Absolute molar mass

Absolute molar mass is a process used to determine the characteristics of molecules. Absolute molar mass is a process used to determine the characteristics of molecules. 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. The most useful of these were membrane osmometry and sedimentation. Another absolute instrumental approach was also possible with the development of light scattering theory by Einstein, Raman, Debye, Zimm, and others. The problem with measurements made using membrane osmometry and sedimentation was that they only characterized the bulk properties of the polymer sample. Moreover, the measurements were excessively time consuming and prone to operator error. In order to gain information about a polydisperse mixture of molar masses, a method for separating the different sizes was developed. This was achieved by the advent of size exclusion chromatography (SEC). 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 result is that a sample is separated according to its hydrodynamic volume V h {displaystyle V_{h}} . As a consequence, the big molecules come out first, and then the small ones follow in the elutent. By choosing a suitable column packing material it is possible to define the resolution of the system. Columns can also be combined in series to increase resolution or the range of sizes studied. The next step is to convert the time at which the samples eluted into a measurement of molar mass. This is possible because if the molar mass of a standard were known, the time at which this standard eluted should be equal to a specific molar mass. Using multiple standards, a calibration curve of time versus molar mass can be developed. This is significant for polymer analysis because a single polymer could be shown to have many different components, and the complexity and distribution of which would also affect the physical properties. However this technique has shortcomings. For example, unknown samples are always measured in relation to known standards, and these standards may or may not have similarities to the sample of interest. The measurements made by SEC are then mathematically converted into data similar to that found by the existing techniques.