Modern high-resolution mass spectrometers are capable of determining very precise molecular weights of substances. Whereas the chemical methods introduced earlier (vapor density method, cryoscopic method, vapor pressure osmometry, and neutralization equivalent) are accurate to within 0.1 to 1% (two or three significant figures), the mass spectrometer has an accuracy of 0.005%.
What does this mean? To illustrate, a molecule with a molecular weight of 60 could be C3H8O, C2H8N2, C2H4O2, or CH4N2O. These compounds would have precise masses as follows:
| C3H8O | 60.05754 |
| C2H8N2 | 60.06884 |
| C2H4O2 | 60.02112 |
| CH4N2O | 60.03242 |
These precise masses are calculated using the precise masses of the elements given in the table below.
| Element | Atomic Weight | Nuclide | Mass |
| Hydrogen | 1.00797 | 1H | 1.00783 |
| 2H | 2.01410 | ||
| Carbon | 12.01115 | 12C | 12.0000 |
| 13C | 13.00336 | ||
| Nitrogen | 14.0067 | 14N | 14.0031 |
| 15N | 15.0001 | ||
| Oxygen | 15.9994 | 16O | 15.9949 |
| 17O | 16.9991 | ||
| 18O | 17.9992 | ||
| Fluorine | 18.9984 | 19F | 18.9984 |
| Silicone | 28.086 | 28Si | 27.9769 |
| 29Si | 28.9765 | ||
| 30Si | 29.9738 | ||
| Phosphorus | 30.974 | 31P | 30.9738 |
| Sulfur | 32.064 | 32S | 31.9721 |
| 33S | 32.9715 | ||
| 34S | 33.9679 | ||
| Chlorine | 35.453 | 35Cl | 34.9689 |
| 37Cl | 36.9659 | ||
| Bromine | 79.909 | 79Br | 78.9183 |
| 81Br | 80.9163 | ||
| Iodine | 126.904 | 127I | 126.9045 |
Return to: Mass Spectrometry Page