MS Isotope Distribution Calculator
Compute the isotope pattern, monoisotopic mass and average mass of a formula for mass spectrometry.
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What it does
The isotope distribution calculator turns a chemical formula into the cluster of peaks a mass spectrometer would record. It reports the monoisotopic mass, the average mass, and the relative intensity of each peak from M+0 up to the chosen mass shift, which is what makes a chlorine or bromine fingerprint, a sulfur M+2, or a carbon M+1 pattern recognisable at a glance. It is aimed at anyone interpreting or predicting a mass spectrum: assigning a molecular ion, checking a formula against an observed isotope envelope, or choosing a mass window.
How it works
The natural abundance of each stable isotope gives a probability distribution for one atom of an element, indexed by the nominal mass offset from the lightest isotope. Raising that distribution to the power of the atom count gives the pattern for all atoms of that element in the molecule, and multiplying the distributions of the different elements — a convolution — gives the pattern for the whole formula. Each peak carries the probability-weighted exact mass, so the cluster reports real m/z values rather than nominal integers. The monoisotopic mass is the sum of the lightest isotope of every element, which is the peak a high-resolution instrument resolves; the average mass is the abundance-weighted sum, which is what the periodic table atomic weights give and what a low-resolution measurement approximates.
Worked example
Caffeine, C₈H₁₀N₄O₂, has a monoisotopic mass of 194.0804 and an average mass of 194.194. Its M+1 peak sits about 10 percent of the base peak, built mainly from one ¹³C among the eight carbons and one ¹⁵N among the four nitrogens, which is why caffeine is a common mass-calibration standard. Dichloromethane, CH₂Cl₂, shows an M+2 peak about two-thirds as tall as M, the signature of two chlorine atoms each contributing a ³⁷Cl at roughly one chance in three.
When to use it
Use it to predict an isotope envelope before acquiring a spectrum, to confirm that an observed pattern fits a proposed formula — a strong M+2 points to chlorine, bromine or sulfur, while a large M+1 points to many carbons — or to pick the monoisotopic peak for exact-mass work. The table holds the common elements, so a formula containing an element without data still gets its average mass but no isotope pattern, and the tool says which elements were excluded.
FAQ
- What is the monoisotopic mass?
- It is the mass of the molecule built entirely from the lightest stable isotope of each element — carbon-12, hydrogen-1, nitrogen-14, oxygen-16. A high-resolution mass spectrometer resolves this peak, so it is the mass used for exact-mass formula assignment.
- Why does a peak appear one mass unit above the main peak?
- A small fraction of carbon is carbon-13, of nitrogen nitrogen-15, and so on, so some molecules are one unit heavier. The M+1 intensity grows with the number of carbon atoms, roughly 1.1 percent of the base peak per carbon, which is why the M+1 to M ratio hints at the carbon count.
- What does a strong M+2 peak indicate?
- A prominent peak two units above M points to an element with two abundant isotopes separated by two mass units — chlorine, bromine or sulfur. Two chlorines give an M+2 about two-thirds of M, and two bromines give one about equal to M, the classic halogen fingerprints.
- Which elements are supported?
- The table covers the common elements: hydrogen, lithium, boron, carbon, nitrogen, oxygen, fluorine, sodium, magnesium, silicon, phosphorus, sulfur, chlorine, potassium, calcium, iron, selenium, bromine and iodine. A formula with an element outside the table still returns its average mass, and the tool lists which elements had no isotope data.