Evaluation of a Dual-Beam Near-Infrared Spectrometer Based on Acousto-Optic Tunable Filters

A dual-beam near-infrared (NIR) spectrometer was constructed and evaluated. The instrument was designed to work with the two monochromatic beams produced by each one of the two acousto-optic tunable filters (AOTFs) evaluated, capable of scanning the wavelength range 800–1600 or 1500–2400 nm. Two temperature-controlled, cooled InAs detectors were employed for comparison of the two monochromatic beams produced by the AOTF. Data were obtained in a controlled ambient temperature, (25 ± 1) °C, and were compared for their intensity and wavelength coincidence for each of the 300 values of radio frequency (85–160 and 56–92 MHz) applied to the AOTF. The results show that the two beams present good concordance as a function of the wavelength (mean Δλ <0.30 nm) for both AOTFs investigated. The normalized intensities of the two beams were quite different, and their ratio values (0.9 to 1.2) depend on the wavelength. Inversion of intensities was observed in the present case and can be attributed to the unmatched position of the two detectors and to the difference in responsivity. On the other hand, the use of the two beams revealed that it can provide a robust way to correct for light-source intensity fluctuation as high as 10%. The mean standard deviation for long-term operation and for a zero absorbance baseline was reduced three times when the dual-beam instrument was employed. The instrument has also been evaluated for determination of water in fuel ethanol. The results for water content demonstrate the good analytical performance of the spectrometer with a relative error of 0.07 and 0.03% (w/w) for water determination in the ranges 0.20–10.0 and 0.20–3.00%, respectively.