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We are developing a spacecraft water quality monitoring system based on measuring the change in diffuse reflectance of indicator disks following exposure to a water sample. The most widely used approach for deriving quantitative information from a diffuse reflectance spectrum involves an initial Kubelka-Munk transformation of the reflectance data. Provided the assumptions underlying the transformation are satisfied, the magnitude of the resulting function varies linearly with analyte concentration. We measure diffuse reflectance spectra using a commercial-off-the-shelf handheld spectrophotometer designed for color matching applications. Spectra are currently downloaded and transformed to obtain the value of the Kubelka-Munk function at a detection wavelength diagnostic of the analyte. This value is then used to quantify the analyte via a standard response curve.