Standardization of Fiber-Optic Probes for Near-Infrared Multivariate Calibrations

The standardization of Fourier transform near-infrared (FT-NIR) spectrometers equipped with fiber-optic probes was investigated. FT-NIR spectra of caustic brines for an industrial process were measured on two different instruments. Calibration transfer across the instruments and probes was studied by employing calibration models built on one instrument to predict properties from spectra measured on the other. The transfer was examined by using spectra without and with preprocessing. The preprocessing methods included a Savitzky–Golay (SG) derivative polynomial filter, a procedure based on a finite impulse response (FIR) filter, and a combination of both. In addition to being a preprocessing technique, the FIR filter is also a standardization method that transforms the instrument response function of one instrument to match that of another. The transformation is performed over a moving processing window without the use of transfer standards. In this study, application of the FIR filter to first-derivative spectra provided the best multivariate calibration models and led to the successful transfer of calibration across different probes and spectrometers.