Molecular Mass - Measurement

Measurement

The molecular mass can also be measured directly using mass spectrometry. In mass spectrometry, the molecular mass of a small molecule is usually reported as the monoisotopic mass, that is, the mass of the molecule containing only the most common isotope of each element. Note that this also differs subtly from the molecular mass in that the choice of isotopes is defined and thus is a single specific molecular mass of the many possible. The masses used to compute the monoisotopic molecular mass are found on a table of isotopic masses and are not found on a typical periodic table. The average molecular mass is often used for larger molecules since molecules with many atoms are unlikely to be composed exclusively of the most abundant isotope of each element. A theoretical average molecular mass can be calculated using the standard atomic weights found on a typical periodic table, since there is likely to be a statistical distribution of atoms representing the isotopes throughout the molecule. This however may differ from the true average molecular mass of the sample due to natural (or artificial) variations in the isotopic distributions.

The basis for determination of molecular weight according to the Staudinger method (since replaced by the more general Mark-Houwink equation) is the fact that relative viscosity of suspensions depends on volumetric proportion of solid particles.

There are several companies that manufacture accurate instruments for determining absolute molecular weight quickly and easily. These companies include Brookhaven Instruments, Malvern Instruments, and Horiba