Vapor phase osmometry (VPO), also known as vapor-pressure osmometry, is an experimental technique for the determination of a polymer's number average molecular weight, Mn. It works by taking advantage of the decrease in vapor pressure that occurs when solutes are added to pure solvent. This technique can be used for polymers with a molecular weight of up to 20,000 though accuracy is best for those below 10,000. Although membrane osmometry is also based on the measurement of colligative properties, it has a lower bound of 25,000 for sample molecular weight that can be measured owing to problems with membrane permeation.
A typical vapor phase osmometer consists of: (1) two thermistors, one with a polymer-solvent solution droplet adhered to it and another with a pure solvent droplet adhered to it; (2) a thermostatted chamber with an interior saturated with solvent vapor; (3) a liquid solvent vessel in the chamber; and (4) an electric circuit to measure the bridge output imbalance difference between the two thermistors. The voltage difference is an accurate way of measuring the temperature difference between the two thermistors, which is a consequence of solvent vapor condensing on the solution droplet (the solution droplet has a lower vapor pressure than the solvent).
The number average molecular weight for a polymer sample is given by the following equation:
is a calibration constant,
is the bridge imbalance output voltage,
is the polymer-solvent solution concentration
It is necessary to calibrate a vapor phase osmometer and it is important to note that K is found for a particular solvent, operational temperature, and type of commercial apparatus. A calibration can be carried out using a standard of known molecular weight. Some possible solvents for VPO include toluene, tetrahydrofuran, or chloroform. Once the experiment is performed, concentration and output voltage data can be graphed on a plot of (?V/c) versus c. The plot can be extrapolated to the y-axis in order to obtain the limit of (?V/c) as c approaches zero. The equation above can then be used to calculate K.
Vapor phase osmometry is well suited for the analysis of oligomers and short polymers while higher polymers can be analyzed using other techniques such as membrane osmometry and light scattering. As of 2008, VPO faces competition from matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS), but VPO still has some advantages when fragmentation of samples for mass spectrometry may be problematic.