The standard atomic weight takes into account the isotopic distribution of the element in a given sample (usually assumed to be "normal").
A theoretical average molecular mass can be calculated using the standard atomic weights found in a typical periodic table.
[4] Contrast from scattered light by a single binding event at the interface between the protein solution and glass slide is detected and is linearly proportional to the mass of the molecule.
To a first approximation, the basis for determination of molecular mass according to Mark–Houwink relations[7] is the fact that the intrinsic viscosity of solutions (or suspensions) of macromolecules depends on volumetric proportion of the dispersed particles in a particular solvent.
Specifically, the hydrodynamic size as related to molecular mass depends on a conversion factor, describing the shape of a particular molecule.
This allows the apparent molecular mass to be described from a range of techniques sensitive to hydrodynamic effects, including DLS, SEC (also known as GPC when the eluent is an organic solvent), viscometry, and diffusion ordered nuclear magnetic resonance spectroscopy (DOSY).
[8] The apparent hydrodynamic size can then be used to approximate molecular mass using a series of macromolecule-specific standards.
[9] As this requires calibration, it's frequently described as a "relative" molecular mass determination method.
It is also possible to determine absolute molecular mass directly from light scattering, traditionally using the Zimm method.