Relating Molecular Properties to the Persistence of Marine Dissolved Organic Matter with Liquid Chromatography-Ultrahigh-Resolution Mass Spectrometry.

Marine dissolved organic matter (DOM) contains a complex mixture of small molecules that eludes rapid biological degradation. Spatial and temporal variations in the abundance of DOM reflect the existence of fractions that are removed from the ocean over different time scales, ranging from seconds to millennia. However, it remains unknown whether the intrinsic chemical properties of these organic components relate to their persistence. Here, we elucidate and compare the molecular compositions of distinct DOM fractions with different lability along a water column in the North Atlantic Gyre. Our analysis utilized ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry at 21 T coupled to liquid chromatography and a novel data pipeline developed in CoreMS that generates molecular formula assignments and metrics of isomeric complexity. Clustering analysis binned 14 857 distinct molecular components into groups that correspond to the depth distribution of semilabile, semirefractory, and refractory fractions of DOM. The more labile fractions were concentrated near the ocean surface and contained more aliphatic, hydrophobic, and reduced molecules than the refractory fraction, which occurred uniformly throughout the water column. These findings suggest that processes that selectively remove hydrophobic compounds, such as aggregation and particle sorption, contribute to variable removal rates of marine DOM.

Evaluating Retention Index Score Assumptions to Refine GC-MS Metabolite Identification.

As metabolomics grows into a high-throughput and high demand research field, current metrics for the identification of small molecules in gas chromatography-mass spectrometry (GC-MS) still require manual verification. Though steps have been taken to improve scoring metrics by combining spectral similarity (SS) and retention index (RI), the problem persists. A large body of literature has analyzed and refined SS scores, but few studies have explicitly studied improvements to RI scores. Here, we examined whether uninvestigated assumptions of the RI score are valid and propose ways to improve them. Query RIs were matched to library RI with a generous window of ±35 to avoid unintentional removal of valid compound identifications. Each match was manually verified as a true positive (TP), true negative, or unknown. Metabolites with at least 30 TP identifications were included in downstream analyses, resulting in a total of 87 metabolites from samples of varying complexity and type (e.g., amino acid mixtures, human urine, fungal species, and so on.). Our results showed that the RI score assumptions of normality, consistent variance across metabolites, and a mean error centered at 0 are often violated. We demonstrated through a cross-validation analysis that modifying these underlying assumptions according to empirical metabolite-specific distributions improved the TP and negative rankings. Further, we statistically determined the minimum number of samples required to estimate distributional parameters for scoring metrics. Overall, this work proposes a robust statistical pipeline to reduce the time bottleneck of metabolite identification by improving RI scores and thus minimize the effort to complete manual verification.

Gaussian Mixture Modeling Extensions for Improved False Discovery Rate Estimation in GC-MS Metabolomics.

The ability to reliably identify small molecules (e.g., metabolites) is key toward driving scientific advancement in metabolomics. Gas chromatography-mass spectrometry (GC-MS) is an analytic method that may be applied to facilitate this process. The typical GC-MS identification workflow involves quantifying the similarity of an observed sample spectrum and other features (e.g., retention index) to that of several references, noting the compound of the best-matching reference spectrum as the identified metabolite. While a deluge of similarity metrics exist, none quantify the error rate of generated identifications, thereby presenting an unknown risk of false identification or discovery. To quantify this unknown risk, we propose a model-based framework for estimating the false discovery rate (FDR) among a set of identifications. Extending a traditional mixture modeling framework, our method incorporates both similarity score and experimental information in estimating the FDR. We apply these models to identification lists derived from across 548 samples of varying complexity and sample type (e.g., fungal species, standard mixtures, etc.), comparing their performance to that of the traditional Gaussian mixture model (GMM). Through simulation, we additionally assess the impact of reference library size on the accuracy of FDR estimates. In comparing the best performing model extensions to the GMM, our results indicate relative decreases in median absolute estimation error (MAE) ranging from 12% to 70%, based on comparisons of the median MAEs across all hit-lists. Results indicate that these relative performance improvements generally hold despite library size; however FDR estimation error typically worsens as the set of reference compounds diminishes.

MetaboDirect: an analytical pipeline for the processing of FT-ICR MS-based metabolomic data.

BACKGROUND: Microbiomes are now recognized as the main drivers of ecosystem function ranging from the oceans and soils to humans and bioreactors. However, a grand challenge in microbiome science is to characterize and quantify the chemical currencies of organic matter (i.e., metabolites) that microbes respond to and alter. Critical to this has been the development of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), which has drastically increased molecular characterization of complex organic matter samples, but challenges users with hundreds of millions of data points where readily available, user-friendly, and customizable software tools are lacking. RESULTS: Here, we build on years of analytical experience with diverse sample types to develop MetaboDirect, an open-source, command-line-based pipeline for the analysis (e.g., chemodiversity analysis, multivariate statistics), visualization (e.g., Van Krevelen diagrams, elemental and molecular class composition plots), and presentation of direct injection high-resolution FT-ICR MS data sets after molecular formula assignment has been performed. When compared to other available FT-ICR MS software, MetaboDirect is superior in that it requires a single line of code to launch a fully automated framework for the generation and visualization of a wide range of plots, with minimal coding experience required. Among the tools evaluated, MetaboDirect is also uniquely able to automatically generate biochemical transformation networks (ab initio) based on mass differences (mass difference network-based approach) that provide an experimental assessment of metabolite connections within a given sample or a complex metabolic system, thereby providing important information about the nature of the samples and the set of microbial reactions or pathways that gave rise to them. Finally, for more experienced users, MetaboDirect allows users to customize plots, outputs, and analyses. CONCLUSION: Application of MetaboDirect to FT-ICR MS-based metabolomic data sets from a marine phage-bacterial infection experiment and a Sphagnum leachate microbiome incubation experiment showcase the exploration capabilities of the pipeline that will enable the research community to evaluate and interpret their data in greater depth and in less time. It will further advance our knowledge of how microbial communities influence and are influenced by the chemical makeup of the surrounding system. The source code and User's guide of MetaboDirect are freely available through ( https://github.com/Coayala/MetaboDirect ) and ( https://metabodirect.readthedocs.io/en/latest/ ), respectively. Video Abstract.

Online Coupling of Liquid Chromatography with Fourier Transform Ion Cyclotron Resonance Mass Spectrometry at 21 T Provides Fast and Unique Insight into Crude Oil Composition.

High magnetic field Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry provides the highest mass resolving power and mass measurement accuracy for detailed characterization of complex chemical mixtures. Here, we report the coupling of online liquid chromatography of complex mixtures with a 21 tesla FT-ICR mass spectrometer. The high magnetic field enables large ion populations to be analyzed for each spectrum for a high dynamic range, with 3.2 million mass resolving power at m/z 400 (6.2 s transient duration) or 1.6 million (3.1 s transient duration) while maintaining high mass accuracy for molecular formula assignment (root-mean-square assignment error < 0.150 ppm). Thousands of unique elemental compositions are assigned per mass spectrum, which can be grouped by the heteroatom class, double bond equivalents (the number of rings and double bonds to carbon), and carbon number. Figures of merit are discussed, as well as characterization of an Arabian heavy vacuum gas oil in terms of the ring number, compound class, double bond equivalents, and ion type. Consideration of elemental composition and retention order provides additional structural information.

4 Years after the Deepwater Horizon Spill: Molecular Transformation of Macondo Well Oil in Louisiana Salt Marsh Sediments Revealed by FT-ICR Mass Spectrometry.

Gulf of Mexico saltmarsh sediments were heavily impacted by Macondo well oil (MWO) released from the 2010 Deepwater Horizon (DWH) oil spill. Detailed molecular-level characterization of sediment extracts collected over 48 months post-spill highlights the chemical complexity of highly polar, oxygen-containing compounds that remain environmentally persistent. Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), combined with chromatographic prefractionation, correlates bulk chemical properties to elemental compositions of oil-transformation products as a function of time. Carboxylic acid incorporation into parent MWO hydrocarbons detected in sediment extracts (corrected for mass loss relative to C30 hopane) proceeds with an increase of ∼3-fold in O2 species after 9 months to a maximum of a ∼5.5-fold increase after 36 months, compared to the parent MWO. More importantly, higher-order oxygenated compounds (O4-O6) not detected in the parent MWO increase in relative abundance with time as lower-order oxygenated species are transformed into highly polar, oxygen-containing compounds (Ox, where x > 3). Here, we present the first molecular-level characterization of temporal compositional changes that occur in Deepwater Horizon derived oil contamination deposited in a saltmarsh ecosystem from 9 to 48 months post-spill and identify highly oxidized Macondo well oil compounds that are not detectable by routine gas-chromatography-based techniques.

Chromatographic enrichment and subsequent separation of nickel and vanadyl porphyrins from natural seeps and molecular characterization by positive electrospray ionization FT-ICR mass spectrometry.

We report a novel chromatographic method to enrich and separate nickel and vanadyl porphyrins from a natural seep sample and combine molecular level characterization by positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Vanadyl and nickel porphyrin model compound elution from primary secondary amine (PSA) stationary phase combined with UV-vis spectroscopy confirms enrichment and subsequent fractionation of nickel and vanadyl porphyrins into polarity-based subfractions. A more than 100-fold increase in signal-to-noise ratio for nickel porphyrins enables unequivocal elemental composition assignment confirmed by isotopic fine structure for all isotopes >1% relative abundance, and the first mass spectral identification of (61)Ni porphyrin isotopologues derived from natural seeps. Oxygen-containing vanadyl porphyrins and sulfur-containing vanadyl porphyrins are isolated in the same fraction simultaneously from the same sample. We provide the first chromatographic evidence of carboxylic acid functionalities peripheral to the porphyrin core, in agreement with previous studies.

Baseline resolution of isomers by traveling wave ion mobility mass spectrometry: investigating the effects of polarizable drift gases and ionic charge distribution.

We have studied the behavior of isomers and analogues by traveling wave ion mobility mass spectrometry (TWIM-MS) using drift-gases with varying masses and polarizabilities. Despite the reduced length of the cell (18 cm), a pair of constitutional isomers, N-butylaniline and para-butylaniline, with theoretical collision cross-section values in helium (ΩHe ) differing by as little as 1.2 Å(2) (1.5%) but possessing contrasting charge distribution, showed baseline peak-to-peak resolution (Rp-p ) for their protonated molecules, using carbon dioxide (CO2), nitrous oxide (N2O) and ethene (C2H4 ) as the TWIM drift-gas. Near baseline Rp-p was also obtained in CO2 for a group of protonated haloanilines (para-chloroaniline, para-bromoaniline and para-iodoaniline) which display contrasting masses and theoretical ΩHe , which differ by as much as 15.7 Å(2) (19.5%) but similar charge distributions. The deprotonated isomeric pair of trans-oleic acid and cis-oleic acid possessing nearly identical theoretical ΩHe and ΩN2 as well as similar charge distributions, remained unresolved. Interestingly, an inversion of drift-times were observed for the 1,3-dialkylimidazolium ions when comparing He, N2 and N2O. Using density functional theory as a means of examining the ions electronic structure, and He and N2-based trajectory method algorithm, we discuss the effect of the long-range charge induced dipole attractive and short-range Van der Waals forces involved in the TWIM separation in drift-gases of differing polarizabilities. We therefore propose that examining the electronic structure of the ions under investigation may potentially indicate whether the use of more polarizable drift-gases could improve separation and the overall success of TWIM-MS analysis.

Oil spill source identification by principal component analysis of electrospray ionization Fourier transform ion cyclotron resonance mass spectra.

One fundamental challenge with either acute or chronic oil spills is to identify the source, especially in highly polluted areas, near natural oil seeps, when the source contains more than one petroleum product or when extensive weathering has occurred. Here we focus on heavy fuel oil that spilled (~200,000 L) from two suspected fuel tanks that were ruptured on the motor vessel (M/V) Cosco Busan when it struck the San Francisco-Oakland Bay Bridge in November 2007. We highlight the utility of principal component analysis (PCA) of elemental composition data obtained by high resolution FT-ICR mass spectrometry to correctly identify the source of environmental contamination caused by the unintended release of heavy fuel oil (HFO). Using ultrahigh resolution electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry, we uniquely assigned thousands of elemental compositions of heteroatom-containing species in neat samples from both tanks and then applied principal component analysis. The components were based on double bond equivalents for constituents of elemental composition, CcHhN1S1. To determine if the fidelity of our source identification was affected by weathering, field samples were collected at various intervals up to two years after the spill. We are able to identify a suite of polar petroleum markers that are environmentally persistent, enabling us to confidently identify that only one tank was the source of the spilled oil: in fact, a single principal component could account for 98% of the variance. Although identification is unaffected by the presence of higher polarity, petrogenic oxidation (weathering) products, future studies may require removal of such species by anion exchange chromatography prior to mass spectral analysis due to their preferential ionization by ESI.

Exploring the intrinsic polar [4+2(+)] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions.

Gas-phase reactions of model carbosulfonium ions (CH(3)-S(+)=CH(2;) CH(3)CH(2)-S(+)=CH(2) and Ph-S(+)=CH(2)) and an O-analogue carboxonium ion (CH(3)-O(+)=CH(2)) with acyclic (isoprene, 1,3-butadiene, methyl vinyl ketone) and cyclic (1,3-cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6-311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4+2(+)] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH(+) transfer) products. In great contrast to its S-analogues, CH(3)-O(+)=CH(2) (as well as C(2)H(5)-O(+)=CH(2) and Ph-O(+)=CH(2) in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH(2)=CH-O(+)=CH(2) ion forms an abundant [4+2(+)] cycloadduct with isoprene, but similar to the behavior of such α,ß-unsaturated carboxonium ions in solution, seems to occur across the C=C bond.

Intrinsic mobility of gaseous cationic and anionic aggregates of ionic liquids.

Travelling-wave ion mobility mass spectrometry was used to measure the intrinsic mobility of a series of gaseous supra-cation and supra-anion aggregates of several ionic liquids. Close mobilities were observed in a T-wave cell filled with helium at ca. 0.8 mbar for [(DAI)(n+1)(X)(n)](+) (DAI is the 1,3-dialkylimidazolium cation and X is the anion) as compared to the respective anions [(DAI)(n)(X)(n+1)](-) for n=0 to 9. The anomalous behavior reported before in the condensed phase seems therefore to be related to the unique structural organization of pure ionic liquids that provides both polar and non-polar regions with directionality in which the anionic species are more retained than the cationic species in the salt network.

Fast screening and secure confirmation of milk powder adulteration with maltodextrin via electrospray ionization-mass spectrometry [ESI(+)-MS] and selective enzymatic hydrolysis.

Direct-infusion electrospray ionization-mass spectrometry [ESI(+)-MS] of several milk powder samples, confiscated by the Brazilian Federal Police, showed ions accounting for sodiated and potassiated molecules of disaccharides (m/z 365 and 381) as well as trisaccharides (m/z 527 and 543), whereas monosaccharide ions were not detected. The trisaccharide ions were not detected in samples of genuine milk powder, raising the suspicion that their presence indicates adulteration by the addition of maltodextrin. In control samples, maltose and maltotriose were hydrolyzed by alpha-glucosidase and not beta-galactosidase, whereas lactose was resistant to alpha-glucosidase but was hydrolyzed with beta-galactosidase. Samples suspected of being adulterated behaved in the same fashion, confirming the presence of maltose and maltotriose or maltodextrin. Direct-infusion ESI-MS is shown therefore to provide rapid screening of milk powder for adulteration with maltodextrin, whereas its combination with selective enzymatic hydrolysis provides highly reliable confirmation for unambiguous results.

Sesquiterpene lactones from Vernonia scorpioides and their in vitro cytotoxicity.

Fresh leaves of Vernonia scorpioides are widely used in Brazil to treat a variety of skin disorders. Previous in vivo studies with extracts of this species had also demonstrated a high antitumor potential. This paper reports isolation of four sesquiterpene lactones (hirsutinolides and glaucolides), together with diacetylpiptocarphol, 8-acetyl-13-etoxypiptocarphol, luteolin, apigenin, and ethyl caffeate from fresh leaves and flowers of Vernonia scorpioides. The hypothesis that hirsutinolide 3 is formed during extraction was verified theoretically using Density Functional Theory. The effects of isolated compounds on in vitro tumor cells were investigated, as well as their genotoxicity by means of an in vitro comet assay. The results indicate that glaucolide 2 and hirsutinolide 4 are toxic to HeLa cells. These compounds were genotoxic in vitro, a property that appears to be related to the presence of their epoxy groups, which has been a more reliable indication of toxicity than substitution on C-13 or the presence of alpha,beta-unsaturated keto-groups. These results need to be replicated in vivo in order to ascertain their toxicity.

Intrinsic acidity and electrophilicity of gaseous propargyl/allenyl carbocations.

The ion/molecule chemistry of four representative propagyl/allenyl cations 1-4 of the general formula R(1)CH(+)-C[triple bond]C-R (a) <--> R(1)CH=C=C(+)-R (b), that is, the reactive C(3)H(3)(+) ions of m/z 39 from EI of propargyl chloride (H(2)C(+)-C[triple bond]C-H, 1a), isomeric C(4)H(5)(+) ions of m/z 53 from EI of 3-butyne-2-ol (2a, H(2)C(+)-C[triple bond]C-CH(3)) and 2-butyne-1-ol (CH(3)-CH(+)-C[triple bond]C-H, 3a), and Ph-C(3)H(2)(+) ions of m/z 115 from 3-phenyl-2-propyn-1-ol (H(2)C(+)-C[triple bond]C-Ph, 4a) was studied via pentaquadrupole mass spectrometry. With pyridine, proton transfer was observed as the predominant process for 1 and the sole reaction channel for the isomeric 2 and 3, whereas 4 reacted preferentially by adduct formation. These outcomes were rationalized using DFT calculations from isodesmic proton transfer reactions. Similar reaction tendencies were observed with acetonitrile and acrylonitrile, with adduct formation appearing again as a minor pathway for 1, 2 and 4, and as a major reaction channel for 4. With 1,3-dioxolane, hydride abstraction was a dominant reaction, with proton transfer and adduct formation competing as side reactions. With 2,2-dimethyl-1,3-dioxolane, an interplay of reactions including methyl anion abstraction, proton transfer, hydride abstraction and adduct formation were observed depending on the ion structure, with 4 reacting again mainly by adduct formation. Proton transfer was also observed as a dominant process in reactions with ethanol for 1, 2 and 3, with 4 being nearly unreactive whereas no adduct formation was observed for any of the carbocations studied. Limited reactivity was exhibited by these ions in cycloaddition reaction with isoprene.

Petroleomics by EASI(+/-) FT-ICR MS.

An ambient ionization/desorption technique, namely, easy ambient sonic-spray ionization mass spectrometry (EASI), has been applied to crude oil samples. From a single droplet of the sample placed on an inert surface, EASI(+/-) is shown to promote efficient desorption and ionization of a myriad of polar components via the action of its cloud of very minute supersonic bipolar charged droplets. The gaseous [M + H](+) and [M - H](-) ions concurrently formed by EASI(+/-) were analyzed by Fourier transform mass spectrometry (FT-ICR MS), and a total of approximately 6000 acidic and basic components have been attributed. EASI(+/-) FT-ICR MS of crude oils is show to be almost as fast as ESI(+)/ESI(-) FT-ICR MS, providing similar compositional information of polar components and spectral quality comparable to that of a commercial nonochip-based robotic ESI device. EASI(+/-) requires no sample workup thus eliminating risks of contamination during sample manipulation and memory effects because of carry over in pumping ESI lines. More importantly, EASI(+/-) is a voltage-free ionization technique therefore eliminating risks of redox processes or duality of ionization mechanisms that can be observed in voltage-assisted processes. Data visualization via typical petroleomic plots confirms the similarity of the compositional information provided by EASI(+/-) compared to ESI(+)/ESI(-). The ambient EASI(+/-) FT-ICR MS method requires no voltage switching in changing the ion polarity mode, offering a workup, heating and voltage-free protocol for petroleomic studies performed at open atmosphere directly on the undisturbed crude oil sample.

From monomers to geometry-constrained molecules: one step further toward cyanide bridged wires.

We report on the synthesis and properties of a family of linear cyanide bridged mixed-valence heptanuclear complexes with the formula: trans-[L(4)Ru(II){(mu-NC)Fe(III)(NC)(4)(mu-CN)Ru(II)L'(4)(mu-NC)Fe(III)(CN)(5)}(2)](6-) (with L and L' a para substituted pyridine). We also report on the properties of a related pentanuclear complex. These oligomers were purified by size exclusion chromatography, characterized by electrospray ionization (ESI) mass spectrometry and elemental analysis, and their linear shape was confirmed by scanning tunneling microscopy (STM). These complexes present a rich electrochemistry associated with the seven redox active centers. The redox potential split of identical fragments indicates that there is considerable communication along the cyanide bridged backbone of the compounds, even for centers more than 3 nm apart. This small attenuation of the interaction at long distances make these cyanide bridged compounds good candidates for molecular wires. Interestingly, the extent of the communication depends on the relative energy of the fragments, as evaluated by their redox potentials, providing a guide for improvement of this interesting property.

Dimerization of ionized 4-(methyl mercapto)-phenol during ESI, APCI and APPI mass spectrometry.

A novel ion/molecule reaction was observed to occur under electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photo ionization (APPI) conditions, leading to dimerization of ionized 4-(methyl mercapto)-phenol followed by fast H(*) loss. The reaction is particularly favored during ESI, which suggests that this ion/molecule reaction can occur both in the solution inside the ESI-charged droplets and in the gas-phase environment of most other atmospheric pressure ionization techniques. The dimerization reaction is inherent to the electrolytic process during ESI, whereas it is more by ion/molecule chemistry in nature during APCI and APPI. From the tandem mass spectrometry (MS/MS) data, accurate mass measurements, hydrogen/deuterium (H/D) exchange experiments and density functional theory (DFT) calculations, two methyl sulfonium ions appear to be the most likely products of this electrophilic aromatic substitution reaction. The possible occurrence of this unexpected reaction complicates mass spectral data interpretation and can be misleading in terms of structural assignment as reported herein for 4-(methyl mercapto)-phenol.

Recognition and resolution of isomeric alkyl anilines by mass spectrometry.

Two MS techniques have been used to recognize and resolve a representative isomeric pair of N-alkyl and ring-alkyl substituted anilines. The first technique (1) uses MS/MS to perform ion/molecule reactions of structurally-diagnostic fragment ions (SDFI) whereas the second (2) uses traveling wave ion mobility spectrometry (TWIMS) of the pair of protonated molecules followed by on-line collision-induced dissociation (CID), that is, MS/TWIMS-CID/MS. Isomeric C(7)H(7)N(+) ions of m/z 106 (1' from 4-butylaniline and 2 from N-butylaniline) are formed as abundant fragments by 70 eV EI of the anilines, and found to function as suitable SDFI. Ions 1' and 2 display nearly identical unimolecular dissociation chemistry, but contrasting bimolecular reactivity with ethyl vinyl ether, isoprene, acrolein, and 2-methyl-1,3-dioxolane. Ion 2 forms adducts to a large extent whereas 1' is nearly inert towards all reactants tested. The intact protonated anilines are readily resolved and recognized by MS/TWIMS-CID/MS in a SYNAPT mass spectrometer (Waters Corporation, Manchester, UK). The protonated N-butyl aniline (the more compact isomer) displays shorter drift time and higher lability towards CID than its 4-butyl isomer. The general application of SDFI 1' and 2 and other homologous and analogous ions and MS/TWIMS-CID/MS for absolute recognition and resolution of isomeric families of N-alkyl and ring-alkyl mono-substituted anilines and analogues is discussed.

Recognizing alpha-, beta- or gamma-substitution in pyridines by mass spectrometry.

A general mass spectrometric method able to recognize the site of substitution of monosubstituted pyridines is described. The method requires that the molecule under investigation forms, upon ionization and dissociation, the respective alpha-, beta- or gamma- pyridinium ion of m/z 78. Pyridinium ions are stable and common fragments of ionized and protonated pyridines and are found to function as appropriate structurally diagnostic fragment ions. They can be identified by their characteristic and nearly identical collision-induced dissociation behavior and distinguished by the combined use of two structurally diagnostic ion/molecule reactions with acetonitrile and 2-methyl-1,3-dioxolane. alpha-, beta- or gamma-substitution in pyridines can, therefore, be securely recognized via an MS-only method based on structurally diagnostic ions and by the inspection of a single molecule (no need for intracomparisons within the whole set of isomers).