Comparison Between Liquid and Paste Karl Fischer Extractions for NIRS Water Calibrations

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Abstract
The acceptance of near infrared reflectance spectroscopy (NIRS) for the routine analysis of water in forage was made possible by the application of Karl Fischer (KF) water analysis as the primary reference method during calibration. Water is extracted by ball milling the sample in the presence of methanol (1:50), and an aliquot of the liquid phase is sampled and titrated. An alternative extraction utilizes a 20 g sample with 20 mL methanol, and an aliquot of the paste is sampled and titrated. The objectives of this study were to compare extraction procedures (i.e., liquid vs paste) on the determination of water in 75 milled grains and concentrate rations, and to develop and compare NIRS calibrations for water on the basis of both extraction methods. The majority of the samples (N = 61) had a 1.05 percentage unit greater water content (P < 0.05) from the paste method than from the liquid method. Bias between the extraction procedures was positively correlated (r2 = 0.84) to starch content. The lowered extraction efficiency of the liquid method is explained by its inability to account for water bound to starch. NIRS calibration and equation validation statistics were superior with water data from the paste method. The increased calibration errors with the liquid method also appear to reflect the inability of the method to accurately measure the total water concentration of the samples in relation to the water measurement based on the NIRS signals. The study concludes that the KF paste method is a superior primary reference method for the determination of water in flour obtained from cereal grains and concentrate rations.

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