Ion Source Multiplexing on a Single Mass Spectrometer

Untargeted Lipidomics after D2O Administration Reveals the Turnover Rate of Individual Lipids in Various Organs of Living Organisms

The administration of low doses of D₂O to living organisms was used for decades for the investigation of metabolic pathways and for the measurement of the turnover rate for specific compounds. Usually, the investigation of the deuterium uptake in lipids is performed by measuring the deuteration level of the palmitic acid residue using GC-MS instruments, and to our knowledge, the application of the modern untargeted LC-MS/MS lipidomics approaches was only reported a few times. Here, we investigated the deuterium uptake for >500 lipids for 13 organs and body liquids of mice (brain, lung, heart, liver, kidney, spleen, plasma, urine, etc.) after 4 days of 100% D₂O administration. The maximum deuteration level was observed in the liver, plasma, and lung, while in the brain and heart, the deuteration level was lower. Using MS/MS, we demonstrated the incorporation of deuterium in palmitic and stearic fragments in lipids (PC, PE, TAG, PG, etc.) but not in the corresponding free forms. Our results were analyzed based on the metabolic pathways of lipids.

Hydrogen/deuterium exchange mass spectrometry in the world of small molecules

The combined use of hydrogen/deuterium exchange (HDX) and mass spectrometry (MS), referred to as HDX-MS, is a powerful tool for exploring molecular edifices and has been used for over 60 years. Initially for structural and mechanistic investigation of low-molecular weight organic compounds, then to study protein structure and dynamics, then, the craze to study small molecules by HDX-MS accelerated and has not stopped yet. The purpose of this review is to present its different facets with particular emphasis on recent developments and applications. Reversible H/D exchanges of mobilizable protons as well as stable exchanges of non-labile hydrogen are considered whether they are taking place in solution or in the gas phase, or enzymatically in a biological media. Some fundamental principles are restated, especially for gas-phase processes, and an overview of recent applications, ranging from identification to quantification through the study of metabolic pathways, is given.

Hydrogen/Deuterium and 16O/18O-Exchange Mass Spectrometry Boosting the Reliability of Compound Identification

Accurate and reliable identification of chemical compounds is the ultimate goal of mass spectrometry analyses. Currently, identification of compounds is usually based on the measurement of the accurate mass and fragmentation spectrum, chromatographic elution time, and collisional cross section. Unfortunately, despite the growth of databases of experimentally measured MS/MS spectra (such as MzCloud and Metlin) and developing software for predicting MS/MS fragments in silico from SMILES patterns (such as MetFrag, CFM-ID, and Ms-Finder), the problem of identification is still unsolved. The major issue is that the elution time and fragmentation spectra depend considerably on the equipment used and are not the same for different LC-MS systems. It means that any additional descriptors depending only on the structure of the chemical compound will be of big help for LC-MS/MS-based omics. Our approach is based on the characterization of compounds by the number of labile hydrogen and oxygen atoms in the molecule, which can be measured using hydrogen/deuterium and ¹⁶O/¹⁸O-exchange approaches. The number of labile atoms (those from -OH, -NH, ═O, and -COOH groups) can be predicted from SMILES patterns and serves as an additional structural descriptor when performing a database search. In addition, distribution of isotope labels among MS/MS fragments can be roughly predicted by software such as MetFrag or CFM-ID. Here, we present an approach utilizing the selection of structural candidates from a database on the basis of the number of functional groups and analysis of isotope labels distribution among fragments. It was found that our approach allows reduction of the search space by a factor of 10 and considerably increases the reliability of the compound identification.

Investigation of the archeological remains using ultrahigh resolution mass spectrometry

Investigation of the archeological material at the molecular level can reveal the composition of ancient paint, balsamic material, reveal cooking recipes, etc. High-resolution mass spectrometry is a powerful technique with underestimated potential for archeology. Here, we present the investigation of the 3000-year-old archeological remains, identified as parts of internal organs of an Egyptian mummy, using high-resolution Orbitrap mass spectrometry. We observed a diverse number of oxidized classes of compounds: O, O₂, O₃, O₄, O₅, N, NO, NO₂, NO₃, NO₄, NO₅, N₂O, N₂O₂. Such chemical composition is unusual and we never observed it in our previous studies of petroleum, humic substances, products of wood pyrolysis or other natural complex mixtures. It is possible that such compounds are formed via biodegradation of lipids and other organic material used for funeral rites. We did not observe evidence of the presence of mineral bitumen, although there are many historical records of the use of mineral bitumen for mummification.

Frequency division multiplex HPLC-MS for simultaneous analyses

Multiplex high-performance liquid chromatograph-mass spectrometry (HPLC-MS), in which multiple HPLCs and one MS are hyphenated, is an approach for high throughput analysis in HPLC-MS. A general multiplex HPLC-MS method employs a column-switching technology, and only one HPLC is connected to one MS at a time. In the present study, we propose a novel multiplex HPLC-MS system for simultaneous HPLC-MS analyses. In this study, multiple HPLCs are hyphenated with one MS without a column-switching mechanism, and a mixed-chromatogram is observed by the MS. Here, we employ a frequency division multiplexing (FDM) technique used in communication engineering to extract any chromatogram from the mixed-chromatogram. When a modulator (chopper or ion-gate type) is set between each ion source and the MS, each modulator blocks each sample stream with an individual frequency. In theory, each chromatogram can be extracted from the mixed-chromatogram via a signal processing based on a Fourier transform (FT), frequency-based signal extraction, and reversed FT. In the actual experiment, two HPLCs are hyphenated with one MS (2HPLC-1MS). The use of chopper type modulators leads to the extraction and restoration of each chromatogram from the mixed-chromatogram. However, each restored-chromatogram involves signal interference. On the other hand, the ion-gate modulation system successfully resulted in restored-chromatograms without interference. The potential of the novel multiplex HPLC-MS system based on FDM is confirmed with respect to the simultaneous and continuous analyses of plural samples.

High-Resolution Mass Spectrometry Study of the Bio-Oil Samples Produced by Thermal Liquefaction of Microalgae in Different Solvents

We have performed a comparative analysis of the bio-oil produced by thermal liquefaction of microalgae in different solvents using high-resolution Orbitrap mass spectrometry and GC-MS approach. Water, methanol, ethanol, butanol, isopropanol, acetonitrile, toluene, and hexane were used as solvents in which the liquefaction was performed. It was observed that all resulting oils demonstrate a considerable degree of similarity. For all samples, compounds containing 1 and 2 nitrogen atoms dominated in the positive ESI spectra, while a relative contribution of other compounds was small. In negative ESI mode, compounds having 2 to 7 oxygens were observed. Statistical analysis revealed that products can be combined in two groups depending on the solvent used for the liquefaction. To the first group, we can attribute the products obtained by using protic (alcohols) and to the second by using aprotic (acetonitrile, toluene) solvents. Nevertheless, based on our results, we concluded that solvent possesses a minor impact on molecular composition of bio-oil. We suggested that the driving force of the liquefaction reaction is the thermal dehydration of the carbohydrate in algae, resulting in water formation, which could be the trigger of the producing of bio-oil. To prove this hypothesis, we performed the reaction with the dry algae in the absence of the solvent and observed the formation of bio-oil. Graphical Abstract.

Methylene Group Transfer in Carbonyl Compounds Discovered in silico and Detected Experimentally

A previously unknown transformation of aldehydes, ketones, and carboxylic acid derivatives leads to the formation of substituted oxiranes, aziridines, and azirines as shown by DFT and MP2 computations. Formations of 2,2-dimethyloxirane-d₈ from acetone-d₆ , phenylazirine-d₂ from benzonitrile and 2-methyl-2-(4-hydroxyphenyl)-oxirane from 4-hydroxyacetophenone were detected experimentally by electrospray ionization mass-spectrometry with a heated desolvating capillary. This reaction is a truly concerted process characterized by high activation barriers (activation enthalpies 320-480 kJ mol^-1 ).

Hydrogen/deuterium exchange in mass spectrometry

The isotopic exchange approach is in use since the first observation of such reactions in 1933 by Lewis. This approach allows the investigation of the pathways of chemical and biochemical reactions, determination of structure, composition, and conformation of molecules. Mass spectrometry has now become one of the most important analytical tools for the monitoring of the isotopic exchange reactions. Investigation of conformational dynamics of proteins, quantitative measurements, obtaining chemical, and structural information about individual compounds of the complex natural mixtures are mainly based on the use of isotope exchange in combination with high resolution mass spectrometry. The most important reaction is the Hydrogen/Deuterium exchange, which is mainly performed in the solution. Recently we have developed the approach allowing performing of the Hydrogen/Deuterium reaction on-line directly in the ionization source under atmospheric pressure. Such approach simplifies the sample preparation and can accelerate the exchange reaction so that certain hydrogens that are considered as non-labile will also participate in the exchange. The use of in-ionization source H/D exchange in modern mass spectrometry for structural elucidation of molecules serves as the basic theme in this review. We will focus on the mechanisms of the isotopic exchange reactions and on the application of in-ESI, in-APCI, and in-APPI source Hydrogen/Deuterium exchange for the investigation of petroleum, natural organic matter, oligosaccharides, and proteins including protein-protein complexes. The simple scenario for adaptation of H/D exchange reactions into mass spectrometric method is also highlighted along with a couple of examples collected from previous studies.