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
Mass
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).