Molecular mass/weight types

The observation of molecular size or polydispersity and the subsequent determination of relative molecular mass, (MJ or molecular mass (weight) distribution (MWD), is the most common analytical application of SEC. The goal of these types of experiments is to either observe the solvated size of one or more molecular species or to observe the distribution of sizes present in a mixture... 

Molecular weightjmass detectors include light scattering detectors and viscosity detectors. When SEC is used in the characterisation of polymer systems, its main aim will be the production of a molecular mass/weight distribution and where possible absolute molecular weights. Mass calibration is a complicated matter (section 9.3.5.1) in that calibration curves differ for different polymer types, and for many commercial polymers, direct molar mass calibration is not possible because of the lack of suitable, known molecular weight standards. 

A polymer, even in its pure state, corresponds to a distribution of macromolecules with different masses. Exclusion chromatography can determine the distribution in molecular weights and the most probable mass and the mean mass. For this type of application, a calibration curve is made using macromolecules of known masses by plotting the retention times (or volumes) on the abscissa and the logarithms of the molecular masses on the ordinate. As can be seen in Fig. 7.5, a linear relationship is obtained. Using this graph, an approximate mass of the unknown can be determined by use of the retention time (volume). This assumes that the mass and the molecular volumes are directly related. 

Up to four cDNA clones have been isolated for potato BE - one for 91 to 99 kDa.253-255 All these allelic clones have sequences similar to those of the BEI type. Also, the sbelc allele codes for a mature enzyme of 830 amino acids with a MW of 95.18 kDa. The sbelc BE protein product, expressed in E. coli, migrates as a 103 kDa protein.253 It is of interest to note that BE isolated from other plants, bacteria and mammals have molecular masses ranging from 75 to —85 kDa. These molecular weights are consistent with the molecular weights obtained from deduced amino acid sequences obtained from isolated genes or cDNA clones. 

The three types of formula masses correspond to the three types of formula units (1) atomic masses (also called atomic weights), (2) molecular masses (also called molecular weights), and (3) formula masses for ionic compounds (also called formula weights). The term atomic mass may be used whether an atom is combined or not, but it always refers to the mass of one atom of an element. 

The formula mass (formula weight) of a substance is determined by adding the atomic masses (atomic weights) of each atom (not each element) in a formnla unit. Molecular mass is one type of formula mass (for substances that form molecnles) and is calculated in the same way as the formula mass for an ionic compound. For example, the formula mass of NH3 is 17.0 amu, the atomic mass of three hydrogen atoms plus that of one nitrogen atom. Three or more significant digits should be used to report formula masses. (Section 7.1)... 

A9.3.5.10.2 Polymers represent a special kind of complex substance, requiring consideration of the polymer type and their dissolution/dispersal behaviour. Polymers may dissolve as such without change, (true solubility related to particle size), be dispersible, or portions consisting of low molecular weight fractions may go into solution. In the latter case, in effect, the testing of a polymer is a test of the ability of low molecular mass material to leach from the bulk polymer, and whether this leachate is toxic. It can thus be considered in the same way as a complex mixture in that a loading of polymer can best characterize the resultant leachate, and hence the toxicity can be related to this loading. 

Likewise, a one normal solution contains 1 g equivalent weight of solute in 1L of solution for example, 1 mol HCl, 0.5 mol H2SO4, and 0.33 mol H3PO4, each in IL of solution, are one normal solutions. The use of normality is limited in that a given solution may have more than one normality, depending on the type of reaction for which the solution is used. The molarity of a solution, however, is a fixed number because there is only one molecular mass for any substance. Normality is no longer recommended to express concentrations. Nevertheless, the term is included here because it remains in common usage and is related to the equivalent concept sometimes favored for serum electrolyte concentrations in the United States and some other countries. 

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