A Photodiode-Array-Based Near-Infrared Spectrophotometer for the 600–1100 nm Wavelength Region

Abstract

We describe a silicon photodiode-array (PDA)-based near-infrared spectrophotometer for making molecular absorption/diffuse reflectance measurements in the 600–1100 nm wavelength range. Absorptions in this spectral region arise from both low-lying electronic states and vibrational overtones of CH, NH, and OH functional groups and combination bands. One disadvantage of silicon-based array detectors is their decreasing quantum efficiency at wavelengths longer than 950 nm. For transmission measurements, this can be compensated for by the technique of spectral plane masking, which markedly improves the consistency and overall level of baseline noise as well as the dynamic range. The instrument's performance is evaluated in the areas of spectral resolution, baseline noise, stray light, and dynamic range, and a comparison is made with a state-of-the-art mechanically scanned instrument. In the study, the PDA spectrometer attained a signal-to-noise ratio two times better than that of the commercial instrument with a time efficiency advantage of twelve, while achieving a spectral resolution three times greater. Finally, the application of analyzing caustic brine solutions by rapid-scanning NIR spectroscopy is illustrated.