A hand-held near-infrared (NIR) meter (called the Gmeter), based upon gap-second derivative (GSD) theory, was designed, constructed, and performance-tested. The design incorporated narrow-band interference filters for isolating the three wavelengths required by the GSD calculations. A microprocessor was included in the design to facilitate both stand-alone and personal computer (PC) operation. The Gmeter was mounted in a caddy for making measurements within the laboratory. Performance of the Gmeter was compared with the performance of a FOSS NIRSystems Model 6500 spectrophotometer for measuring protein in soy-protein/sugar mixtures and for measuring nitrogen in fescue grass tissue. Two calibrations were developed on both instruments: (1) single-term GSD equations and (2) three-term (log 1/<i>R</i>) multiple linear regression (MLR) equations. Second-derivative calibration experiments on the Model 6500 spectrophotometer formulated the basis for selecting the three filters in the Gmeter. Model 6500 data indicated that the GSD calibration [coefficient of variation (CV) = 5.14%] performed better than a three-term MLR equation (CV = 8.0%). In addition, the Gmeter performed almost as well (CV = 6.30%) as the Model 6500 (CV = 5.14%) for measuring protein in the mixtures using single-term GSD equations. An exciting extra in this study was the fact that measurements from the same three filters selected for determining protein in protein/sugar mixtures could be used for determining nitrogen (CV = 17.2%) in dry-grass tissue.
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