Monoisotopisk

Monoisotopisk is an adjective used in chemistry, biochemistry, and related fields to describe concepts or measurements tied to the monoisotopic mass of a molecule. The monoisotopic mass is the sum of the exact masses of the lightest stable isotopes of each element present in a substance (for example, 1H, 12C, 14N, 16O, and so on). This mass value provides a precise reference point for identifying and characterizing molecules in high‑resolution mass spectrometry.

Monoisotopic mass differs from the average or most abundant isotopic mass. The average mass weights isotopes by their natural abundances, yielding a mean value that reflects typical samples, whereas the monoisotopic mass uses the lightest stable isotopes. In many analytical contexts, especially in tandem mass spectrometry, the monoisotopic mass is used to match observed spectra to theoretical masses with high specificity.

Calculation and use: the monoisotopic mass is obtained by summing the exact masses of the lightest stable isotopes for all elements in a molecule. In practice, software tools and databases provide monoisotopic masses for amino acids, nucleotides, and small molecules, enabling precise identification in proteomics, metabolomics, and related fields. The concept is particularly important for distinguishing closely related compounds and for representing peptide and nucleotide masses in databases.

Limitations and considerations: natural isotope distributions mean that most molecules contain one or more heavier isotopes, so the monoisotopic peak can be less intense for large molecules. In such cases, analysts may rely on M+1, M+2, or average masses, depending on the instrument and workflow. Monoisotopisk measurements are most informative when high mass accuracy is achievable and isotopic composition is well understood.