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🧲 LCMS Adduct Mass Calculator

Calculate the m/z values for common positive and negative mode LC-MS adducts from a neutral monoisotopic mass.

Neutral Monoisotopic Mass (M) Da

Common Adduct m/z Values

Adduct	Mode	m/z

💡 Note: These are calculated from exact (monoisotopic) masses of the adducting species, not average atomic weights — appropriate for high-resolution LC-MS work. [M+H]⁺ and [M-H]⁻ are the most commonly observed adducts in ESI; others appear depending on solvent additives and analyte chemistry.

📖 Example: For M = 300.1200 Da: [M+H]⁺ = 301.1273, [M+Na]⁺ = 323.1092, [M-H]⁻ = 299.1127.

Frequently asked questions

Why are there so many possible adducts?

In electrospray ionization (ESI), a neutral molecule can associate with different charged species present in the mobile phase — protons, sodium or potassium ions from glassware or solvents, ammonium from buffer additives, or chloride/formate in negative mode — each producing a different observed m/z for the same underlying compound.

Which adduct should I expect to see for my compound?

[M+H]+ in positive mode and [M-H]- in negative mode are the most commonly observed adducts for most small molecules. Sodium and potassium adducts are more prominent for compounds with good cation affinity (e.g. those with multiple oxygens), and ammonium adducts often appear when ammonium acetate or ammonium formate is used as a mobile phase additive.

Does this use monoisotopic or average mass?

This calculator uses monoisotopic masses for the adducting species (e.g. exact mass of a proton, not average atomic weight), which is the correct convention for high-resolution mass spectrometry. Make sure the neutral mass (M) you enter is also the monoisotopic mass of your compound, not its average molecular weight.

Accuracy & how this is derived

Derivation: Each adduct m/z is calculated as the neutral monoisotopic mass (multiplied by 1 or 2 for dimeric adducts) plus or minus the exact monoisotopic mass of the adducting species (e.g. a proton = 1.00728 Da, sodium = 22.98922 Da), consistent with standard mass spectrometry adduct mass tables.

Validated against: Standard monoisotopic adduct mass values used throughout LC-MS analytical chemistry (e.g. Waters, Thermo Fisher adduct reference tables).

⚠️ For educational and research support only — verify critical results independently before use in regulated, clinical, or publication-bound work.

✔ Last updated: July 2026 · Report an error

Calculating LC-MS adduct masses

In liquid chromatography-mass spectrometry, a neutral analyte molecule is rarely observed directly; instead, it is detected as an ion formed by association with a charged species during electrospray ionization, such as a proton, sodium ion, or ammonium ion in positive mode, or through loss of a proton or association with chloride or formate in negative mode. Calculating the expected m/z for each common adduct from a compound's monoisotopic mass helps analysts quickly identify which peak in a mass spectrum corresponds to their target compound, particularly when multiple adduct forms appear simultaneously.