JEOL MS Calculator App


  • Enter an elemental composition and calculate the exact mass and relative abundance for the isotope peaks
  • Enter an exact mass and calculate the possible elemental compositions within the specified error tolerances
  • Calculate the m/z values for a given molecular weight
  • Calculate the molecular weight from two successive m/z values
The JEOL MS Calculator App is available for free through the Apple Store.

Tab Pages

Click on the tab control at the bottom of the screen to select which page is displayed. The currently selected tab page is highlighted in blue on the tab controller. There are three tab pages:
  • Elemental compositions and isotope patterns
  • Element limits for composition calculations
  • Multiple charge peak calculations
Elemental Compositions and Isotopes Tab
Multiple Charge Peaks Tab
Element Limits Tab

Isotope Calculations

To calculate an isotope pattern, select the Composition tab (“CHON” icon) and:
  • Enter an elemental composition* (example: “C60” or “C19H38O2”) in the green rectangle labeled “Formula or Mass”.
  • Press the “Calculate” button.
  • The calculated isotope pattern will be displayed in the text box.
* The elemental composition must begin with an elemental symbol. If a numeric value is entered, the program will try to calculate elemental compositions. Element symbols are case-sensitive: “CO” (carbon and oxygen) is different from “Co” (cobalt).
Isotope Calculations

Elemental Composition Calculations

To calculate elemental compositions, select the Composition tab (“CHON” icon) and:
  • To calculate elemental compositions, select the Composition tab (“CHON” icon) and:
  • Press “Calculate” to calculate elemental compositions that have monoisotopic exact masses within the specified limits.

Element limits

The element limits are shown in the box above the “Formula or Mass” box. The text shows element limits for each element in the format: Element symbol {space} Minimum} / Maximum with spaces between each element’s limits. For example “C 0/50 H 0/100 O 0/10 N 0/10” means to calculate compositions with between 0 and 50 carbons, 0 and 100 hydrogens, 0 and 10 oxygens, and 0 and 10 nitrogens. Press “Reset” to display default C,H,O,N limits for small molecules.
  • You can edit the limits string yourself, but it must follow the defined format. 
  • It is generally easier to set the element limits on the element limits tab to ensure the correct format.
  • If you omit an element that may be present, the correct composition will not be displayed.
  • If you include a large number of elements, the list of compositions may be very large, and the calculation may take a very long time.

Ion polarity

Click “+”, “-“, or “0” to specify positive ions, negative ions, or neutral masses.
  • Choosing “+” will subtract the rest mass of an electron (0.000548u) from the calculated neutral mass.
  • Choosing “-“ will add the rest mass of an electron.
  • Choosing “0” will ignore the electron mass.

Ion type

This allows you to limit the number of calculated compositions if you know the ion type:
  • Even-electron ions such as [M +H] +, [M – H] -, [M + Na] +
  • Odd-electron ions such as M +.
  • Both


Enter the measured m/z to calculate all compositions within the specified limits. If your mass accuracy takes the electron mass into account, you should specify the ion polarity. Only single charge ions are currently supported.

Tolerance (mmu)

The mass tolerance for calculating elemental compositions. Please ensure that this tolerance matches the accuracy and precision of your measured m/z values. If you enter “91” for the mass (really, the m/z), specify even-electron ions, and your mass tolerance if 5 mmu (0.005u), the displayed compositions will only include C2H3O4 (m/z 91.003). However, if you enter “91.055”, the compositions will include C7H7 (m/z 91.055) and C2H7O2N2 (m/z 91.051). 

Element Limits Tab

Use this tab to choose elements to be included in the elemental composition calculation, and to specify the minimum and maximum numbers for each element. 
  • Click on the element symbol to include an element, or to remove an included element from the calculation.
  • Elements that are currently included will be highlighted in red.
  • Specify the minimum and maximum for the included elements.
  • If you select “X”, enter the element symbol in the box below (potassium is specified as “K” in the figure above).
  • Press the SAVE LIMITS button to use these element limits for the calculation.
Element Limits Tab

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