Measurement of Water Vapor Pressure and Activity Using Infrared Diode Laser Absorption Spectroscopy

Abstract

A mid-infrared laser technique is described for the accurate determination of water vapor concentration in the headspace above a liquid sample. The method is based on the measurement of the fractional absorption, at line center, of a single ro-vibrational line. A dual-beam optical arrangement, incorporating a reference vapor cell, is used to provide an active calibration for the measurement. It is shown how the absorption signals can be related to the water activity of a liquid sample. The technique has the advantages of being noninvasive, rapid, and signal-specific to water. It does, however, intrinsically have a nonlinear response to activity. This drawback is mainly a consequence of pressure broadening, and has limited the present measurements to sample temperatures in the range 268 to 293 K. In this regime the method gives a precision of ±1% for activities near unity. This limit is imposed by small fluctuations in certain environmental variables. Suitable modifications should enable operation at the ±0.1% level of accuracy over an extended temperature range.