# Mass Accuracy and Resolution

Nearly all elements have more than one naturally occurring isotope. Since each isotope has a unique mass, the mass spectrum of even a single element will often be comprised of a mixture reflecting the relative abundance of each isotope. When elements are combined to form a molecule, the mass spectrometer “sees” the entire distribution of masses from the combination of elements and their isotopes. How well we “see” this distribution and how accurate the masses are is dependent on the available resolving power (resolution) of the mass spectrometer.

With normal resolution mass spectrometers (e.g., ion traps and quadrupoles) our ability to resolve the isotopic pattern ends at approximately 2000-3000 Da. As a result, we may only see a single unresolved m/z peak for a given isotopic distribution for higher mass species on these instruments. However, with the power of high resolution mass spectrometers, we are able to clearly separate isotopic peaks and delve deeper into the data.

Nominal mass is calculated based on the mass of the most abundant isotope of each atom in a compound, and is represented by an integer. E.g. since the most abundant isotope of Hydrogen is 1 and that of Oxygen is 16, the nominal mass of water would be (1+1+16=) 18 Da.

Novatia uses the term “nominal mass” a bit freely. If we report normal resolution data for a small molecule sample (<1000 Da) on one of our ion trap instruments, for example, we usually refer to this as nominal mass data. Although the accuracy of our ion trap mass spectrometers for small molecule samples is typically within 0.1 – 0.2 m/z units of the expected monoisotopic mass value (see below) for the ionic species. The ionic species reported are typically protonated [M+H]+ for positive ions or deprotonated [M-H]- for negative ions, where M represents the monoisotopic mass.

Average mass is calculated by taking the weighted average of all isotopes in a given molecule and adding them together. The average mass of water would be (1.0079 + 1.0079 + 15.9994 =) 18.0152 Da. The molecular weight of a compound is its isotopic average mass. This differs significantly from the monoisotopic mass.

Monoisotopic mass takes into account only the isotope with the lowest mass for each isotopic distribution. For example, based on IUPAC values, the monoisotopic mass of Hydrogen is 1.0078 and that of Oxygen is 15.9949. Thus the mono isotopic mass of water to 4 decimal places would be (1.0078+1.0078+15.9949=) 18.0105 Da. The difference between the average and monoisotopic masses for water does not seem like much, but the difference between these two numbers becomes more significant for large molecules due to the width of the isotopic pattern.