FT-ICR MS analysis

The crude oils were dissolved in toluene to produce a 10 mg/ml solution for ESI FT-ICR MS analysis. A total of 20 μL of solution was further diluted with 1 ml of toluene:methanol (1:1, v-v) solution; 15 μL of ammonium hydroxide solution (28%) was added to facilitate deprotonation of the acids and neutral nitrogen compounds to yield [M-H]⁻ ions. The crude oil and its fractions were analyzed using a Bruker apex-ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet. Sample solutions were infused via an Apollo II electrospray source at 180 μL/h with a syringe pump. Typical operating conditions for negative-ion formation were emitter voltage, 4.0 kV; capillary column introduce voltage, 4.5 kV; and capillary column end voltage, −320 V. Ions accumulated for 0.1 s in a hexapole with 2.4 V of direct current (DC) voltage and 400 Vp-p (volts peak to peak) of radio-frequency (RF) amplitude. The optimized mass for Q1 was m/z 250. An argon-filled hexapole collision pool was operated at 5 MHz and 400 Vp-p of RF amplitude, in which ions accumulated for 0.4 s. The extraction period for ions from the hexapole to the ICR cell was 1.2 ms. The mass spectrometer was calibrated using sodium formate. Mass peaks with a relative abundance greater than 6 times the standard deviation of the baseline noise level were exported to a spreadsheet. Data analysis was performed using custom software, which has been described elsewhere⁵³. Compounds with the same heteroatom class and its isotopes with different values by DBE and carbon number were searched within a set ± 0.001 Kendrick mass defect (KMD) tolerance⁵⁴.