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Vapor pressure meassurements by Headspace-gaschromatograph


With this method the vapor pressure of polymer solutions or of colloid suspensions can be measured. The measuring instrument is made up of a headspace sampler and a gas chromatograph:

Set-up Headspace-Gaschromatograph

Set-up of the headspace autosampler - gas chromatograph


The samples are placed in glass vials and closed with a septum. The injection needle of the headspace sampler injects through the septum in the vapor space of the sample. Via nitrogen an overpressure is produced in the vial, which is relieved afterwards against air-pressure into the sample loop. The filled sample loop is injected via an injector into the gas chromatograph. The substances covered in the vapor space are detected via flame ionization detector or via heat conductivity detector, amplified electronically and registered by means of a printer. The determined peak areas are proportional to the amount of the detected substances and therefore at constant sample loop volume is proportional to the vapor pressure of the solvents. Generally the ratio of the vapor pressure over the sample to the vapor pressure of the pure solvents can be determined as reduced vapor pressure. A typical curve of the reduced vapor pressure as a function of the concentration of the non-volatile component is shown in the following figure.


Reduced vapor pressure

Reduced vapor pressure as a function of composition


Microscope
 
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