Tools for Bark Biorefineries: Studies toward Improved Characterization of Lipophilic Lignocellulosic Extractives by Combining Supercritical Fluid and Gas Chromatography

The bark of trees contains an interesting mixture of bioactive compounds, or so-called extractives. The use of supercritical carbon dioxide (sc-CO₂) eliminates both the need for organic solvents as extractants and the danger that solvent traces might compromise the purity of the extracts. Unfortunately, the complexity and natural variability of extracts’ composition render any utilization attempts rather challenging. Thus, in order to implement exploitation concepts in a meaningful way, appropriate analytical techniques for characterizing extracts must be available beforehand. In our work, we explored gas chromatography coupled to both mass spectrometry and a flame ionization detector (GC-MS/FID), in combination with ultraperformance convergence chromatography and quadrupole time-of-flight mass spectrometry (UPC²-QTof-MS), for the characterization of bark extracts from pine (Pinus sylvestris L.) in both qualitative and quantitative terms. Although the conventional GC-MS/FID approach is a robust method for overall quantification of extractives, it fails to provide ample information about native sterol esters and triglycerides. These data are provided by a new, complementary analytical technique based on supercritical carbon dioxide, as the chromatographic eluant, coupled to a high-resolution mass spectrometer. The combination of both techniques and the use of sc-CO₂ as both an extraction solvent and eluant made this combined tool especially powerful. The most prominent triglycerides in the extract were identified qualitatively and quantitatively, and the dominating sterol esters were identified qualitatively, by UPC²-QTof-MS.