. Halogen patterns:

3. Sulfur: 34S → 4.4% at M+2. M+2 ≈ 4-5% rel M (after 13C2 correction) → ≈ 1 S.
4. Silicon: 29Si (5.1%) + 30Si (3.4%) → distinctive M+1 + M+2 contributions.
5. Compare calc vs observed: Use proposed formula → theoretical pattern → overlay observed → confirm/refute.

→ Pattern analyzed, Cl/Br/S/Si presence determined, consistent w/ proposed formula.

If err: Isotope res insufficient (low-res instrument) → M+2 unresolvable. Note limitation, rely on exact mass + other spectra for elemental comp.

Step 4: Fragmentation Losses + Key Frag Ions

Map pathways → structural info:

1. Catalog major frags: All peaks > 5-10% rel int w/ m/z.
2. Neutral losses from mol ion:

3. Characteristic frag ions:

4. Map frag pathways: Connect frag ions by successive losses → frag tree from M down to low mass.
5. Rearrangement ions: McLafferty (γ-H transfer + β-cleavage) → even-electron ions from carbonyl compounds. Retro-Diels-Alder → characteristic cyclohexene.

→ All major frag ions assigned, neutral losses calc + correlated w/ structure, frag tree built.

If err: Frags don't correspond to simple losses → consider rearrangement. Unassigned frags → unexpected groups, impurities, matrix/BG peaks.

Step 5: Purity + Structure

Evaluate spectrum for purity + assemble proposal:

1. Purity check: GC-MS / LC-MS → examine chrom for add'l peaks. Direct-infusion → look for unexpected ions not frags/adducts of analyte.
2. BG + contaminant peaks: Common: phthalate plasticizers (m/z 149, 167, 279), column bleed (siloxanes 207, 281, 355, 429 in GC-MS), solvent clusters.
3. Structure proposal: Combine formula (Step 2) + isotope (Step 3) + frag (Step 4) → structure / candidate set.
4. Rank candidates: Frag tree → rank. Best = explains most frag ions w/ fewest ad hoc.
5. Cross-validate: Compare vs NMR, IR, UV-Vis. MS alone rarely unambiguous for novel compounds.

→ Purity assessed, contaminants ID'd if present, structural proposal / ranked candidates consistent w/ all MS + cross-validated where poss.

If err: Multiple components w/o chrom sep → flag mixture, recommend LC-MS / GC-MS reanalysis. No satisfactory proposal → ID which add'l data (HRMS, MS/MS, NMR) would resolve.

Check

• Ionization method ID'd + expected ion types documented
• Mol ion located + distinguished from adducts, frags, clusters
• N rule applied + consistent w/ proposed formula
• DBE calc + accounted for in structure
• Isotope pattern matches formula
• Major frag ions assigned w/ neutral losses + structural rationale
• Frag tree built M → low mass
• Contaminant + BG peaks ID'd + excluded
• Proposal cross-validated w/ other spectra

Traps

• Mis-ID mol ion: EI → base peak often frag, not M. M = highest m/z w/ reasonable isotope pattern. ESI adducts ([M+Na]+, [2M+H]+) → mistaken for M.
• Ignore N rule: Odd-mass M → odd # N. Forget → impossible formulas.
• Confuse isobaric losses: Loss 28 = CO or C2H4; loss 29 = CHO or C2H5. HRMS / add'l frag → distinguish.
• Neglect multiply charged: ESI → 2+/3+ at half/third expected m/z. Non-integer spacing between isotope peaks → multi charge diagnostic.
• Over-interpret low-abundance: Peaks < 1-2% rel int → noise, isotope contribs, minor contaminants, not real frags.
• Assume pure: Many real spectra = mixtures. Check chrom purity + look for ions inconsistent w/ proposed structure.

→

• interpret-nmr-spectrum — connectivity + H environments → structural confirm
• interpret-ir-spectrum — func groups explaining observed frag
• interpret-uv-vis-spectrum — chromophores in analyte
• interpret-raman-spectrum — complementary vibrational
• plan-spectroscopic-analysis — select + sequence techniques pre-acquisition
• interpret-chromatogram — GC/LC chrom data coupled w/ MS