Organic Chemistry Text Book (CHEM 3401 and 3402)

In assigning mass values to atoms and molecules, we have assumed integral values for isotopic masses. However, accurate measurements show that this is not strictly true. Because the strong nuclear forces that bind the components of an atomic nucleus together vary, the actual mass of a given isotope deviates from its nominal integer by a small but characteristic amount (remember E = mc²). Thus, relative to ¹²C at 12.0000, the isotopic mass of ¹⁶O is 15.9949 amu (not 16) and  ¹⁴N is 14.0031 amu (not 14). 

By designing mass spectrometers that can determine m/z values accurately to four decimal places, it is possible to distinguish different formulas having the same nominal mass. The table on the right illustrates this important feature, and a double-focusing high-resolution mass spectrometer easily distinguishes ions having these compositions. Mass spectrometry therefore not only provides a specific molecular mass value, but it may also establish the molecular formula of an unknown compound.
Tables of precise mass values for any molecule or ion are available in libraries; however, the mass calculator provided below serves the same purpose. Since a given nominal mass may correspond to several molecular formulas, lists of such possibilities are especially useful when evaluating the spectrum of an unknown compound. Composition tables are available for this purpose, and a particularly useful program for calculating all possible combinations of H, C, N & O that give a specific nominal mass has been written by Jef Rozenski. To use this calculator Click Here. 

The mass calculator on the right may be used to calculate the exact mass of a molecule based on its elemental composition. Simply enter an appropriate subscript number to the right of each symbol, leaving those elements not present blank, and press the "Calculate" button. Only the mass of the most abundant isotope, relative to C (12.0000), is used for these calculations. For compounds of chlorine and bromine, increments of 1.997 and 1.998 respectively must be added for each halogen to arrive at the higher mass isotope values.