Analysis of organic and inorganic acids in biomass pyrolysis process samples by ion chromatography-mass spectrometry

Biomass pyrolysis is a sustainable process that converts organic matter into value‑added products such as biochar, bio‑oil, and syngas. The composition of pyrolysis products varies widely depending on biomass type, age, moisture content, and operating conditions. Monitoring compositional changes during pyrolysis is therefore essential for improving product quality, maintaining process efficiency, and enabling by‑product valorisation. A new ion chromatography-mass spectrometry (IC‑MS) method is presented for the quantitative determination of organic and inorganic acids released during biomass pyrolysis, namely acetic, formic, sulfuric, oxalic, and phosphoric acids. Sample preparation involved simple dilution and filtration, with chromatographic separation achieved using an IonPac AS24 column within a 30 min run time. Quantification was carried out by ESI-MS using time-Selected Ion Monitoring (t-SIM) and the standard addition method (SAM). Suppressed conductivity detection provided complementary monitoring of overall ionic species, while Full Scan acquisition supported broader screening. Instrumental limits of detection ranged from 0.2 to 3.4 ng mL⁻¹ across all analytes. Intra‑day and inter‑day repeatability was better than 12 % for all analytes, except oxalic acid, which showed inter‑day variability below 20 %. The method was applied to analyse 215 samples collected from two commercial pyrolysis process sampling points over an eight‑month operating period, providing detailed insight into temporal and spatial changes in acid profiles during this processing period. Acetic and formic acids were identified as markers for by‑product levels, while elevated sulfuric acid concentration was associated with decreased main product yield. This study demonstrates that the developed IC‑MS method is a rapid, robust, and industry‑ready tool for process monitoring, optimisation, and product quality assessment.