Isotope Distribution Calculator

In mass spectrometry, M refers to the monoisotopic peak (all atoms are the lightest isotope). M+1 is the peak one mass unit higher (one atom is a heavier isotope, typically C-13 or N-15). M+2 is two units higher (one atom of Cl-37, S-34, or Br-81, or two C-13 atoms). The M+1 and M+2 relative intensities help determine molecular formula.

More atoms create a broader isotope envelope with more peaks. For one Cl atom: 2 peaks (3:1). For two Cl atoms: 3 peaks (9:6:1). For three Cl atoms: 4 peaks (27:27:9:1). The envelope approaches a Gaussian distribution for large numbers of atoms, with the width proportional to the square root of n. This is why protein mass spectra show broad, smooth isotope envelopes.

Yes. Molecules with the same nominal mass but different formulas often have distinctly different isotope patterns. For example, C2H4O (MW 44) and CO2 (MW 44) have different M+1 intensities because C2H4O has more carbon atoms contributing to C-13 peaks. High-resolution mass spectrometry combined with isotope pattern analysis can unambiguously determine molecular formulas.

Isotope dilution adds a known amount of an enriched isotope to a sample to quantify the element or compound. By measuring the change in isotope ratio, the original amount can be calculated with very high accuracy. This technique, pioneered by Hevesy and Paneth, is considered a primary analytical method and is used for certified reference materials and forensic analysis.

Monoisotopic elements (F, Na, Al, P, Mn, Co, As, Au, etc.) contribute no isotope peaks because they have only one stable isotope. Molecules composed entirely of monoisotopic elements show a single peak with no isotope envelope. This simplifies mass spectral interpretation but provides less structural information than polyisotopic elements.