RESUMO
RATIONALE: Characterization of crude oil represents a challenge for researchers due to its complexity. While Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is the method of choice for such complex matrices the high number of ions present limits the efficiency of the analysis due to charge competition and space charge effects. One way to solve this problem is the direct coupling of FT-ICR MS with high-performance liquid chromatography (HPLC). METHODS: Normal-phase liquid chromatography was applied on a deasphalted crude oil sample by using a polar aminocyano-bonded stationary phase with n-hexane and isopropyl alcohol as a mobile phase. The HPLC system was coupled online to a 12 T ultrahigh-resolution FT-ICR mass spectrometer. Ion chromatograms were obtained with electrospray ionization (ESI), atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure laser ionization (APLI). RESULTS: The chromatographic separation yielded a group separation into two peaks according to the polarity of the components. Each ionization technique was able to uniquely assign components differing in polarity and aromaticity. Additionally, an increase in aromaticity in the course of retention time for nonpolar species in the first peak was observed. Monitoring the ratio between protonated and radical mono-nitrogen species was achieved. CONCLUSIONS: For the analysis of a crude oil sample, online coupling of a normal-phase HPLC system to an FT-ICR mass spectrometer was achieved. The results of the different ionization techniques were compared with each other, which enables a detailed characterization of the complex sample. Copyright © 2014 John Wiley & Sons, Ltd.
RESUMO
The high complexity of crude oil makes the use of chromatographic separation an important tool especially for sample simplification. The coupling of normal-phase high-performance liquid chromatography (HPLC) using a polar aminocyano column to a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer offers the best attributes of good separation prior to ultrahigh resolution mass spectrometry (MS) detection. Atmospheric pressure laser ionization (APLI) was used as an ionization technique to analyze the nitrogen-containing aromatic compounds in a deasphalted crude oil due to its unique selectivity toward aromatic compounds and also due to its sensitivity. Two main chromatographic peaks were observed during this separation indicating a class-based separation. Mass spectra obtained from fractions were collected along the entire retention time and compared with each other to assign the unique constituents. By coupling the HPLC system directly to the FTICR mass spectrometer, comparable ion and UV chromatograms were obtained, reflecting the scan-to-scan sensitivity of the coupling system. The results show that it is possible to calculate reconstructed class chromatograms (RCC), allowing differences in class composition to be traced along the retention time. As an example, radical and protonated nitrogen species generated by APLI were detected along the retention time which enabled a differentiation between basic and nonbasic species in the same polar peak, thus overcoming the limitation of chromatographic resolution. This report represents the first online LC-FTICR MS coupling in the field of crude oil analysis.