Thermal degradation of spruce needles studied by time-resolved mass spectrometry and multivariate data analysis

Abstract

In a study related to the impact of air pollution on forests, needles from a healthy and a severely damaged Norway spruce tree were analysed by temperature-programmed pyrolysis/field ionization (FI) mass spectrometry. Dried and pulverized spruce needles were heated at a rate of 0.6°C s −1 to 450°C in the high vacuum of a FI ion source. Over 100 mass spectra were recorded electrically during each analysis. From each mass spectrum, average molecular weights of the pyrolysis products were calculated; their variation with pyrolysis temperature is discussed. The mass spectra in the range m/z 100–600 are used to calculate partial weight-loss curves. The FI mass spectra are evaluated by principal component analysis and factor rotation. The three-factor spectra based on loadings of the rotated principal components show typical FI signals which are produced during pyrolysis at low, medium and high temperatures. These signal patterns are interpreted as molecular ions of thermally stable, relatively volatile plant constituents and molecular ions of thermal degradation products derived from the thermolysis of carbohydrates, lignin and other biopolymers which occur in conifer needles. Medium- and high-temperature products of lignin can be distinguished. Principal component scores can be used to simulate the appearance of single FI signals, i.e., pyrolysis products. The evaluation of time-resolved pyrolysis and soft ionization mass spectrometric data from a single sample by principal component analysis and factor rotation appears to be suitable for characterization of the major chemical components and their thermal behaviour in complex biological samples.