Optimization of instrumentation precision and wavelength resolution for the performance of NIR calibrations of sucrose in a water-cellulose matrix

Instrumentation for near-infrared (NIR) spectroscopic applications should b e optimized for the intended application. The influence of signal precision and wavelength resolution was considered for the application of the noninv asive assessment [NIR 700-1050 nm, partial least-squares (PLS) calibration] of the sugar content of fruit, with the use of a model system of sucrose s olution on cellulose. The precision (as estimated at the maximum count of t he spectrum) of an MMSI Zeiss spectrometer (Carl Zeiss Pty. Ltd.) was varie d by altering the number of scans averaged per spectrum, as well as the sig nal level. Wavelength resolution was varied between 8 and 20 nm (as estimat ed of the 912 nm Ar peak) by adjustment of the entrance slit of a prototype spectrometer employing a Hamamatsu S4874Q photodiode array as the detector , constructed on an optical bench. PLS calibrations were developed from int eractance spectra of 0-20% w/v sucrose solution soaked filter papers, and c ompared on the basis of standard error of cross-validation (SECV) and coeff icient of correlation-validation (R-v). The optimum measurement precision f or calibration development was lower than expected at a coefficient of vari ation (CV) of 0.02 [signal-to-noise ratio (SNR) 5000:1]. Calibration perfor mance was poorer at a resolution of <8 nm full width at half-maximum (FWHM) in the NIR region, but not significantly different at resolutions of betwe en 8 and 20 nm. Further work is required to define the upper threshold of w avelength resolution. We conclude that instrumentation for the application of fruit sorting can have a relatively poor resolution, and can afford to o perate at signal-to-noise levels considered low for a photodiode array dete ctor.