What is the Resolution of a Mass Ion Peak?

For a general object, if the density and the volume of that object are known, the mass of that object can be derived using the following equation:

  m =ρ x V;

where m= mass, ρ = density, and V = volume.

See also: 

However, in a mass spectrometer, ions are acquired as m/z values. In matrix-assisted laser desorption time of flight mass spectrometer instruments, the major ions for peptides analyzed are usually parent ions for which z =1. The shape of the mass or ion peak determines the resolution. Based on the shape of the peak the resolution can be mathematically described using the formula:

Resolution = M/ΔM  

where M = mass and ΔM (dMass or delta Mass) = mass range at 50% intensity of the peak.

The resolving power in a mass spectrometer is defined as the ability of the instrument or measurement to distinguish between two peaks at m/z values differing by a small amount, expressed as the peak width in mass units. See also:  https://en.wikipedia.org/wiki/Full_width_at_half_maximum.

Example 1: Resolving Power and Accuracy

Legend: M = mass; FWHM = ΔM = full width at half maximum; ΔM =  delta Mass.

However, the accuracy of mass measurements depends on its resolution. Therefore, a higher resolution results in a better mass accuracy.

Example 2: Impact of Resolution and Error of Measurement


Calculating Peptide Masses

      Sum the monoisotopic residue masses

      Add mass of H2O (18.01056)

      Add mass of H+ (1.00785 to get M+H)

      If Met is oxidized add 15.99491

      If Cys has acrylamide adduct add 71.0371

      If Cys is iodoacetylated add 58.0071

      Other modifications are listed at


      Mostly consider peptides with masses > 400 for proteomic experiments

Example 3: Mass Calculation for a Glycine Ion