Screening of imidazoles in atmospheric aerosol particles using a hybrid targeted and untargeted method based on ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry

Abstract The method for identification and quantification of imidazoles in atmospheric aerosol particles with an aerodynamic diameter up to 2.5 μm (PM2.5) is scarce, and the existing method focus on only a few imidazoles. With the goal of measuring more imidazoles, especially some previously unidentified ones, we developed a screening workflow based on data-dependent acquisition (DDA) auto MS/MS with a preferred targeted list containing 421 imidazoles using ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS). To enable our method to effectively and accurately detect as many imidazoles as possible, we optimized and validated the method based on specificity, limit of detection (LOD), limit of quantification (LOQ), linearity, accuracy, precision and matrix effects using 20 imidazole standards with different functional groups. The method exhibited excellent performance with LOD and LOQ of 0.5–2 ng/mL and 1.5–6 ng/mL, respectively, and spiked recoveries ranging from 64.7–98.7% with standard deviations less than 16.0%, and with relatively shorter analytical time. The established method was then used to screen imidazoles in 37 ambient PM2.5 samples. Ten targeted imidazoles were identified and quantified using imidazole standards, while five suspected imidazoles were identified without standards, and three imidazoles have not been reported before. Concentrations of the 10 targeted imidazoles ranged from 0.13–0.42 ng/m3. The established method enabled us to identify a wide range of imidazoles in ambient aerosol particles with and without using standards.