Dual-beam near-infrared Hadamard spectrophotometer

A dual-beam Hadamard multiplexed spectrophotometer is described. The instru ment 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 c ontaining 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 t he multiplexed beam exiting from the mask, which can then be split by using either an integrating sphere, a bifurcated optical bundle, or a beamsplitt er, Two cooled PbS or InAs detectors were employed to collect the 255 multi plexed 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 languag e. The instrument can successfully correct for the drift of the light sourc e 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 x 1 0(-3) absorbance units, a value that is about three times lower than that o btained for the single-beam multiplexed approach. The instrument has been a pplied to the determination of water in fuel ethanol with the use of partia l least-squares modeling. The absolute standard error of prediction was 0.0 7% (w/w).