Vapor phase osmometry calibration

The objective of this present work was to investigate the feasibility of using GPC/DV for absolute molecular weight determination of hydroxypropylated lignins. In order to verify the validity of the universal calibration method, vapor phase osmometry (VPO) was used to provide reference number average molecular weight values. Comparisons with LALLS results have also been made and will be reported in another publication. 

Vapor Phase Osmometry. A Wescan Model 233 vapor phase osmometer was used to obtain number average molecular weights. The lignin solutions were made up with HPLC grade tetrahydrofuran (THF) and shaken manually until the solutions were clear. The experiments were conducted at 30°C. Number average molecular weights were determined by multistandard calibration (41), a procedure found to greatly enhance reproducibility and accuracy of the results. Experiments were conducted immediately after sample preparation and three days later. 

Fig. 8.1.5. Calibration of styragel columns with polystyrene standards ( ) and with lignin samples (o). The molecular weights of the latter were determined by vapor phase osmometry. (Mansson 1981)...

A special group of detectors are those which continuously measure the molar mass of sample components in the eluate. Their application allows to omit the calibration of the gel chromatographic system. So far, vapor phase osmometry for solutes with lower molar masses, and viscosity and light scattering measurements for polymers have been applied. 

Since ks cannot be derived theoretically, Ke is usually determined by calibration with substances of known molar mass. As with all the other molar-mass-determination methods, only apparent molar masses Afapp are obtained for finite concentrations (see Sections 6.4 and 6.5) because of the effects of the virial coefficients and/or association, A/app must therefore be extrapolated to the concentration a 0. In vapor phase osmometry, low amounts of nonvolatile impurities interfere with the result, but volatile impurities do not, since they pass into the vapor phase. 

The number average molecular weights of the oligomers, obtained by reacting DOMS-SA mixtures at different times, were calculated by means of vapor phase osmometry in chloroform at 37°C with a Knauer osmometer. Monodispersed polystyrene standards were used to generate the calibration curve. 

Vapor phase osmometry R Mn <3 X 10 Suitable standards for accurate calibration required... 

Vapor-pressure osmometiy is, from its name, compared with membrane osmometry by considering the vapor phase to act like the semipermeable membrane, however, from its principles it is based on vapor pressure lowering or boiling temperature elevation. Since the direct measure of vapor pressure lowering of dilute polymer solutions is impractical because of the extreme sensitivity that is required, VPO is in widespread use for oligomer solutions (M less than 20,000 g/mol) by employing the thermoelectric method as developed by Hill in 1930. In the thermoelectric method, two matched temperature-sensitive thermistors are placed in a chamber that is thermostated to the measuring temperature and where the atmosphere is saturated with solvent vapor. If drops of pure solvent are placed on both thermistors, the thermistors will be at the same temperature (zero point calibration). If a solution drop is placed on one thermistor, a temperature difference AT occurs which is caused by condensation of solvent vapor onto the solution drop. From equihbrixun thermodynamics follows that this temperature increase has its theoretical limit when the vapor pressure of... 

Vapor pressure osmometry [34—36] constitutes a very helpful nonequilibrium method for obtaining thermodynamic information for solutions of oligomers and polymers of low molar mass, for which osmometry and light scattering experiments do no longer yield reUable data. Such experiments are based on the establishment of stationary states for the transport of solvent via the gas phase from a drop of pure solvent fixed on one thermistor to the drop of oligomer solution positioned on another thermistor. Because of the heats of vaporization and of condensation, respectively, this transport process causes a time-independent temperature difference from which the required information is available after calibrating the equipment. 

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