There has been interest in the quantitative infrared spectroscopy of aqueous solutions for many years, but these studies have not been numerous, because even qualitative applications of this type have been regarded as difficult. This has been due to the poor sensitivity of dispersive infrared instruments, combined with the high-infrared absorptivity of water. Past applications have been largely limited to concentrated solutions in thinpathlength transmission cells, but some attenuated total reflectance [ATR] work has been reported. However, the greater sensitivity of Fourier transform instruments, combined with the use of high-throughput ATR liquid cells, enables quantitative measurements to be made in aqueous solutions at and below the 1 % level. A typical ATR cell, giving about 10 reflections, provides a reproducible, effective pathlength of about 12 microns in the mid-infrared when used with aqueous solutions. The crystal may be constructed from Germanium or ZnSe, and the accessory may be designed as a flowcell for continuous monitoring purposes. FT-IR liquid cell ATR has been used in conjunction with a number of studies of biological systems, and has recently been employed in the quantitative analysis of ethanol in gasohol. In this paper, the analysis of a model fermentation broth by liquid cell ATR is described.
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