The purpose of the work was to determine the absorption isotherms of 58 paper samples and to find out how the water content affects the NIR spectra. Results of mass measurements on the MX/UMX microbalance of fifty-eight samples at different relative humidities: 0%, 20%, 40%, 60%, 80% showed, as expected, that it increases with humidity. Absorption isotherms of less absorptive and more absorptive material differed according to water content, as expected. The results of measurements in the near infrared range with a Labspec spectrometer showed that the absorbance of the O-H hydrogen bond peak is not directly or inversely proportional to relative humidity. The results are not in line with expectations, because the structure of the molecules in the sample is determined from the position of the peaks, while the concentration of individual molecules in the sample is determined from the height of the peaks. We would expect the peak heights to increase with humidity in some kind of interdependence. It is worth emphasizing that the peaks of water around 1900 nm represent the vibration of the OH group, which is present in cellulose and proteins, which are the building blocks of paper.
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