Dual-Beam Near-Infrared Hadamard Spectrophotometer

Abstract

A dual-beam Hadamard multiplexed spectrophotometer is described. The instrument is intended to work in the near-infrared region of the electromagnetic spectrum (900–1800 nm) and is based on the use of a linear Hadamard mask containing 255 multiplexing elements. Simple symmetric Czerny–Turner optics were employed based on 10 cm diameter, 20 cm focus spherical mirrors and a plane grating containing 295 grooves mm−1. The dual-path system employs the multiplexed beam exiting from the mask, which can then be split by using either an integrating sphere, a bifurcated optical bundle, or a beamsplitter. Two cooled PbS or InAs detectors were employed to collect the 255 multiplexed intensities in about 8 s. Signal was obtained by linear displacement of the mask, which was controlled by software written in VisualBasic 4.0. Spectral data were demultiplexed with a program written in the same language. The instrument can successfully correct for the drift of the light source intensity. The wavelength precision is 0.4 nm, while the average standard deviation for absorbance measurements taken from 900 to 1800 nm is 1.5 × 10−3 absorbance units, a value that is about three times lower than that obtained for the single-beam multiplexed approach. The instrument has been applied to the determination of water in fuel ethanol with the use of partial least-squares modeling. The absolute standard error of prediction was 0.07% (w/w).