Measurement of water in a multiple alcohol and brine solution with a ClearView db photometer.
A fiber optic diode array NIR spectrometer and 2 low-OH optical fibers were used to obtain spectra in a 5mm cuvette at 30°C of the samples shown in Table 1. Near-infrared (NIR) spectra are rich in chemical information. These spectra are complex with overlapping peaks from water and alcohols, as shown in Figure 2.
These results indicate that a ClearView db fiber optic filter photometer with an analyzing wavelength of 1820 nm and a reference wavelength near 1050 nm can analyze water in the presence of alcohols and salt.
A careful analysis of the spectra reveals that a single wavelength at 1820 nm measures water with minimal interference from ethanol and isopropanol (see Figure 2). This wavelength is sensitive to water be- cause it lies between the O-H peak from water at 1440 nm and the intense H-O-H peak near 1930 nm. It is, however, at a longer wavelength than the nearby alcohol peaks. It is also much less intense than the main water peak at 1440 nm, thereby allowing a probe to be used with a longer optical path, such as 5 mm, for better sensitivity. Lastly, an added benefit in using this longer wavelength for water is its low sensitivity to sample temperature changes. Variation of ±5°C changes the predicted water content by less than 0.7% (without correcting for temperature). ClearView db can measure the sample temperature input from an RTD and make this correction, if needed.