Electron Ionization Spectra Research Articles

Machine learning of cellular metabolic rewiring.

Metabolic rewiring allows cells to adapt their metabolism in response to evolving environmental conditions. Traditional metabolomics techniques, whether targeted or untargeted, often struggle to interpret these adaptive shifts. Here, we introduce MetaboLiteLearner, a lightweight machine learning framework that harnesses the detailed fragmentation patterns from electron ionization (EI) collected in scan mode during gas chromatography/mass spectrometry to predict changes in the metabolite composition of metabolically adapted cells. When tested on breast cancer cells with different preferences to metastasize to specific organs, MetaboLiteLearner predicted the impact of metabolic rewiring on metabolites withheld from the training dataset using only the EI spectra, without metabolite identification or pre-existing knowledge of metabolic networks. Despite its simplicity, the model learned captured shared and unique metabolomic shifts between brain- and lung-homing metastatic lineages, suggesting cellular adaptations associated with metastasis to specific organs. Integrating machine learning and metabolomics paves the way for new insights into complex cellular adaptations.

Electronic Characterization of Glycolaldehyde: Experimental and Theoretical Insights from the Core- and Valence-Level Spectroscopy.

The core-level electron excitation and ionization spectra of glycolaldehyde have been investigated by photoabsorption and photoemission spectroscopy at both carbon and oxygen K-edges; the valence ionization spectra were also recorded by photoelectron spectroscopy in the UV-vis region. The spectra are interpreted by means of ab initio calculations based on the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) and coupled cluster singles, doubles, and perturbative are in good agreement with the experimental results, and many of the observed features are assigned. The photoabsorption spectra are not only dominated by transitions from core-level orbitals to unoccupied π and σ orbitals but also show structures due to Rydberg transitions.

Hammett correlation in competition experiments in dissociation of ionised substituted benzophenones and dibenzylideneacetones.

A convenient method of applying competition experiments to devise a Hammett correlation in the dissociation by α-cleavage of 17 ionised 3- and 4-substituted benzophenones, YC6H4COC6H5 [Y=F, Cl, Br, CH3, CH3O, NH2, CF3, OH, NO2, CN and N(CH3)2] is reported and discussed. The results given by this approach, which rely on the relative abundance of [M-C6H5]+ and [M-C6H4Y]+ ions in the electron ionisation spectra of the substituted benzophenones, are compared with those obtained by previous methods. Various refinements of the method are considered, including reducing the ionising electron energy, making allowance for the relative abundance of ions such as C6H5+ and C6H4Y+, which may be formed to some extent by secondary fragmentation, and using substituent constants other than the standard σ constants. The reaction constant, ρ, of 1.08, which is in good agreement with that deduced previously, is consistent with a considerable reduction in electron density (corresponding to an increase in positive charge) at the carbon of the carbonyl group during fragmentation. This method has been successfully extended to the corresponding cleavage of 12 ionised substituted dibenzylideneacetones, YC6H4CH=CHCOCH=CHC6H5 (Y=F, Cl, CH3, OCH3, CF3, and NO2), which may fragment to form either a substituted cinnamoyl cation, [YC6H4CH=CHCO]+, or the cinnamoyl cation, [C6H5CH=CHCO]+. The derived ρ value of 0.76 indicates that the substituent, Y, influences the stability of the cinnamoyl cation somewhat less strongly than it does the analogous benzoyl cation.

Identifying reliable ions for the statistical differentiation of structurally similar fentanyl analogs.

Definitive identification of fentanyl analogs based on mass spectral comparison is challenging given the high degree of structural and, hence, spectral similarity. To address this, a statistical method was previously developed in which two electron-ionization (EI) mass spectra are compared using the unequal variance t-test. Normalized intensities of corresponding ions are compared, testing the null hypothesis (H0 ) that the difference in intensity is equal to zero. If H0 is accepted at all m/z values, the two spectra are statistically equivalent at the specified confidence level. If H0 is not accepted at any m/z value, then there is a significant difference in intensity at that m/z value between the two spectra. In this work, the statistical comparison method is applied to distinguish EI spectra of valeryl fentanyl, isovaleryl fentanyl, and pivaloyl fentanyl. Spectra of the three analogs were collected over a 9-month period and at different concentrations. At the 99.9% confidence level, the spectra of corresponding isomers were statistically associated. Spectra of different isomers were statistically distinct, and ions responsible for discrimination were identified in each comparison. To account for inherent instrument variations, discriminating ions for each pairwise comparison were ranked based on the magnitude of the calculated t-statistic (tcalc ) value. For a given comparison, ions with higher tcalc values are those with the greatest difference in intensity between the two spectra and, therefore, are considered more reliable for discrimination. Using these methods, objective discrimination among the spectra was achieved and ions considered most reliable for discrimination of these isomers were identified.

Mass-spectra of new heterocycles: XXIV. Electron impact and chemical ionization study of <i>N</i>-[3-alkoxyand 3-(1<i>H</i>-pyrrol-1-yl)-2-thienyl]imidothiocarbamates

The behavior of a wide range of previously unknown N -(2-thienyl)imidothiocarbamates synthesized in a single preparative step from heterosubstituted allenes (methoxy-, 1-ethoxyethoxy- and 1 H -pyrrol-1-ylallenes) and aliphatic, cycloaliphatic, and aromatic isothiocyanates was studied under conditions of electron (70 eV) and chemical (reagent gas - methane) ionization. Under the electron impact, the studied compounds form an unstable molecular ion, the main direction of fragmentation of which is associated with the breaking of the C-N bond in the imidoformamide fragment with charge localization at the imine nitrogen atom. A similar decay channel of a molecular ion is also observed in the chemical ionization mass spectra of the studied compounds. In this case, characteristic, but low-intensity ions in the electron ionization spectra of N -(3-methoxy-2-thienyl)imidothiocarbamates become dominant in the chemical ionization spectra. In the chemical ionization spectra of N -[3-(1 H -pyrrol-1-yl)-2-thienyl]imidothiocarbamates, the maximum intensity peak belongs to the [ M - SMe]+ ion. For N -[3-(1-ethoxyethoxy)-2-thienyl]imidothiocarbamate, the peak of the ion formed upon successive elimination of ethoxyethene and methanethiol molecules from the [ M + H]+ ion has the maximum intensity.

Complementary Mass Spectral Analysis of Isomeric O-bearing Organic Compounds and Fragmentation Differences through Analog Techniques for Spaceborne Mass Spectrometers

Mass spectrometers on board spacecraft typically use either impact ionization or electron ionization (EI) as ion sources. Understanding the similarities and differences in the spectral signatures and fragmentation patterns produced by different techniques in mass spectrometry could elucidate the composition of organic compounds. Here we present a comparison between the mass spectra obtained through laser-induced liquid beam ion desorption (LILBID; proven to simulate the impact ionization mass spectra of ice grains) and EI mass spectra of pairs of low-mass, isomeric aldehydes and ketones. Our comparison confirms that EI produces more fragmentation of carbonyl compounds, particularly aldehydes, than LILBID. We find protonated molecular ions [M+H]+ in LILBID but molecular ions [M]+ in EI spectra. From the evaluated species, LILBID generally produces oxygen-carrying fragment ions (e.g., [CHO]+ and [C2H3O]+) in the mass ranges 26–30 and 39–44 u, while in EI, most ions in these ranges correspond to hydrocarbon fragments. The LILBID spectra additionally show mostly protonated oxygen-bearing fragments [CH3O]+ and [C2H5O]+ at m/z 31 and 45, less commonly observed in EI spectra. We observe a decrease in the relative intensities of cation fragment mass lines between m/z 26 and 33 and an increase between m/z 39 and 45, with an increasing carbon number for ketones and aldehydes with LILBID and EI, respectively. Our study provides a basis for complementary compositional analysis to identify the structural properties of organic species in a space environment using different spaceborne mass spectrometers (e.g., SUrface Dust Analyzer and MAss Spectrometer for Planetary EXploration) on board NASA’s future Europa Clipper space mission.

Protocol for Structure Determination of Unknowns by EI Mass Spectrometry. I. Diagnostic Ions for Acyclic Compounds with up to One Functional Group

This Review covers wide-ranging electron ionization (EI) dissociation reactions for various acyclic compounds and their derivatives, such as alcohols, aldehydes, ketones, carboxylic acids, amines, halides, thiols, thiones, esters, thioesters, amides, and more. Common derivatives of monofunctional compounds, such as trialkylsilyl, acyl, perfluoroacyl, oxazoline, and nicotinyl derivatives, are also discussed. The behavior of these under mass spectrometry (MS) conditions is determined, structures and stabilities of product ions are considered, and the ions of diagnostic power in their EI spectra are highlighted. Characteristic dissociation pathways for specific structural elements and their application for spectra/structure correlations are presented. Fundamental approaches for identifying unknowns are given. The advantages and limitations of EI-MS are emphasized. This knowledge is the key for successful applications of the exceptional capabilities of EI-MS for initial structure elucidation and then reliable structure determination of unknowns.

Controlled Formation of Protonated and Radical Cation Precursor Ions by Atmospheric Pressure Photoionization with μLC-MS Enables Electron Ionization and MS/MS Library Search.

Atmospheric pressure photoionization (APPI) was developed as an alternative to electrospray ionization (ESI) for the generation of protonated molecules using liquid chromatography and optimized using dopants such as toluene, which predominantly forms protonated molecules, and chlorobenzene, which favors the formation of radical cations, although the latter has not been fully exploited. Based on 40 diverse low-molecular-weight compounds and micro liquid chromatography (μLC) coupled with APPI tandem mass spectrometry (APPI-MS/MS), the potential of radical cations was investigated. Chromatographic and ionization conditions were decoupled by post-column addition of methanol, allowing separate study and optimization. Due to the mass flow sensitive behavior of APPI, sensitivity is not affected by post-column dilution, and for 8 of 35 analytes, the radical cation response with μLC-APPI is better than for protonated molecules using μLC-ESI. Collision-induced fragmentation (CID) of radical cations produced within a collision energy range from 10-115 eV have, in the median, 65% of the fragments found in electron ionization (EI) spectra. This similarity allowed identification of 86% of the analytes using data-dependent acquisition (DDA) of radical cations and NIST EI library searches. We propose a workflow that uses multimodal DDA of protonated precursor molecules using ESI or APPI with toluene as a dopant, and radical cations produced by chlorobenzene-assisted μLC-APPI with post-column addition of methanol. This increases the confidence of molecular identification by allowing orthogonal library searches using MS/MS libraries for protonated precursor CID spectra and EI libraries for radical cation CID spectra.

Evaluating the Accuracy of the QCEIMS Approach for Computational Prediction of Electron Ionization Mass Spectra of Purines and Pyrimidines.

Mass spectrometry is the most commonly used method for compound annotation in metabolomics. However, most mass spectra in untargeted assays cannot be annotated with specific compound structures because reference mass spectral libraries are far smaller than the complement of known molecules. Theoretically predicted mass spectra might be used as a substitute for experimental spectra especially for compounds that are not commercially available. For example, the Quantum Chemistry Electron Ionization Mass Spectra (QCEIMS) method can predict 70 eV electron ionization mass spectra from any given input molecular structure. In this work, we investigated the accuracy of QCEIMS predictions of electron ionization (EI) mass spectra for 80 purine and pyrimidine derivatives in comparison to experimental data in the NIST 17 database. Similarity scores between every pair of predicted and experimental spectra revealed that 45% of the compounds were found as the correct top hit when QCEIMS predicted spectra were matched against the NIST17 library of >267,000 EI spectra, and 74% of the compounds were found within the top 10 hits. We then investigated the impact of matching, missing, and additional fragment ions in predicted EI mass spectra versus ion abundances in MS similarity scores. We further include detailed studies of fragmentation pathways such as retro Diels–Alder reactions to predict neutral losses of (iso)cyanic acid, hydrogen cyanide, or cyanamide in the mass spectra of purines and pyrimidines. We describe how trends in prediction accuracy correlate with the chemistry of the input compounds to better understand how mechanisms of QCEIMS predictions could be improved in future developments. We conclude that QCEIMS is useful for generating large-scale predicted mass spectral libraries for identification of compounds that are absent from experimental libraries and that are not commercially available.

Efficient implementation of the single-reference algebraic diagrammatic construction theory for charged excitations: Applications to the TEMPO radical and DNA base pairs.

We present an efficient implementation of the second- and third-order single-reference algebraic diagrammatic construction (ADC) theory for electron attachment and ionization energies and spectra [EA/IP-ADC(n), n = 2, 3]. Our new EA/IP-ADC program features spin adaptation for closed-shell systems, density fitting for efficient handling of the two-electron integral tensors, and vectorized and parallel implementation of tensor contractions. We demonstrate capabilities of our efficient implementation by applying the EA/IP-ADC(n) (n = 2, 3) methods to compute the photoelectron spectrum of the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) radical, as well as the vertical and adiabatic electron affinities of TEMPO and two DNA base pairs (guanine-cytosine and adenine-thymine). The spectra and electron affinities computed using large diffuse basis sets with up to 1028 molecular orbitals are found to be in good agreement with the best available results from the experiment and theoretical simulations.

Predicting in silico electron ionization mass spectra using quantum chemistry

Compound identification by mass spectrometry needs reference mass spectra. While there are over 102 million compounds in PubChem, less than 300,000 curated electron ionization (EI) mass spectra are available from NIST or MoNA mass spectral databases. Here, we test quantum chemistry methods (QCEIMS) to generate in silico EI mass spectra (MS) by combining molecular dynamics (MD) with statistical methods. To test the accuracy of predictions, in silico mass spectra of 451 small molecules were generated and compared to experimental spectra from the NIST 17 mass spectral library. The compounds covered 43 chemical classes, ranging up to 358 Da. Organic oxygen compounds had a lower matching accuracy, while computation time exponentially increased with molecular size. The parameter space was probed to increase prediction accuracy including initial temperatures, the number of MD trajectories and impact excess energy (IEE). Conformational flexibility was not correlated to the accuracy of predictions. Overall, QCEIMS can predict 70 eV electron ionization spectra of chemicals from first principles. Improved methods to calculate potential energy surfaces (PES) are still needed before QCEIMS mass spectra of novel molecules can be generated at large scale.

Characterization of aerosol nitroaromatic compounds: Validation of an experimental method.

The analytical capabilities associated with the use of silylation reactions have been extended to a new class of organic molecules, nitroaromatic compounds (NACs). These compounds are a possible contributor to urban particulate matter of secondary origin which would make them important analytes due to their (1) detrimental health effects, (2) potential to affect aerosol optical properties, and (3) and usefulness for identifying PM2.5 from biomass burning. The technique is based on derivatization of the parent NACs by using N,O-bis-(trimethylsilyl)-trifluoro acetamide, one of the most prevalent derivatization reagent for analyzing hydroxylated molecules, followed by gas chromatography-mass spectrometry using electron ionization (EI) and methane chemical ionization (CI). This method is evaluated for 32 NACs including nitrophenols, methyl-/methoxy-nitrophenols, nitrobenzoic acids, and nitrobenzyl alcohols. Electron ionization spectra were characterized by a high abundance of ions corresponding to [M+ ] or [M+ -15]. Chemical ionization spectra exhibited high abundance for [M+ +1], [M+ -15], and [M+ +29] ions. Both EI and CI spectra exhibit ions specific to nitro group(s) for [M+ -31], [M+ -45], and [M+ -60]. The strong abundance observed for [M+ ] (EI), [M+ -15] (EI/CI), or [M+ +1] (CI) ions is consistent with the high charge stabilizing ability associated with aromatic compounds. The combination of EI and CI ionization offers strong capabilities for detection and identification of NACs. Spectra associated with NACs, containing hydrogen, carbon, oxygen, and nitrogen atoms only, as silylated derivatives show fragment/adduct ions at either (a) odd or (b) even masses that indicate either (a) odd or (b) even number of nitro groups, respectively. Mass spectra associated with silylated NACs exhibited 3 distinct regions where characteristic fragmentation with a specific pattern associated with (1) ─OH and/or ─COOH groups, (2) ─NO2 group(s), and (3) benzene ring(s). These findings were confirmed with applications to chamber aerosol and ambient PM2.5 .

GC‐MS Characterization of Hydroxy Fatty Acids Generated From Lipid Oxidation in Vegetable Oils

Lipid oxidation has long been described as following a radical chain reaction mechanism, where hydrogen abstraction is considered the preferred pathway. Hydroxy compounds are, in theory, major products formed from hydrogen abstraction but their presence is rarely monitored. In this study, a GC‐MS technique to characterize hydroxy fatty acids (FA) formed during the oxidation of sunflower and canola oils is described. First, hydroxy FA in oxidized oils are methylated and isolated from non‐oxygenated structures using solid phase extraction (SPE). Then they are converted into their trimethylsilyl (TMS) derivatives using a N,O‐bis (trimethylsilyl) trifluoroacetamide (BSTFA)‐pyridine method and identified by their electron ionization (EI) and positive chemical ionization (PCI) spectra. Separation of most isomeric hydroxy FA with very similar structures is accomplished using a DB‐23 capillary column with (50%‐cyanopropyl)‐methylpolysiloxane phase. The fragmentation patterns of the TMS derivatives are discussed in detail and several easily applicable rules for spectral interpretations are presented. The major hydroxy FA arising from oxidation of sunflower and canola oils are allylic and conjugated structures, specifically 8‐, 9‐, 10‐, and 11‐hydroxyoctadecenoic acid (OH‐C18:1) and 9‐ and 13‐hydroxyoctadecadienoic acid (OH‐C18:2).Practical Applications: Unsaturated hydroxy FA are potential indicators of hydrogen abstraction reactions by FA alkoxyl radicals during lipid oxidation. A prerequisite for the quantification of these hydroxy FA is knowledge of their structures. In this work a GC‐MS method, coupled with SPE and TMS derivatization, to characterize the hydroxy FA derived from oxidation of vegetable oils, which will enable future quantifications of these compounds is employed. In addition, the use of EI and PCI spectra provided clear strategies to interpret mass spectra for both saturated and unsaturated hydroxy FA. The fragmentation patterns in EI and PCI presented in this paper will benefit future studies on identification or quantification of both saturated and unsaturated hydroxy FA in other lipid samples.Hydroxy fatty acids in oils are methylated and isolated using solid phase extraction. They are converted into their trimethylsilyl (TMS) derivatives, which enabled identification by the electron ionization and positive chemical ionization spectra. Here, the fragmentation patterns of the TMS derivatives are discussed in detail and several easily applicable rules for spectral interpretations are presented.

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

Electron ionisation mass spectra of N-methylimidazolidin-4-one organocatalysts were studied by experimental and theoretical means. The molecular ions mostly undergo alpha cleavages of exocyclic substituents that leave the five-membered ring intact. The type of substituent strongly dominates the appearance of the spectra. Fragmentation cascades are corroborated by metastable ion mass spectra. Quantum Chemistry Electron Ionisation Mass Spectra calculations correlate reasonably well with the experimental electron ionisation spectra and reveal mechanistic details of fragmentation pathways. The drawbacks and benefits of such calculations are discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Differentiation of the six dimethoxypyrovalerone regioisomers: GC-MS, GC-MS/MS and GC-IR

Multiple and complementary analytical methods are often necessary for the identification of a specific compound from a series of closely related structural isomers. Gas chromatography-mass spectrometry (GC-MS), gas chromatography-product ion mass spectrometry (GC-MS/MS) and gas chromatography-infrared spectroscopy (GC-IR) were used to differentiate between the six dimethoxypyrrovalerone (DMPV) regioisomers. The six regioisomeric aminoketones were separated on a 50% phenyl stationary phase and the elution order is related to the positioning of substituents on the aromatic ring. These six DMPV regioisomers yield essentially identical mass spectral data in both chemical ionization (CI-MS) and electron ionization (EI-MS) spectra as well as identical product ion MS/MS spectra of the iminium cation base peak (m/z 126). These various mass spectral techniques provide data to identify all major structural features of these molecules except the dimethoxy substitution pattern of the aromatic ring. The region of the vapor phase infrared spectra between 1600cm−1 and 1000cm−1 provides a significant number of unique absorption bands characteristic of each individual DMPV regioisomer.

The role of ultra high performance liquid chromatography with time of flight detection for the identification of synthetic cannabinoids in seized drugs

Separation and mass spectrometric techniques are integral parts of forensic drug analysis for both screening and confirmation. The Scientific Working Group for the Analysis of Seized Drugs (SWGDRUG), which is responsible for setting standards for drug analysis, requires for drug identification a Category A test such as mass spectrometry with an additional test from either Category B or C. If a Category A method is not used at least two uncorrelated tests from Category B must be included, for which separation techniques such as gas chromatography and liquid chromatography would qualify. The utility and validity of using ultra high performance liquid chromatography (UHPLC) and time-of-flight (TOF) mass spectrometry (MS) for the analysis of synthetic cannabinoids is presented. The separation of 32 solutes, including 23 controlled substances and nine non-controlled positional isomers of JWH-018, are compared using UHPLC with TOF detection and capillary GC with electron ionization (EI). For these solutes, the reversed phase UHPLC separation on three different 2.1mm×150mm×2.7μm superficially porous (SPP) columns (C18, Phenyl-Hexyl and Dimethylpentafluorophenylpropyl (PFP)) compared favorably with the capillary gas chromatography (GC) separation using an Elite-5MS column 0.25mm×30m×0.25μm. Principal component analysis revealed that all three UHPLC separations for the separation of the controlled substances are orthogonal to the capillary GC separation. It was also revealed by principal component analysis that the separation of JWH-018 and the nine non-controlled positional isomers for the various techniques were significantly more correlated than the separation of the controlled substances. Although most of the controlled synthetic cannabinoids gave unique TOF in-source collision-induced dissociation MS spectra and EI spectra, it was not possible to discriminate among the geometric isomers (CP47, 497, Epi CP47, 497; Cp47, 497 C8 homologue, Epi CP47, 497 C8 homologue). JWH-018 could be distinguished from the non-controlled isomers based on its EI spectra. In contrast, several of the non-controlled JWH-018 isomers give identical TOF in-source collision-induced dissociation MS spectra to JWH-018.

Identification of natural indigo in historical textiles by GC-MS.

The possibility of successfully applying a common GC-MS procedure for identification in one step of all types of dyes from plants of unknown origin and from historical objects is particularly attractive due to the high separation efficiency of the capillary columns, the MS detection sensitivity and the reproducibility of results. In this work, GC-MS analysis, previously and successfully used for the characterization of anthraquinones, flavonoids and tannins from plant extracts and historical samples, has been tested on indigoid dyestuffs. An analytical procedure based on the silylating agent N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA) with 1% trimethylchlorosilane (TMCS) was applied to pure molecules of indigotin and indirubin and to plant extracts of Indigofera tinctoria L. and Isatis tinctoria L. Preliminary tests have been done to establish the chromatographic conditions and the derivatization amounts most suitable for the simultaneous detection of indigoid molecules and of the other natural compounds, such as fatty acids, carboxylic acids and sugars, contained within the plant extracts. In order to assess the capacity and the sensitivity of the analytical procedure in typical archaeometric applications, wool samples dyed in the laboratory with indigo were analysed by mimicking the sample amounts typically available with historical objects. The electron ionization (EI) spectra of the main silylated derivatives of indigoid molecules obtained in this way constitute the necessary data set for the characterization of natural extracts and historical works of art. Subsequently, the procedure has been applied to historical samples for the detection of indigo and of other dyestuffs eventually contained in samples. Additional information, useful for restoration and preservation of works of art, could be also obtained on the nature of stains and smudges present on the sampled textile material. The GC-MS method turns out to be an efficient and fast analytical tool also for the identification of natural indigo in plants and textile artefacts, providing results complementary to those from high-performance liquid chromatography (HPLC).

Quality control for building libraries from electrospray ionization tandem mass spectra.

Electrospray ionization (ESI) tandem mass spectrometry coupled with liquid chromatography is a routine technique for identifying and quantifying compounds in complex mixtures. The identification step can be aided by matching acquired tandem mass spectra (MS(2)) against reference library spectra as is routine for electron ionization (EI) spectra from gas chromatography/mass spectrometry (GC/MS). However, unlike the latter spectra, ESI MS(2) spectra are likely to originate from various precursor ions for a given target molecule and may be acquired at varying energies and resolutions and have characteristic noise signatures, requiring processing methods very different from EI to obtain complete and high quality reference spectra for individual analytes. This paper presents procedures developed for creating a tandem mass spectral library that addresses these factors. Library building begins by acquiring MS(2) spectra for all major MS(1) peaks in an infusion run, followed by assigning MS(2) spectra to clusters and creating a consensus spectrum for each. Intensity-based constraints for cluster membership were developed, as well as peak testing to recognize and eliminate suspect peaks and reduce noise. Consensus spectra were then examined by a human evaluator using a number of criteria, including a fraction of annotated peaks and consistency of spectra for a given ion at different energies. These methods have been developed and used to build a library from >9000 compounds, yielding 230,000 spectra.

Investigation of 1,1′-disubstituted 4,4′-bipyridinium salts by various mass spectrometry techniques

The mass spectrometric behavior of symmetrically disubstituted 4,4′-bipyridinium salts under electron ionization (EI), electrospray ionization (ESI), direct analysis in real time (DART) ionization, and matrix-assisted laser desorption-ionization (MALDI) has been investigated. Electron ionization spectra were recorded using the direct injection of the probe at elevated temperature, when the salts were decomposed with the loss of counter-ions. In the case of ESI, primary ions are radical cations [Cat]+· and cations [Cat-H]+, decomposed further by the loss of N-substituents. More complicated DART spectra revealed the peaks characterizing both bipyridyl group and organic substituents. MALDI spectra were rather simple and contained the peaks for [Cat]+ · ions that are structurally identical to radical cations formed during the reduction of the salts in solution. Among all the methods, DART and MALDI are suitable for the qualitative analysis of such bis-quaternary salts, while ESI is convenient for the quantitative analysis of these analytes.

Gas Chromatography/Electron Ionization Mass Spectrometric Analysis of Oligomeric Polyethylene Glycol Mono Alkyl Ethers

Polyethylene Glycol Monoalkyl Ethers, CxH2x+1 (OC2H4)n OH, (PEGMAE), are polar compounds like Polyethylene Glycols (PEG) and they undergo microbial degradation which produces toxic substances that are potentially dangerous to the environment. Therefore, the purpose of this study is to carry out proper identification and characterization of these compounds. The Electron Ionization (EI) and Chemical Ionization (CI) spectra of various PEGMAE were obtained by Gas Chromatography/Mass Spectrometry (GC/MS) and were used to identify and characterize these compounds. The characteristic cleavages in the EI and CI reactions of PEGMAEs were also studied. The results obtained showed that the methane CI mass spectra of the PEGMAE contain MH + ions and fragment ions similar to those found in their EI mass spectra. The relative abundances of the MH + ions are low, variable and increase with increasing sample size across the chromatographic peaks; but do not increase with increasing x or n. The base peaks of the low mass oligomers (x3) are PEG related (e.g., m/z 45, 59) but those of higher mass oligomers (x4) include the ion at m/z 63 (HOC2H4OH) H + or m/z 107 (HO (C2H4O)2 H) H + . There are no (MH-H2O) + ions or protonated dimer ions (n2, x2) in the spectra of PEGMAE. The Relative Molar Sensitivities (RMS) or the Relative Sensitivity per Gram (RSG) increases linearly with molecular polarizabilty or molecular weight with a non-zero intercept.