Molar mass determination resolution

Since no adequate SEC standards were available, linear polystyrene was used as standard. As expected, the determined molar masses were not in agreement with the theoretical molar masses. This could be explained by the differences in hydro-dynamic volume between linear polystyrene standards and the dendritic polyesters. SEC analyses showed polydispersity values (A/ /M ) below 1.02 for dendrimers Dl, D2, and D3, which was the maximum resolution of the column (Table 2, Figure 4). 

The classical approach is based on the dependence of copolymer solubility on composition and chain length. A solvent/nonsolvent combination fractionating solely by molar mass would be appropriate for the evaluation of MMD, another one separating with respect to chemical composition would be suited for determining CCD or FTD. However, in reality, precipitation fractionation yields fractions which vary both in chemical composition and molar mass. Even high resolution fractionation would not improve the result. Narrower fractions can be obtained by cross-fractionation separating in two different directions. However, even in this case, it is almost impossible to obtain perfectly homogeneous fractions. 

SEC coupled with FTIR becomes an inevitable tool when blends comprising copolymers have to be analyzed. Very frequently components of similar molar masses are used in polymer blends. In these cases resolution obtained by SEC is not sufficient to resolve all component peaks (see Fig. 31 for a model binary blend containing an additive). The elution peaks of the polymer components 1 and 2 overlap and, thus, the molar masses cannot be determined directly. Only the additive peak 3 at the low molar mass end of the chromatogram is well separated and can be quantified. 

Our discussion thus far has assumed that a mass spectrometer can distinguish between ions that differ in mass by one amu, where the m/z values are measured to the nearest whole number. In the jargon of MS, these are called low-resolution mass spectrometers. Much more expensive instruments, called high-resolution mass spectrometers, can measure the m/z ratios to 0.001 or 0.0001 amu and provide very accurate measurements of molar masses. These data may then be used to determine elemental composition, which is extremely valuable information when you are trying to assign a structure to an unknown compound. 

The research chemist usually obtains IR and NMR spectra of an unknown substance even before obtaining an elemental analysis or performing solubility tests because these types of spectra are easily and quickly measured on a small amount of sample. Only after analyzing them does the researcher undertake other experimental approaches for determining the structure. This process can save many hours of unnecessary laboratory work. Other useful information about a compound may be obtained from mass spectrometry (Sec. 8.5), which provides the molar mass of the compound and, if high-resolution data are available, its elemental composition. 

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