The first absolute measurements of molecular weights (i.e. made without reference to standards) were based on fundamental physical characteristics and their relation to the molar mass.
Another absolute instrumental approach was also possible with the development of light scattering theory by Albert Einstein, Chandrasekhara Venkata Raman, Peter Debye, Bruno H. Zimm, and others.
In order to gain information about a polydisperse mixture of molar masses, a method for separating the different sizes was developed.
SEC is based on the fact that the pores in the packing material of chromatography columns could be made small enough for molecules to become temporarily lodged in their interstitial spaces.
As the sample makes its way through a column the smaller molecules spend more time traveling in these void spaces than the larger ones, which have fewer places to "wander".
The problem was that the system was calibrated according to the Vh characteristics of polymer standards that are not directly related to the molar mass.
Thus, to be accurate, the calibration must use the same polymer, of the same conformation, in the same eluent and have the same interaction with the solvent as the hydration layer changes Vh.
This was especially important in pharmaceutical applications where slight changes in molar mass (e.g. aggregation) or shape may result in different biological activity.
A low angle light scattering system was developed in the early 1970s that allowed a single measurement to be used to calculate the molar mass.
When the low-angle laser light scattering (LALLS) became popular in the 1970s and mid-1980s, good quality disposable filters were not readily available and hence multi-angle measurements gained favour.
Multi-angle light scattering was invented in the mid-1980s and instruments like that were able to make measurements at the different angles simultaneously but it was not until the later 1980s (10-12)[clarify] that the connection of multi-angle laser light scattering (MALS) detectors to SEC systems was a practical proposition enabling both molar mass and size to be determined from each slice of the polymer fraction.
The addition of an SLS detector coupled downstream to a chromatographic system allows the utility of SEC or similar separation combined with the advantage of an absolute detection method.
LALS does not need any model to fit the angular dependence and hence is giving more reliable molecular weights measurements for large molecules.