ESI-MS Adduct m/z Calculator ([M+H]+, [M+Na]+, Neutral Mass)

A mass spectrometer measures the mass-to-charge ratio m/z, not molecular weight directly. In electrospray ionization (ESI), neutral molecules gain or lose one or more protons in solution, forming charged ions that enter the detector. A single molecule can produce multiple adduct ions, each with a different m/z. Misassigning adducts shifts the molecular weight calculation and causes database searches to return wrong compounds or no hits at all.

Core Formulas

Forward — calculate adduct m/z from neutral exact mass:

m/z = (M + Σ m_adduct) / |z|

M is the monoisotopic exact mass of the neutral molecule (Da); Σ m_adduct is the summed mass contribution from all adduct species; |z| is the absolute charge state.

Common adduct mass contributions (Da):

Positive ion mode (ESI+)
  [M+H]+          M + 1.007276          z = 1
  [M+Na]+         M + 22.989218         z = 1
  [M+K]+          M + 38.963158         z = 1
  [M+NH4]+        M + 18.033825         z = 1
  [M+2H]2+        M + 2 × 1.007276      z = 2
  [M+H+Na]2+      M + 1.007276 + 22.989218   z = 2
  [M+3H]3+        M + 3 × 1.007276      z = 3

Negative ion mode (ESI−)
  [M−H]−          M − 1.007276          z = 1
  [M+Cl]−         M + 34.969402         z = 1
  [M+HCOO]−       M + 44.998201         z = 1
  [M+CH3COO]−     M + 59.013851         z = 1
  [M−2H]2−        M − 2 × 1.007276      z = 2

Reverse — calculate neutral mass from observed m/z:

M = m/z × |z| − Σ m_adduct

Worked Examples

Caffeine (C₈H₁₀N₄O₂, M = 194.080376 Da), positive ion mode:

[M+H]+:   (194.080376 + 1.007276) / 1 = 195.0877 Da  ← PubChem CID 2519 theoretical value
[M+Na]+:  (194.080376 + 22.989218) / 1 = 217.0696 Da  ← agrees with literature value 217.0695
[M+2H]2+: (194.080376 + 2 × 1.007276) / 2 = 98.0475 Da

Reverse check: given m/z = 195.0877, z = 1, adduct [M+H]+, then M = 195.0877 × 1 − 1.007276 = 194.0804 Da ✓

Aspirin (C₉H₈O₄, M = 180.042259 Da), negative ion mode:

[M−H]−:   (180.042259 − 1.007276) / 1 = 179.0350 Da  ← literature value 179.0350
[M+Cl]−:  (180.042259 + 34.969402) / 1 = 215.0117 Da  ← literature value 215.0116

Why Proton Mass, Not Hydrogen Atom Mass

ESI ionization transfers a proton (H⁺), which carries no electron and has a mass of 1.007276 Da (NIST CODATA 2018). A hydrogen atom has one additional electron and a mass of 1.007825 Da; the difference is 0.000549 Da (one electron mass). High-resolution mass spectrometers typically achieve ≤ 5 ppm accuracy; for a 200 Da compound, 1 ppm = 0.2 mDa, and the proton–hydrogen difference of 0.55 mDa ≈ 2.7 ppm is large enough to cause wrong or missed hits in exact-mass database searches. Proton mass must be used.

Monoisotopic Mass vs. Average Molecular Weight

If you only have the average molecular weight (e.g. from a reagent bottle label), there will be a significant offset in HRMS searches. Confirm the monoisotopic mass in PubChem or ChemDraw before querying.

Scope

• Applicable: ESI ionization; small organic molecules < 2000 Da (drugs, metabolites, lipids, natural products); short peptides < 5 kDa with a known charge state.
• Not applicable: Intact proteins (require deconvolution software); MALDI (complex matrix adducts, require separate treatment); EI ionization (generates radical cations M⁺·, mass = M − 0.000549 Da, different rules).
• This tool computes theoretical m/z only. It does not process spectral files or perform isotope envelope analysis.

Common Mistakes

• Using average molecular weight instead of monoisotopic mass: the two differ by 0.1–2 Da; HRMS searches return wrong compounds or miss entirely.
• Treating m/z as molecular weight: the m/z of [M+2H]²+ is roughly half the molecular weight — that is the same molecule carrying two charges, not a different, smaller compound.
• Overlooking sodium adducts: in positive ion mode, even without added sodium salt, trace sodium from glassware and skin produces a [M+Na]⁺ peak 21.982 Da above [M+H]⁺; beginners often flag it as an impurity.