LC-MS Adduct Calculator: Monoisotopic Mass and m/z from Molecular Formula

Type a molecular formula and get the exact mass and the m/z of common ESI adducts.

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Molecular formula (or a neutral monoisotopic mass)

Example: paracetamol, C8H9NO2. Parentheses are supported, e.g. C2H4(OH)2.

Monoisotopic mass–

Average MW–

m/z = (M + n·m_adduct)/|z| · uses monoisotopic masses with electron mass correction

How it works

High-resolution LC-MS works with monoisotopic masses (the most abundant isotope of each element), not the average molecular weight printed on a certificate. The m/z of an ion also depends on the adduct that carries the charge, and the mass of the electron (0.00055 Da) matters at ppm accuracy.

Seeing an unexpected peak at M+22 relative to [M+H]⁺ usually means a sodium adduct; at M+17, an ammonium adduct. Adduct patterns change with the mobile-phase additive and glassware, so they are a common source of confusion in impurity identification.