A mid-IR multivariate analysis study on the gross calorific value in longleaf pine: impact on correlations with lignin and extractive contents

Twenty 70-year-old longleaf pine trees from a spacing, thinning, and pruning study were harvested, from which samples were analyzed for gross calorific value (GCV). A strong correlation was found between GCV and extractive contents for the unextracted wood samples. Although lignin content should impact GCV, no correlation was found between the variation in GCV with lignin content for the extractive-free wood samples. Mid-IR spectroscopy coupled with multivariate analysis provided strong correlations between the mid-IR-predicted and calorimetry-determined values for the unextracted wood samples. Plotting the regression coefficients for GCV and extractive contents showed that the same mid-IR bands were responsible for the strength of these models. Spectral differences were observed between the different extract samples, and relative peak intensities appeared to be dependent upon the extractive contents from the wood samples. Thus, models were also built based on the corresponding wood extractive contents using the actual extracts. These provided good correlations, suggesting a proportional change in extractive compositions coinciding with the total amount of extractives present in the unextracted wood samples.