Near Infrared Spectroscopy for Agricultural Materials: An Instrument Comparison

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Abstract
The selection of a spectrophotometer for the measurement of constituents of agricultural materials with acceptable accuracy and cost effectiveness requires a comparative study of the performance of different spectrophotometers. Four commercially available spectrophotometers were evaluated, based on measurements performed on three agricultural materials. These spectrophotometers, differing mainly in wavelength range and measurement principles, comprised a diode array (DA) of 300–1700 nm, a combination of diode array and scanning monochromator (DASM) of 350–2500 nm, a Fourier transform (FT) of 750–2500 nm and a scanning monochromator (SM) of 400–2500 nm spectrophotometers. They were used to measure the moisture content of soil, the chemical constituents of hog manure and to detect bruising in apples. Three spectral pre-treatments were considered. Calibrations were developed using partial least squares (PLS) regression with the leave-one-out cross-validation technique for soil and manure and principal component analysis (PCA) for apple. The four instruments provided good predictions for soil moisture content, with the largest coefficient of determination ( r2) values between 0.84–0.86 and with the largest ratio of prediction to deviation ( RPD) of standard deviation ( SD) to root mean square error of cross-validation ( RMSECV) ranging from 2.53 to 2.75. The DASM and SM were comparable and slightly better than the DA and FT. For hog manure, total nitrogen was predicted more accurately with the four instruments ( r2 = 0.83–0.89 and RPD = 2.43–3.01) than the phosphorus ( r2 = 0.66–0.85 and RPD = 1.72–2.61) and potassium ( r2 = 0.70–0.84 and RPD = 1.83–2.50). The order of accuracy of the four spectrophotometers for the measurement of total nitrogen was: FT–SM–DA–DASM. For phosphorus and potassium the order was: DA–DASM–FT–SM and SM–FT–DASM–DA, respectively. The DA performed better than the DASM and FT for discrimination of bi-colour and single-colour bruised apple, respectively. Therefore, selection of a spectrophotometer depends mainly on the type of material analysed and the constituent to be measured. Wavelengths above 1700 nm were found unnecessary for the applications considered and the DA spectrophotometer was of sufficient accuracy, as it is robust, significantly cheaper and can be used in the field for on-line measurements.

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