Mass Defect

The mass defect is the difference between the calculated mass and the nominal mass (integer). Combining the masses of all the components of the compound uses calculated mass. The masses of the elements come from the periodic table, with the mass of 12C  being exactly 12.0000000 amu. Most of the other elements have masses close to their nominal elemental mass:

You can calculate the mass of a compound as follows:

Therefore, as you get to larger compounds containing more atoms, you will add all the "defects" of the individual atoms together and the difference may be substantial, especially if you display the mass as an integer. In the examples above, the first would be displayed as an integer (150 amu) either way. In the second example, the actual mass in integer format would be 703 amu; that is, a whole mass unit from the nominal mass of 702 amu.

If you have organic compounds containing nitrogens and hydrogens and the only other element is carbon, the result is a positive mass defect. Since many hydrocarbon compounds have a lot of hydrogen, the defect can be very large.

If there is also a substantial amount of oxygen, its negative mass defect can offset the nitrogen and hydrogen and reduce the mass defect. That is why the user sets the type and magnitude of the mass defect depending upon what kind or class of compounds he is working with. The correction for mass defect is generally set as x mmu per 100 amu where mmu is millimass unit or one thousandth of a mass unit.

If the correction is set to 100 mmu/100 amu (it is usually set from 30 to 120 mmu/100 amu), the correction for the examples above would be:

Thus, the correction factor "modifies" the data so that people used to working with integer masses for their elements in a compound obtain the expected results.

 Note: 

In Chromeleon, atomic masses are always indicated in [m/z] (= mass/charge number).