Isotope patterns and exact mass

Masses — the monoisotopic mass uses the most abundant isotope of each element (¹²C = 12 exactly); the average mass uses natural abundances; the nominal mass is the integer sum. Masses and abundances are NIST "Atomic Weights and Isotopic Compositions" values.

Isotope pattern — computed exactly by multiplying out each element’s isotope polynomial (repeated squaring for large counts), dropping peaks below 10⁻⁷ and merging peaks within 0.5 u (unit resolution) or 0.0005 u (fine structure, as an Orbitrap or FT-ICR would resolve). Intensities are relative to the tallest peak. Chlorine and bromine give the familiar M+2 patterns; ethanol’s M+1 is about 2.2 % of M.

Ions — m/z = (n·M + adduct masses − charge × electron mass) ÷ |z| for [M+H]⁺, [M+Na]⁺, [M+NH₄]⁺, [M+2H]²⁺, [M−H]⁻, [M+Cl]⁻, [M+HCOO]⁻, radical cations, and others.

ppm error — enter a measured m/z: error = (measured − theoretical) ÷ theoretical × 10⁶.