A commercial Fourier transform infrared spectrometer (modified to improve the purge) and a long-path gas cell were used to demonstrate a detection limit of about 10 ppb for water in N-2, HCl, and HBr. This technology is being developed as an in-line monitor to extend the life of gas delivery systems and improve wafer yields in the semiconductor industry. The reported detection limit includes an improvement of about a factor of three achieved by applying quantitative multivariate calibration to the problem. Methods are discussed to compensate for background moisture in the beam path. Also, discussed are the choice of operating parameters to optimize the instrumental performance. Surprisingly, resolving the narrow water absorption bands (FWHM approximate to 0.20 cm(-1)) is not necessary to achieve optimal sensitivity In fact optimal sensitivity is achieved at 2 to 4 cm(-1) resolution, allowing the use of an inexpensive interferometer in the final instrument. A calibration over the range of 40 to 2000 ppb of water in N, has been performed and found to be in good agreement with published spectral data. Finally, an improved instrument design is discussed which is projected to have a lower detection limit and a shorter collection time.