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📶Isotope Pattern Calculator - Mass Spec Isotope Distribution, Exact Mass & Adduct m/z

Most elements have more than one stable isotope, so a molecule appears in a mass spectrum as a cluster of peaks: the M peak contains only the most abundant isotope of each element, while M+1, M+2 and so on come from heavier isotopes such as ¹³C, ²H, ¹⁵N, ¹⁸O, ³⁴S, ³⁷Cl and ⁸¹Br. This calculator writes each element's isotope abundances as a polynomial, raises it to the number of atoms and multiplies the elements together (convolution by repeated squaring, pruning peaks below 10⁻⁹), then normalises the tallest peak to 100. Glucose, C₆H₁₂O₆, has a monoisotopic mass of 180.0634 u, and its [M+Na]+ ion appears at m/z 180.06339 + 22.98977 − 0.00055 (electron) = 203.0526. Chlorobenzene shows M : M+2 ≈ 100 : 32, and dibromomethane M : M+2 : M+4 ≈ 51 : 100 : 49. You can choose M, M+•, [M+H]+, [M+Na]+, [M+K]+, [M+NH4]+, [M−H]−, [M+Cl]− and z = 1–3. Unit-resolution mode sums peaks at each nominal mass, and fine-structure mode separates ¹³C from ¹⁵N or ²H a few mDa apart. A peak table, a Cl/Br pattern cheat sheet and a helper that estimates carbon count from the M+1/M ratio are included. Exact masses come from NIST/AME, with IUPAC representative isotopic abundances.

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How to use Isotope Pattern Calculator

  1. 1Enter a molecular formula such as C6H12O6 or C6H5Cl, or click an example
  2. 2Pick the ion type (M, [M+H]+, [M+Na]+, [M−H]− …), charge z and unit or fine-structure display
  3. 3Read the monoisotopic m/z, average mass, isotope plot and peak list (copyable), and use the Cl/Br sheet or the M+1 carbon estimator below

FAQ

What is the difference between monoisotopic, average and nominal mass?
Monoisotopic mass uses the most abundant isotope of every element (¹²C, ¹H, ³⁵Cl…) and is where the M peak sits in a high-resolution spectrum, e.g. 180.0634 u for glucose. Average mass uses standard atomic weights and matches the molar mass you weigh out, 180.156 for glucose. Nominal mass is the integer sum of mass numbers, 180 for glucose, as used in low-resolution MS.
Why do chlorinated and brominated compounds show a strong M+2 peak?
Chlorine's isotopes ³⁵Cl and ³⁷Cl occur at roughly 3 : 1 and bromine's ⁷⁹Br and ⁸¹Br at roughly 1 : 1, and each pair differs by two mass units. One Cl gives M : M+2 ≈ 3 : 1 and one Br about 1 : 1; two Cl give 9 : 6 : 1, two Br 1 : 2 : 1 and ClBr 3 : 4 : 1. These patterns are the quickest way to spot halogens in a spectrum.
How do I estimate the number of carbons from the M+1 peak?
¹³C makes up about 1.1% of natural carbon, so each carbon has a 1.1% chance of being heavy and the M+1/M intensity ratio is roughly n(C) × 1.1%. If M+1 is 8.8% of M, the molecule has about 8 carbons. Nitrogen (0.37% each) and hydrogen add a little to M+1, so the estimate tends to run slightly high; use it as a clue when proposing a formula.
Why correct for the electron mass?
A cation has lost electrons and an anion has gained them. One electron weighs 0.000549 u, so the correct m/z of [M+H]+ is M + m(H) − mₑ, i.e. M plus the mass of a proton. High-resolution instruments are accurate to a few ppm, and 0.00055 u is about 2.7 ppm at m/z 200, enough to matter. The correction is on by default and can be switched off to compare with older software.