Experimental Evaluation of Mass Discrimination Effects in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: A Case Study of Straight-Chain Fatty Acids

• • Absolute FT-ICR-MS intensity is not linearly proportional to concentration; internal standard intensity dropped up to 53% despite constant concentration, and C30 fatty acid intensity was ~40-fold lower than C12 at equal concentrations, causing severe underestimation of high-m/z species. • • Response ratios (fatty acid/internal standard) versus concentration ratios showed excellent linearity (R² > 0.9), enabling derivation of deviation coefficients (F) that increased exponentially with m/z (R² > 0.999), reaching >50 for C30. • • Applying F correction to PM2.5 fatty acids increased long-chain (C20–C30) abundances by 3.6-fold and shifted CPI from 1.8 to 3.4, altering source interpretation from fossil fuel to higher plant dominance—critical for accurate source apportionment. • • Without mass discrimination correction, response-weighted metrics (e.g., average carbon number, O/C, H/C ratios) and source apportionment based on relative abundances are systematically biased, particularly for high-m/z compounds, impacting environmental and geochemical conclusions.