VAPOR-PRESSURE OF AGROCHEMICALS BY THE KNUDSEN EFFUSION METHOD USING A QUARTZ-CRYSTAL MICROBALANCE

Vapor pressure determination of agrochemicals is required for EPA and EC registration. The Knudsen effusion method is an approved regulatory method of performing vapor pressure measurements. The Knudsen effusio n method is a dynamic technique based on the rate of escape of vapor m olecules through an orifice into a vacuum. We have modified the Knudse n effusion apparatus, replacing a conventional microbalance with a qua rtz crystal microbalance (QCM). The QCM has a sensitivity of less than 1 ng; therefore, small mass rate losses can be observed, reducing exp erimental time. The use of the QCM allows rapid determination of vapor pressures as low as 10 mu Pa. The vapor pressure of several agrochemi cals has been measured using this modified method, and the results agr ee with previously reported values. The results and methodology are di scussed.