Isotopic Mass

Posted : admin On 1/29/2022

Isotopic Abundances. Since 1931, the Commission regularly publishes the critical evaluation of isotopic abundances of elements. These values form basis for the standard atomic weights of elements. Below is the current table of Isotopic Compositions of the Elements. Isotopic Compositions of the Elements 2019. Isotopic mass synonyms, Isotopic mass pronunciation, Isotopic mass translation, English dictionary definition of Isotopic mass. The mass of an atom, usually expressed in atomic mass units. American HeritageĀ® Dictionary of the English Language, Fifth Edition.

  • The mass spectrum of strontium has four different peaks, varying in intensity. The four peaks indicate that there are four isotopes of strontium. The four isotopes of strontium have isotopic mass numbers of 84, 86, 87, and 88, and relative abundances of 0.56%, 9.86%, 7.00%, and 82.58%, respectively.
  • The atomic weights are available for elements 1 through 118 and isotopic compositions or abundances are given when appropriate. The atomic weights data were published by J. Meija et al in Atomic Weights of the Elements 2013, and the isotopic compositions data were published by M. Berglund and M.E. Wieser in Isotopic Compositions of the.

'an IonSource.Com tutorial'
-Slide 8


Monoisotopic and Average Mass


-Figure A.


Figure A. shows a simulated isotopic distribution of the [M+H]+ ion of a compound with the following elemental composition, C48 H82 N16 O17 (Poly-Alanine)

The Program IsoPro ver 3.1 was used for the simulation. The resolving power was set to 10,000.


Monoistopic Mass = 1155.6

Average Mass = 1156.3 (calculated)

As shown in Figure B. the monoisotoptic mass of this compound is 1155.6. For a given compound the monoisotopic mass is the mass of the isotopic peak whose elemental composition is composed of the most abundant isotopes of those elements. The monoisotopic mass can be calculated using the atomic masses of the isotopes.

The average mass is the weighted average of the isotopic masses weighted by the isotopic abundances. The average mass can be calculated using the atomic weights of the elements. The calculated average mass is shown in Figure B with the red line.

Figure B.


Figure C.-

Average mass can be measured by attempting to centroid a low resolution profile of a compound. The better the centroiding program the closer the measured mass will be to the theoretical average mass. The yellow envelope, shown in figure C., depicts how the peak might look at low resolution (perhaps with a low res. quadrupole instrument). A good centroiding program will be able to find the center of mass of this peak (yellow blob). A not so good program (that's a euphemism) will only pick the peak top. So with bad software, the reported mass for this compound would be closer to the monoisotopic mass. As the A+1 peak grows with increasing mass the peak top of the low resolution peak will inch closer to the theoretical average mass, and the program will begin to report values closer to the average mass. One might attempt to measure the masses and the abundances of the isotopes (at a higher resolution) then calculate the average mass of the compound but this exercise is seldom performed in routine analysis.

Once again we would like to reiterate that at lower resolution a peak labeling program might report masses closer to the monoisotopic mass when the compound mass is below 1500 amu and may report masses closer to the calculated average mass when the compound is larger than 1500 amu.

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Isotopic Mass Unit

[MD, Info, DF]= isotopicdist(SeqAA) analyzesa peptide sequence and returns a matrix containing the expected massdistribution; a structure containing the monoisotopic mass, averagemass, most abundant mass, nominal mass, and empirical formula; anda matrix containing the expected density function.

[MD, Info, DF]= isotopicdist(Compound) analyzesa compound specified by a numeric vector or matrix.

[MD, Info, DF]= isotopicdist(Formula) analyzesa compound specified by an empirical chemical formula representedby the structure Formula. The field namesin Formula must be valid element symbolsand are case sensitive. The respective values in Formula arethe number of atoms for each element. Formula canalso be an array of structures that specifies multiple formulas. Thefield names can be in any order within a structure. However, if thereare multiple structures, the order must be the same in each.

isotopicdist(..., 'PropertyName', PropertyValue,...) calls isotopicdist with optionalproperties that use property name/property value pairs. You can specifyone or more properties in any order. Enclose each PropertyName insingle quotation marks. Each PropertyName iscase insensitive. These property name/property value pairs are asfollows:

IsotopicIsotopic mass table

isotopicdist(..., 'NTerminal', NTerminalValue,...) modifies the N-terminal of the peptide.


isotopicdist(..., 'CTerminal', CTerminalValue,...) modifies the C-terminal of the peptide.

Isotopic Mass Definition

isotopicdist(..., 'Resolution', ResolutionValue,...) specifies the approximate resolution of the instrument,given as the Gaussian width (in daltons) at full width at half height(FWHH).

isotopicdist(..., 'FFTResolution', FFTResolutionValue,...) specifies the number of data points per dalton, tocompute the fast Fourier transform (FFT) algorithm.

isotopicdist(..., 'FFTRange', FFTRangeValue,...) specifies the absolute range (window size) in daltonsfor the FFT algorithm and output density function.

isotopicdist(..., 'FFTLocation', FFTLocationValue,...) specifies the location of the FFT range (window)defined by FFTRangeValue. It specifiesthis location by setting the location of the lower limit of the range,relative to the location of the monoisotopic peak, which is computedby isotopicdist.

isotopicdist(..., 'NoiseThreshold', NoiseThresholdValue,...) removes points in the mass distribution that aresmaller than 1/NoiseThresholdValue timesthe most abundant mass.

isotopicdist(..., 'ShowPlot', ShowPlotValue,...) controls the display of a plot of the mass distribution.