Pressurized solvent extraction of secondary char from hydrothermal carbonization: sample preparation and analytical strategy shape GC-MS results

Abstract

Pressurized solvent extraction (PSE) reduces extraction time and solvent use compared to ambient extraction. This study examines the potential to use PSE to extract secondary char, the amorphous tarry phase formed during hydrothermal carbonization (HTC) from hydrochar, and two key sample preparation factors affecting SC characterization. Hydrochar was produced via HTC of cellulose, apple pomace, miscanthus, and a representative food waste. Solvent drying and reconstitution had minimal impact on the secondary char composition, except for minor losses of short-chain acids in cellulose-derived samples. Derivatization improved detection of polar compounds, particularly alcohols, though 5-hydroxymethylfurfural became undetectable, likely due to polymerization. Critically, the common practice of using GC-MS peak areas as biofuel concentration proxies may introduce substantial error when comparing relative abundances of compounds and functional groups across samples or studies. Ratios of chromatogram areas should only be used to indicate relative concentration within the same analytical group, not to compare absolute yields across samples or disparate studies. Overall, while sample preparation decisions modestly affect GC-MS analysis of PSE secondary char, data analysis decisions profoundly influence interpretation. We recommend the transition from qualitative peak-area comparisons to quantitative GC-MS methods to ensure accurate characterization of PSE-derived secondary char and thermochemically derived biofuels in general.