Abstract

Tunable diode laser absorption measurements of pressure and ${\mathrm{H}}_2\mathrm{O}$ concentration in the headspace of vials using a distributed-feedback (DFB) diode laser near 1.4 μm are reported. A ${\mathrm{H}}_2\mathrm{O}$ line located near $7161.41{\mathrm{cm}}^{-1}$ is selected based on its strong absorption strength and isolation from interference of neighboring transitions. Direct absorption spectra of ${\mathrm{H}}_2\mathrm{O}$ are obtained for the measurement path as well as the reference path by scanning the laser wavelength. The pressure and ${\mathrm{H}}_2\mathrm{O}$ vapor concentration in the headspace of a vial are inferred from a differential absorption signal, which is the difference between the measured and the referenced absorbance spectra. This sensor is calibration-free and no purge gas is needed. The demonstrated capability would enable measurements of pressure and ${\mathrm{H}}_2\mathrm{O}$ concentration in the headspace of vials within 2.21% and 2.86%, respectively. A precision of 1.02 Torr and 390 ppm is found for the pressure and ${\mathrm{H}}_2\mathrm{O}$ concentration, respectively. A set of measurements for commercial freeze-dried products are also performed to illustrate the usefulness of this sensor.

© 2013 Optical Society of America

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