Determination of carboxyl content in high-yield kraft pulps using photoacoustic rapid-scan Fourier transform infrared spectroscopy

Pinus radiata kraft pulps with varying carboxyl content were studied using Fourier transform infrared photoacoustic spectroscopy (FT-IR-PAS). The examined pulp samples, with Kappa number ranging from 20.8 to 128, originated from pulping experiments conducted in flow-through reactors utilizing varying effective alkali, temperature, and cooking time. A partial least-squares (PLS) analysis was used to formulate a model that correlates the spectral data with the carboxyl content of pulp. Using three principal components, the resultant PLS model could explain similar to 98.5% of the variance in the X-matrix (spectral features) and 96.8% of the variance in the Y-matrix (measured carboxyl content). The FT-IR-PAS technique in combination with PLS analysis predicts the carboxyl content of the pulps with a high degree of accuracy. This method is much faster than the conventional titration methods and also not destructive to the pulp sample.