The precursor ion and fragmentation of acid standard derivatives were studied by direct infusion of solutions containing 20 µM of individual derivatives using a syringe pump with a flow rate of 40 µL min combined with a 400 µL min -1 auxiliary flow of ACN:H2O (50:50, v/v) with 1% FA to mimic the detection during the chromatographic process. The experiments were measured in full scan mode (200 -1000 m/z) and all-ion fragmentation (AIF) mode to identify the carboxylic derivative with the ion-pair fragment of 169 and 171 m/z. The normalized collision energy (NCE) was optimized at 35 arbitrary units. For optimization experiments, the responses of acid derivatives were recorded with selected ion monitoring mode (Targeted-SIM), using a precursor ion inclusion list provided from direct infusion study (Table ). For profiling of unknown carboxylic acids, we applied the full MS/AIF scanning mode with a sequence of full MS scan (without high-energy collision dissociation cell -HCD fragmentation) followed by AIF with NCE applied. Full scan spectra (200 -1000 m/z, resolution 120 000) and AIF spectra (144 -194 m/z, resolution 120 000 and NCE = 35) were acquired. Chromatograms and mass spectra data were processed using the XCalibur software (Thermo Scientific) and exported as comma separated files for further processing as described below.