Ion Source Complementarity for Characterization of Complex Organic Mixtures Using Fourier Transform Mass Spectrometry: A Review.

Complex organic mixtures are found in many areas of research, such as energy, environment, health, planetology, and cultural heritage, to name but a few. However, due to their complex chemical composition, which holds an extensive potential of information at the molecular level, their molecular characterization is challenging. In mass spectrometry, the ionization step is the key step, as it determines which species will be detected. This review presents an overview of the main ionization sources employed to characterize these kinds of samples in Fourier transform mass spectrometry (FT-MS), namely electrospray (ESI), atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI), atmospheric pressure laser ionization (APLI), and (matrix-assisted) laser desorption ionization ((MA)LDI), and their complementarity in the characterization of complex organic mixtures. First, the ionization techniques are examined in the common direct introduction (DI) usage. Second, these approaches are discussed in the context of coupling chromatographic techniques such as gas chromatography, liquid chromatography, and supercritical fluid chromatography.

GC-FTICR mass spectrometry with dopant assisted atmospheric pressure photoionization: application to the characterization of plastic pyrolysis oil.

Pyrolysis is a promising way to convert plastic waste into valuable resources. However, for downstream upgrading processes, many undesirable species, such as conjugated diolefins or heteroatom-containing compounds, can be generated during this pyrolysis. In-depth chemical characterization is therefore required to improve conversion and valorization. Because of the high molecular diversity found in these samples, advanced analytical instrumentation is needed to provide accurate and complete characterization. Generally, direct infusion Fourier transform mass spectrometry is used to gather information at the molecular level, but it has the disadvantage of limited structural insights. To overcome this drawback, gas chromatography has been coupled to Fourier transform ion cyclotron resonance mass spectrometry. By taking advantage of soft atmospheric pressure photoionization, which preserves molecular information, and the use of different dopants (pyrrole, toluene, and benzene), selective ionization of different chemical families was achieved. Differences in the ionization energy of the dopants will only allow the ionization of the molecules of the pyrolysis oil which have lower ionization energy, or which are accessible via specific chemical ionization pathways. With a selective focus on hydrocarbon species and especially hydrocarbon species having a double bond equivalent (DBE) value of 2, pyrrole is prone to better ionize low-mass molecules with lower retention times compared to the dopant benzene, which allowed better ionization of high-mass molecules with higher retention times. The toluene dopant presented the advantage of ionizing both low and high mass molecules.

Contribution of LDI and MALDI for the Characterization of a Lignocellulosic-Based Pyrolysis Bio-Oil.

In recent years, various alternatives to fossil fuels have been developed. One of them involves the production of bio-oils from lignocellulosic-based biomass through pyrolysis. However, bio-oils present numerous heteroatoms and, in particular, oxygen atoms that need to be removed by an upgrading process. To optimize these processes, it is necessary to have good knowledge of the composition of the bio-oils at the molecular level. This work aims to establish the usefulness of laser desorption ionization (LDI) and matrix-assisted laser desorption/ionization (MALDI) techniques on lignocellulosic biomass-based bio-oils. Using a Fourier transform ion cyclotron mass spectrometer (FTICR MS), we showed that MALDI gives more information than LDI. The selectivity of a series of MALDI matrices was investigated, showing that some matrices are selective toward compound families and others ionize a wider range of compounds. In this study, nine proton-transfer matrices and three electron-transfer matrices were used and compared to results obtained in LDI. Dithranol, acetosyringone, and graphene oxide were the three promising matrices selected from all matrices, giving an overall characterization of oxygenated classes in a bio-oil. They allowed the ionization of many more species covering a wide range of polarity, aromaticity, and mass with a homogeneous relative intensity for all molecular classes such as lignin-derivative species, sugars, and lipid-derivative species.

Speciation and Semiquantification of Nitrogen-Containing Species in Complex Mixtures: Application to Plastic Pyrolysis Oil.

Plastic pyrolysis oil is of particular interest for waste management in the current context of a circular economy. Due to their uncontrolled origin, these oils may contain significant amount of unwanted compounds such as nitrogen-containing species. These compounds are known to be catalyst poisons during refining processes. Therefore, the removal of these species is crucial, and their characterization from structural and quantification points of view is essential for this purpose. This study presents a method to specify and quantify nitrogen-containing classes in a plastic pyrolysis oil by direct infusion mass spectrometry. Two steps were used, namely structural characterization to select suitable standards and semiquantification. The structural speciation of nitrogen-containing compounds was first performed by electrospray ionization Fourier transform mass spectrometry, followed by tandem mass spectrometry using high-resolution mass isolation and infrared multiphoton dissociation fragmentation. A semiquantification is then performed by the standard addition method, which is very appropriate for such complex matrices. Aromatic cores such as quinoline and quinoxaline were evidenced for both N1 and N2 classes, allowing 2-methylquinoxaline and 2-butylquinoline to be proposed as standards for the semiquantification of N2- and N1-containing compounds, respectively. The amount of nitrogen detected from the sum of the individual species was consistent with the bulk analysis. The reported methodology can be applied to numerous other families of compounds in various other complex matrices.