Identification Of Isomers Research Articles

The differentiation of 2,5-dimethoxy-N-(N-methoxybenzyl)phenethylamine (NBOMe) isomers using GC retention indices and multivariate analysis of ion abundances in electron ionization mass spectra

Synthetic phenethylamine derivatives known as 2,5-dimethoxy-N-(N-methoxybenzyl)phenethylamines (NBOMes) are a common class of novel psychoactive substances (NPS) that are causing many accidental deaths across the United States. Many derivatives are now banned at the federal and state levels, but such control requires reliable identification of the different positional isomers. This manuscript helps establish retention indices and characteristic ion ratios that can be used to distinguish between the positional isomers of 25C-NBOMe and 25I-NBOMe. This manuscript also provides additional support for the ortho effect as a reliable, general, fragmentation mechanism to differentiate positional isomers of NBOMes in electron ionization (EI) mass spectra.The retention indices and fragment ion abundances of the positional isomers of 25C-NBOMe and 25I-NBOMe were measured on two instruments using three different GC columns and parameters. The measured retention indices for the six compounds on three different 5% diphenyl columns are as follows: ortho-25C-NBOMe = 2614 ± 15; meta-25C-NBOMe = 2666 ± 13; para-25C-NBOMe = 2692 ± 13; ortho-25I-NBOMe = 2821 ± 16; meta-25I-NBOMe = 2877 ± 15; and para-25I-NBOMe = 2904 ± 12, where the errors represent the 95% confidence interval of the measurements. Principal component analysis (PCA) and canonical discriminant analysis (CDA) were used, respectively, to assess the variance and classification of NBOMe isomers based on the 15 most abundant ions relative to the base peak. The CDA classification accuracy for the six NBOMe compounds was 99.5% when the data set included spectra from three instrumental setups and the widest range of concentrations. Isomer classification was greater than 99.9% within an instrument and excluding low abundance spectra. These results support the use of chemometric approaches for the classification of unknown compounds, even when non-ideal lower-abundance spectra are used for classification.

Identification of closely related new psychoactive substances (NPS) using solid deposition gas-chromatography infra-red detection (GC–IRD) spectroscopy

The challenges associated with the identification of new NPS have become more apparent with the increasing number of new drugs in the market and the need to identify the specific isomer due to legislation concomitant with the lack of reference standards for comparison. A recent new tandem technique, solid deposition gas chromatography–infra red detection spectroscopy (GC–IRD), which incorporates the GC for the separation of the different components in the sample matrix and infrared red (IR) spectroscopy which provides unique IR spectra of each component, has provided the necessary discrimination for the identification of isomers. This paper presents the identification and the application of an algorithm-based criteria (ABC) for the evaluation of the quality match factor (QMF) as an objective critical criteria in determining the correctness of an identification of close analogues in four classes of compounds; namely the JWH-018 and its structural isomers, AM-2201 and its fluoro positional isomers in the pentyl group, methylmethcathinone (MMC) and its methyl positional isomers in the phenyl ring, and dibutylone and its close analogues.

Effective Liquid Chromatography-Trapped Ion Mobility Spectrometry-Mass Spectrometry Separation of Isomeric Lipid Species.

Lipids are a major class of molecules that play key roles in different biological processes. Understanding their biological roles and mechanisms remains analytically challenging due to their high isomeric content (e.g., varying acyl chain positions and/or double bond locations/geometries) in eukaryotic cells. In the present work, a combination of liquid chromatography (LC) followed by high resolution trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) was used to investigate common isomeric glycerophosphocholine (PC) and diacylglycerol (DG) lipid species from human plasma. The LC dimension was effective for the separation of isomeric lipid species presenting distinct double bond locations or geometries but was not able to differentiate lipid isomers with distinct acyl chain positions. High resolution TIMS-MS resulted in the identification of lipid isomers that differ in the double bond locations/geometries as well as in the position of the acyl chain with resolving power ( R) up to ∼410 ( R ∼ 320 needed on average). Extremely small structural differences exhibiting collision cross sections (CCS) of less than 1% (down to 0.2%) are sufficient for the discrimination of the isomeric lipid species using TIMS-MS. The same level of performance was maintained in the complex biological mixture for the biologically relevant PC 16:0/18:1 lipid isomers. These results suggest several advantages of using complementary LC-TIMS-MS separations for regular lipidomic analysis, with the main emphasis in the elucidation of isomer-specific lipid biological activities.

Identification and quantification of perfluorooctane sulfonamide isomers by liquid chromatography–tandem mass spectrometry

Characterization of the isomers perfluorooctane sulfonamide (PFOSA), a key intermediate of the perfluorooctane sulfonate (PFOS) precursors, is a prerequisite to understand the contribution of precursors to PFOS in the environmental and biological matrices. However, the lack of commercial standards makes quantification of the PFOSA isomers in complex matrices a big challenge. For the first time, a liquid chromatography-tandem mass spectrometry (LC–MS/MS) method was established to identify and quantify PFOSA isomers. Improvement on chromatographic separation with selected reaction monitoring allowed full resolution of six monomethyl branched isomers of PFOSA. The isomers were identified and quantified using a series of characteristic fragment ions and the specific product ions of molecular anion m/z 498: m/z 78 (n-), m/z 169 (iso-), m/z 419 (1m-), m/z 164 (2m-), m/z 259 (3m-), m/z 269 (4m-) and m/z 219 (5m-PFOSA). With the aid of 19F nuclear magnetic resonance to define one technical product as standard, the developed method was used to quantify PFOSA isomers in other technical products and spiked fish blood samples. The results indicated that the developed method displayed strong application prospect for measuring trace level of PFOSA isomers in complex samples. The method detection limits for all isomers were in the range of 0.1–1 pg/g ww for blood samples.

Quantitative evaluation of sulfation position prevalence in chondroitin sulphate by Raman spectroscopy

AbstractChondroitin sulphate (CS) as a major component of the extracellular matrix of numerous connective tissues is responsible for biomechanical properties such as resistance and elasticity. It occurs in different isomeric forms with different sites and degrees of sulfation. The characterization of crude CS and quantitative isomeric identification by high‐performance liquid chromatography (HPLC) requires specific enzymes, expensive reagents, and a complicated analytical process. Poor reproducibility and imprecise quantification were found with other methodologies. Raman spectroscopy allows rapid and reproducible identification of CS isomers offering specificity and avoids the need for sample pretreatment or external markers. In the present work, a quantitative identification of major contribution of isomeric chondroitin 4 and 6‐sulphate in crude CS by Raman spectroscopy has been performed. Two quantitative indices have been proposed based on areas of specific bands of interest related to main skeletal modes, an aromatic ring with axial orientation in the case of the major contribution of 4‐sulphate and C‐O‐(S) vibration, which is heavily detected in the case of the major contribution of 6‐sulphate. Both mammalian and fish sources of CS were used to validate the ability of mentioned indices to discriminate between prevalence of 4‐sulphate, 6‐sulphate, or none of them. Correspondence of given results with HPLC was demonstrated.

Nuclear decay studies of rare isotopes

This article overviews recent developments and achievements on decay spectroscopic studies at the Radioactive-Isotope Beam Factory (RIBF) in RIKEN Nishina Center, which has come online in 2007 as the first 3rd-generation in-flight separator facility. The research territory on the chart of nuclides has been dramatically expanded both towards the proton-rich and neutron-rich frontiers with the advent of RIBF. Taking advantage of the world’s currently strongest RI beams, in conjunction with new experimental techniques and state-of-the-art detector systems, more than 400 species of rare isotopes have been explored in experimental programs dedicated to radioactive-decay measurements performed in the last decade. Some selected topics are introduced in this review. Excited levels investigated by $\gamma$-ray measurements following $\beta$ decay have provided significant information on the evolution of shell structures and nuclear shapes, and thus serve as a stringent benchmark for testing microscopic and macroscopic nuclear model calculations. Systematic studies of $ \beta$-decay half-lives of very neutron-rich nuclei have an enormous impact on a better understanding of explosive nucleosynthesis in the rapid neutron-capture (r) process. On the proton-rich side of the $ \beta$-stability line, new proton emitters have been identified, allowing an exploration of atomic nuclei beyond the proton drip line. New results obtained in a series of decay experiments include various types of nuclear isomers, for example, seniority, spin-trap, shape, and K isomers. The identification and characterization of such exotic isomers have revealed the underlying single-particle orbits and nucleon-nucleon correlations, as well as the change of nuclear shape and spin orientation. These new results will offer further insight into exotic nuclei and contribute to the development of new theories of nuclear structure and nucleosynthesis. In this review article, some of the ongoing research projects and future prospects for decay studies at RIBF are also presented.

Stereochemical features of low-coordinate organophosphorus compounds

Various kinds of sterically protected low-coordinate phosphorus compounds with Mes* (2,4,6-tri-tert-butylphenyl) group such as diphosphenes, 1-phosphaethenes, 1-phosphaallenes, 1,3-diphosphaallenes, and 3,4-diphosphinidenecyclobutenes are discussed in terms of stereochemistry including E/Z photo-isomerization around the P = P or P = C bond and R/S identification of rotational isomers around the crowded P–C bonds. (π-Allyl)palladium complexes ligated with 3,4-diphosphinidenecyclobutenes are useful in catalytic organic reactions such as the Tsuji-Trost reaction and the mechanism is proposed to interpret products.

Identification of alkylpyrazines by gas chromatography mass spectrometry (GC-MS)

Pyrazines are an important group of natural products widely used as food additives and fragrants. Gas chromatography-mass spectrometry (GCMS) is the most widely applied analytical technique for characterization of alkylpyrazines. However, mass spectra of many positional isomers of alkylpyrazines are very similar. Consequently, an unambiguous identification of each positional isomer by spectral interpretation or database search protocols is practically unfeasible. In fact, there are many misidentifications in literature. To identify alkylpyrazines, chemists often resort to gas chromatographic retention indices (RIs). Although there are many compilations of retention indices of alkylpyrazines, these databases are often incomplete and the values reported are sometimes inconsistent. Herein, we present retention indices of fifty-six alkylpyrazines recorded on DB-1, ZB-5MS, DB-624, and ZB-WAXplus stationary phases, and compare them with those available in the NIST-2017 MS-RI database. Furthermore, we demonstrate how RI values can be used, together with mass spectral interpretations, to identify certain alkylpyrazines unambiguously.

Identification of Double Bond Position Isomers in Unsaturated Lipids by m-CPBA Epoxidation and Mass Spectrometry Fragmentation.

Lipids are highly diverse biomolecules associated with several biological functions including structural constituent, energy storage, and signal transduction. It is essential to characterize lipid structural isomers and further understand their biological roles. Unsaturated lipids contain one or multiple carbon-carbon double bonds. Identifying double bond position presents a major challenge in unsaturated lipid characterization. Recently, several advancements have been made for double bond localization by mass spectrometry (MS) analysis. However, many of these studies require complex chemical reactions or advanced mass spectrometers with special fragmentation techniques, which limits the application in lipidomics study. Here, an innovative meta-chloroperoxybenzoic acid ( m-CPBA) epoxidation reaction coupling with collision-induced dissociation (CID)-MS/MS strategy provides a new tool for unsaturated lipidomics analysis. The rapid epoxidation reaction was carried out by m-CPBA with high specificity. Complete derivatization was achieved in minutes without overoxidized byproduct. Moreover, diagnostic ion pair with 16 Da mass difference indicated localization of carbon-carbon double bond in MS/MS spectra. Multiple lipid classes were evaluated with this strategy and generated abundant fragments for structural analysis. Unsaturated lipid analysis of yeast extract using this strategy took less than 30 min, demonstrating the potential for high-throughput lipidomics analysis by this approach. This study opens a door for high throughput unsaturated lipid analysis with minimal requirement for instrumentation, which could be widely applied in lipidomics analysis.

Theoretical investigation on the low-energy isomer identification, structural evolution, stability, and electronic properties of Al10–xBex (x = 1–9) nanoalloys

Numerous isomeric equilibrium structures have been identified for the Al10–xBex (x = 1–9) nanoalloy clusters by using the stochastic search procedure in combination with density functional theory calculations. The relative stability and various electronic properties of the lowest-energy Al10–xBex (x = 0–10) clusters have been systematically studied by using the B3LYP and CCSD(T) methods with the aug-cc-pVDZ basis set. The evolution of the binding energies, the second difference in energy, HOMO–LUMO gaps, vertical detachment energies, vertical ionization potentials, vertical electron affinities, and hardness with the increasing number of Be atoms in the most stable Al10–xBex (x = 0–10) clusters demonstrates that the global minimum of Al8Be2 cluster possesses a special stability. Thus, the electronic structure of the lowest-energy Al8Be2 cluster has been also detected in detail. In addition, it is found that the polarizabilities gradually decrease with increasing number of Be atoms, and the charges always transfer from Al to Be atoms in these nanoalloy clusters. We hope this work could provide helpful insight into the composition-dependent electronic properties of BeAl alloy at the nanoscale, serving as powerful guidelines for future experimental research.

Large-scale profiling of carotenoids by using non aqueous reversed phase liquid chromatography – photodiode array detection – triple quadrupole linear ion trap mass spectrometry: Application to some varieties of sweet pepper (Capsicum annuum L.)

Analysis of carotenoids is very complex and demanding in terms of both separation and detection. In this article, an analytical strategy relying on high-performance liquid chromatography-photodiode array detection-tandem mass spectrometry (HPLC-PAD-MS/MS) is presented for the large-scale screening of these phytochemicals. Separation was realized by means of Non-Aqueous Reversed Phase (NARP) chromatography on a triacontyl stationary phase kept at subambient temperature, using a mobile phase compatible with atmospheric pressure chemical ionization (APCI). The standards of 14 analytes were used to optimize the method and to predict the chromatographic behaviour of untargeted carotenoids. MS and MS/MS data, obtained during Information Dependent Acquisition (IDA) experiments, were utilized to set up a sensitive HPLC- selected reaction monitoring (SRM) method. Relative abundance between SRM ion currents (ion ratio) allowed the MS distinction of structural isomers (for example, bicyclic, monocyclic and acyclic isomers), while the identification of geometrical isomers was based on Qratio and fine structure, as calculated from UV–vis spectra. The comparison of LC-PAD- SRM chromatograms, acquired after applying two different extraction procedures (matrix solid-phase dispersion and overnight cold saponification), allowed verifying that sweet peppers are a good source of xanthophylls, prevalently occurring as esterified forms. The overall strategy could identify more than 40 carotenoids in some sweet pepper varieties (Jolly, horn and sweet chili pepper) available on the Italian and European food market.

Critical Comparison of Liquid Chromatography Coupled to Mass Spectrometry and Three Different Ion Mobility Spectrometry Systems on Their Separation Capability for Small Isomeric Compounds

The very fast separation and identification of isomeric small substances with a molecular weight under 800 Da is still a challenge for high-throughput analysis. Inadequate chromatographic or spectrometric separation hampers an unequivocal identification of isomers, which may be recorded as a sum parameter. Reversed-phase liquid chromatography can be considered a generic method for the separation of isomers. However, a separation is usually achieved within minutes and not milliseconds as is typical for ion mobility spectrometry. The aim of this study, therefore, was to investigate the potential of IMS to separate small isomeric compounds. Hence, 23 substances divided into 11 isomeric groups have been selected. Among them, cancer drugs, hormones, pain relievers and others are contained. Three ion mobility spectrometers with different separation principles were compared with respect to their resolving performance. These systems comprised a traveling wave ion mobility spectrometer, a differential ion mobility spectrometer and a differential mobility analyzer. For reference, the chromatographic resolution and peak capacity was determined by high-performance liquid chromatography using a reversed phase.

Investigation of mycobacteria fatty acid profile using different ionization energies in GC-MS.

Gas chromatography (GC) coupled with electron ionization (EI) mass spectrometry (MS) is a well-established technique for the analysis of volatile and semi-volatile compounds. The main advantage is the highly repeatable fragmentation of the compounds into the ion source, generating intense and diagnostic fragmentation when the ionization is performed at 70 eV; this is considered the standard ionization condition and has been used for creating many established databases, which are of great support in the analyte identification process. However, such an intense fragmentation often causes the loss of the molecular ion or more diagnostic ions, which can be detrimental for the identification of homologous series or isomers, as for instance fatty acids. To obtain this information chemical or soft ionization can be used, but dedicated ion sources and conditions are required. In this work, we explored different ionization voltages in GC-EI-MS to preserve the intensity of the molecular ion using a conventional quadrupole MS. Twenty, 30, 50, and 70 eV were tested using a mixture of fatty acid methyl esters standards. Intensity and repeatability of the most informative ions were compared. Twenty and 70 eV were then used to analyze the fatty acid composition of six different strains of mycobacteria. Two approaches were used for elaborating the data: (1) a single average spectrum of the entire chromatogram was derived, which can be considered (in terms of concept) as a direct EI-MS analysis; (2) the actual chromatographic separation of the compounds was considered after automatic alignment. The results obtained are discussed herein. Graphical abstract ᅟ.

Low-Temperature Oxidation of Ethylene by Ozone in a Jet-Stirred Reactor.

Ethylene oxidation initiated by ozone addition (ozonolysis) is carried out in a jet-stirred reactor from 300 to 1000 K to explore the kinetic pathways relevant to low-temperature oxidation. The temperature dependencies of species' mole fractions are quantified using molecular-beam mass spectrometry with electron ionization and single-photon ionization employing tunable synchrotron-generated vacuum-ultraviolet radiation. Upon ozone addition, significant ethylene oxidation is found in the low-temperature regime from 300 to 600 K. Here, we provide new insights into the ethylene ozonolysis reaction network via identification and quantification of previously elusive intermediates by combining experimental photoionization energy scans and ab initio threshold energy calculations for isomer identification. Specifically, the C2H4 + O3 adduct C2H4O3 is identified as a keto-hydroperoxide (hydroperoxy-acetaldehyde, HOOCH2CHO) based on the calculated and experimentally observed ionization energy of 9.80 (±0.05) eV. Quantification using a photoionization cross-section of 5 Mb at 10.5 eV results in 5 ppm at atmospheric conditions, which decreases monotonically with temperature until 550 K. Other hydroperoxide species that contribute in larger amounts to the low-temperature oxidation of C2H4, like H2O2, CH3OOH, and C2H5OOH, are identified and their temperature-dependent mole fractions are reported. The experimental evidence for additional oxygenated species such as methanol, ketene, acetaldehyde, and hydroxy-acetaldehyde suggest multiple active oxidation routes. This experimental investigation closes the gap between ozonolysis at atmospheric and elevated temperature conditions and provides a database for future modeling.

Photoinduced Heterocyclic Ring Opening of Furfural: Distinct Open-Chain Product Identification by Ultrafast X-ray Transient Absorption Spectroscopy.

The ultraviolet-induced photochemistry of five-membered heterocyclic rings often involves ring opening as a prominent excited-state relaxation pathway. The identification of this particular photoinduced mechanism, however, presents a challenge for many experimental methods. We show that femtosecond X-ray transient absorption spectroscopy at the carbon K-edge (∼284 eV) provides core-to-valence spectral fingerprints that enable the unambiguous identification of ring-opened isomers of organic heterocycles. The unique differences in the electronic structure between a carbon atom bonded to the oxygen in the ring versus a carbon atom set free of the oxygen in the ring-opened product are readily apparent in the X-ray spectra. Ultrafast ring opening via C-O bond fission occurs within ∼350 fs in 266-nm photoexcited furfural, as evidenced by fingerprint core (carbon 1s) electronic transitions into a nonbonding orbital of the open-chain carbene intermediate at 283.3 eV. The lack of recovery of the 1sπ* ground-state depletion in furfural at 286.4 eV indicates that internal conversion to the ground state is a minor channel. These experimental results, augmented by recent advances in the generation of isolated attosecond pulses at the carbon K-edge, will pave the way for probing ring-opened conical intersection dynamics in the future.

Glycan Isomer Identification Using Ultraviolet Photodissociation Initiated Radical Chemistry.

Glycans are fundamental biological macromolecules, yet despite their prevalence and recognized importance, a number of unique challenges hinder routine characterization. The multiplicity of OH groups in glycan monomers easily afford branched structures and alternate linkage sites, which can result in isomeric structures that differ by minute details. Herein, radical chemistry is employed in conjunction with mass spectrometry to enable rapid, accurate, and high throughput identification of a challenging series of closely related glycan isomers. The results are compared with analysis by collision-induced dissociation, higher-energy collisional dissociation, and ultraviolet photodissociation (UVPD) at 213 nm. In general, collision-based activation struggles to produce characteristic fragmentation patterns, while UVPD and radical-directed dissociation (RDD) can distinguish all isomers. In the case of RDD, structural differentiation derives from radical mobility and subsequent fragmentation. For glycans, the energetic landscape for radical migration is flat, increasing the importance of the three-dimensional structure. RDD is therefore a powerful and straightforward method for characterizing glycan isomers.

Online Separation and Identification of Isomers Using Infrared Multiple Photon Dissociation Ion Spectroscopy Coupled to Liquid Chromatography: Application to the Analysis of Disaccharides Regio-Isomers and Monosaccharide Anomers.

The vast array of molecular isomerisms which form the complex molecular structure of carbohydrates is the foundation of their biological versatility but defies the analytical chemist. Hyphenations of mass spectrometry with orthogonal structural characterization, such as ion mobility or ion spectroscopy, have recently shown great promise for distinction between closely related molecular structures. Yet, the lack of analytical strategies for identification of isomers present in mixtures remains a major obstacle to routine carbohydrate sequencing. In this context, an ideal workflow for glycomics would combine isomer separation and individual characterization of the molecular structure with atomistic resolution. Here we report the implementation of such a multidimensional analytical strategy, which consists of the first online coupling of high-performance liquid chromatography (HPLC)-MS and infrared multiple photon dissociation (IRMPD) spectroscopy. The performance of this novel workflow is exemplified in the case of monosaccharides (anomers) and disaccharides (regioisomers) standards. We report that the LC-MS-IRMPD approach offers a robust advanced MS diagnostic of mixtures of isomers, including carbohydrate anomers, which is critical for carbohydrate sequencing. Our results also explain the bimodal character generally observed in LC chromatograms of carbohydrates. More generally, this multidimensional analytical strategy opens the gateway to rapid identification of molecular isoforms with potential application in the "omics" fields.

LC-MS3 method for the analysis of Lipid Peroxidation Products (LPPs) positional isomers

Lipid peroxidation is an enzymatic or non-enzymatic transformation of polyunsaturated fatty acids (PUFAs) leading to the formation of various lipid peroxidation products (LPPs) including low molecular weight aldehydes (i.e. 4-hydroxy-trans-2- nonenal, acrolein), truncated lipids (i.e. alkanals, alkenals), and hydroxy-alka(e)nals), isoprostanes, hydroperoxy-, hydroxy-, keto- and epoxy-derivatives. Numerous studies shown the involvement of lipid peroxidation in the onset and progression of inflammatory based diseases such as diabetes, Alzheimer, Parkinson diseas and cardiovascular diseases [1]. LPPs are known to modulate different cellular signaling pathways by inducing changes in the biological membranes [2], protein lipoxidation, and interaction with cell surface (e.g. scavenger receptors and TLRs) [3] and intracellular (e.g. PPARs) receptors [4]. Various biological activities of LPPs are mainly determined by their chemical diversity. In order to understand LPPs structure-functional relationships, specific and sensitive analytical tools allowing separation and identification of structural isomers are required. Liquid chromatography coupled on-line to mass spectrometry (LC-MS) allows high-throughput characterization of LPPs in biological samples. However, majority of the methods are not capable to distinguish LPP structural isomers. Here a novel LC-MS method for analysis of isomeric LPPs was developed. MS and tandem MS analysis was performed in negative ion mode. Anions of oxidized PUFAs, produced by collision induced dissociation of LPP precursors, were further used for data-driven MS3 analysis yielding structure specific fragment ions necessary to assign type and position of oxidation within PUFA alkyl chain. Method was validated using in vitro oxidized standard PLs and further translated for analysis of complex biological samples. Thus, using novel LC-MS3 based approach it was possible to reveal structural diversity of LPPs in a high-throughput manner.

Ultrafast Deep-Ultraviolet Laser Ionization Mass Spectrometry Applicable To Identify Phenylenediamine Isomers

The application of low-fragmentation mass spectrometry to identify chemicals has been recognized to be of particular importance in chemistry, biomedicine, and materials science. Utilizing a customized all-solid-state picosecond-pulsed deep-ultraviolet (DUV) laser, here we present new advances into photoionization mass spectrometry. The DUV laser ionization mass spectrometry (DUV-LIMS) results in very clean spectra pertaining to minimized structure relaxation and fragmentation under the ultrafast ionization process. Typical DUV-LIMS applications are illustrated not only for small organic molecules but also for long-chain unsaturated hydrocarbons and clusters of benzene. The unique advantages of DUV-LIMS enable us to detect and analyze confusing organic compound mixtures, indicating promising applications. DUV-LIMS is also found to be applicable in the identification of phenylenediamine isomers. An in-depth analysis of reaction dynamics is provided showing how hydrogen-atom-transfer (HAT) initiates the distinguishable photodissociation of phenylenediamines under near-resonant excitation. In particular, ortho-phenylenediamine (OPD) finds a remarkable dehydrogenation product with comparable intensity to the molecular ion peak, which is associated with the quantum tunnelling tautomers, providing new subjects for studying intramolecular noncovalent interactions.

Identification of Isomers by Multidimensional Isotopic Shifts in High-Field Ion Mobility Spectra.

Nearly all molecules incorporate elements with stable isotopes. The resulting isotopologue envelopes in mass spectra tell the exact stoichiometry but nothing about the geometry. Chromatography and electrophoresis at high resolution also can distinguish isotopologues, again without revealing structural information. In high-definition differential ion mobility (FAIMS) spectra, these envelopes universally split in a structure-specific manner, providing a new general approach to isomer delineation. Here, we show that the peak shifts from instances of the same isotope are equal and can be averaged into characteristic elemental shifts, namely 13C and 37Cl for dichloroanilines (DCA). Matrices of these shifts, including the gas composition dimension, are unique to the structure. Hence, all six DCA isomers (with four making two unresolved pairs) are readily delineated in the 13C/37Cl maps with He/CO2 buffer gases. Mixtures of coeluting isomers are also distinguished from pure components.