Effect of Water and Physical State on Near- and Mid-Infrared Partial Least Squares Calibrations for Multi-Component Carbohydrate Mixtures

The objective was to determine the effects of water and physical state on near- and mid-infrared calibrations for carbohydrate mixtures. Seventy-five mixtures of cellulose, starch, pectin, locust gum, glucose and xylitol were scanned in the near- and mid-infrared in three physical states (wet, simple mixtures of dry constituents and mixtures dried together). These final samples (mixtures dried together) were prepared by drying wet mixtures/solutions while under constant stirring to prevent separation of the insoluble polymers by settling, which occurs during normal oven drying. Final drying of the resulting solid mass was carried out in a vacuum oven at 60°C. The dried mass was then ground and scanned in the spectrometer. All dry samples were ground to a flour-like consistency using a A-10 Tekmar Analytical Mill to avoid effects due to gross differences in particle size. The wet samples were produced by adding sufficient water to result in samples of a putty like consistency. Spectra were taken on a Digilab FTS-65 Fourier transform spectrometer equipped with near- and mid-infrared benches by diffuse reflectance. Results demonstrated that the effects of water and physical state (matrix effects), seen previously with simple model systems, can be extrapolated to draw some general conclusions about real world calibrations, but that developing model systems to either predict the feasibility of developing a calibration or to determine the reason for the specific results of a calibration must be done with extreme care. For example, using simple mixtures of components to determine whether a calibration for the same components in a different physical condition (melted together, dried together etc.) might well lead to erroneous conclusions about its usefulness. Also, from the results, it would appear that factors (e.g. physical state) other than the mere presence of large amounts on water in samples can account for the decrease in calibration accuracy often seen in samples with high moisture contents as compared to their dried counterparts. Finally, the results seen using mid-infrared spectra indicate that mid-infrared diffuse reflectance spectroscopy might be feasible even with samples containing high percentages of water.