Method for building a portable near infrared photometer based on LEDs and interference filters chosen by a spectral variable selection algorithm

Abstract

A new procedure is described for the development of photometers that operate in the NIR spectral region. The instrument was based on a set of LEDs as radiation source and associate interference filters (IF) to narrow the selected wavelength. The procedure involves obtaining the spectra using a FT-NIR spectrophotometer from a sample set that has spectral variations of a given analyte concentration. Based on the combined LED/IF spectral bandwidths, the high resolution FT-NIR spectra were processed to obtain a deresolved data set with lower resolution. Multiple Linear Regression combined with a variable selection method were used to select the most significant variables, among those present in the deresolved spectral data, to quantify a desired analyte. The optimized calibration results employing these selected variables guided the choice of how many LEDs and which IFs should be used in the photometer. In order to demonstrate the proposed procedure feasibility, a photometer was built to quantify biodiesel content in diesel. Seven pairs of LED/IF were selected for the photometer, which were chosen according to the selected variables by the proposed procedure. In addition, lenses were used to conduct the LED/IF pulsed radiation to a single photodiode detector. The pulsed LED protocol allows the use of Sequential and Hadamard matrix-based multiplexing methods for spectral data acquisition. The results obtained for biodiesel content ranging from 0.50% to 20.00% blended with diesel and vegetable oil presented a RMSEP of 0.53% v/v, LOD range of 0.23%–0.55% v/v, and R2 of 0.99. These results show that the selected variables are not affected by the vegetable oil, a compound with very similar chemical (and spectral) characteristics to the fatty acid esters (biodiesel). The performance reached with the photometer was in accordance with that recommended by the National Agency of Petroleum, Natural Gas and Biofuels (ANP), which regulates and monitors fuel quality in Brazil.