The even-electron rule in electrospray mass spectra of pesticides

A study of the fragmentation and ion formation of three major families of pesticides (including herbicides, insecticides, and fungicides) by liquid chromatography/time-of-flight mass spectrometry (LC/TOF-MS) and liquid chromatography/quadrupole time-of-flight mass spectrometry (LC/Q-TOF-MS) was carried out using positive electrospray ionization (ESI) and the results compared with those by gas chromatography (GC)/TOF-MS with electron ionization (EI) in order to test the validity of the even-electron rule in electrospray ionization. First, the majority of the fragmentations by positive ion ESI were even electron (EE) ions (93% of the fragment ions). Secondly, the formation of odd-electron (OE) fragment ions was greater with EI, where the fragment ions were present in a ratio of approximately 1:2 (35% OE ions and 65% EE ions). Thirdly, in-source collision-induced dissociation (CID) fragmentation by LC/TOF-MS and CID fragmentation in the collision cell by LC/Q-TOF-MS/MS resulted in 95% of the fragment ions being identical between the two types of fragmentation. As ESI in the positive ion mode yields an EE precursor ion (normally a protonated molecule), this commonly leads to EE fragment ions by elimination of molecules - a process called the even-electron rule. Neutral radical losses were less common in ESI but were common in the EI spectra of the same compounds. The structures that did lead to OE ions in ESI (exceptions to the even-electron rule approximately 7% of all ESI ions) favored electronegative radical losses in approximately the following order: .SO(2)CH(3), .NO(2), .CH(3), .Cl, .SCH(3), .CH(2)CH, and .OH.     

Effects of ionization mode on charge-site-remote and related fragmentation reactions of long-chain quaternary ammonium ions

Comparison of collisionally activated fragment spectra of long-chain quaternary ammonium ions, formed by liquid-assisted secondary ion mass spectrometry (LSIMS) and electrospray ionization (ESI), shows the latter are dominated by radical cations while the former yield mainly even-electron charge-site-remote (CSR) fragments, similar to the report for different precursors by Cheng et al., J. Am. Soc. Mass Spectrom. 1998, 9, 840. Here, mixed-site fragmentation products (formal loss of a radical directly bonded to the nitrogen plus a radical derived from the long chain) are of comparable importance for both ionization techniques. These observations are difficult to understand if the CSR ions are formed by a concerted rearrangement-elimination reaction, since precollision internal energies of the ESI ions are much lower than those of the ions from LSIMS. Alternatively, if one discards the concerted mechanism for high-energy CA, and assumes that the even-electron fragments are predominantly formed via homolytic bond cleavage, the colder radical cations from ESI survive to the detector while the more energized counterparts from LSIMS preferentially lose a hydrogen atom to yield the CSR ions, as proposed by Wysocki and Ross (Int. J. Mass Spectrom. Ion Processes 1991, 104, 179). The present work also attempts to reconcile discrepancies involving critical energies and known structures for neutral fragments.     

Structures of α-type ions formed in the 157 nm photodissociation of singly-charged peptide ions

One hundred fifty-seven nm photodissociation of singly-charged peptide ions induces the cleavage of alpha-carbon to carbonyl-carbon bonds along the backbone. a(n) + 1 radical ions are observed as the primary photolysis products of peptides with N-terminal arginines in a linear ion trap mass spectrometer. The radical elimination pathways undertaken by the a(n) + 1 radical ions to form more stable even-electron species are studied in hydrogen-deuterium (H/D) exchange experiments. Two types of a(n) ions along with d-type ions are observed as secondary elimination products. The relative abundance of each depends on the C-terminal residue of the radical fragment ion.     

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.     

On the Origin of the Methyl Radical Loss from Deprotonated Ferulic and Isoferulic Acids: Electronic Excitation of a Transient Structure

Formation of radical fragments from even-electron ions is an exception to the “even-electron rule”. In this work, ferulic acid (FA) and isoferulic acid (IFA) were used as the model compounds to probe the fragmentation mechanisms and the isomeric effects on homolytic cleavage. Elimination of methyl radical and CO₂ are the two competing reactions observed in the CID-MS of [FA – H]^? and [IFA – H]^?, of which losing methyl radical violates the “even-electron rule”. The relative intensity of their product ions is significantly different, and thereby the two isomeric compounds can be differentiated by tandem MS. Theoretical calculations indicate that both the singlet-triplet gap and the excitation energy decrease in the transient structures, as the breaking C–O bond is lengthened. The methyl radical elimination has been rationalized as the intramolecular electronic excitation of a transient structure with an elongating C–O bond. The potential energy diagrams, completed by the addition of the energy barrier of the radical elimination, have provided a reasonable explanation of the different CID-MS behaviors of [FA – H]^? and [IFA – H]^?.

Even-electron ions: a systematic study of the neutral species lost in the dissociation of quasi-molecular ions

The collision-induced dissociations of the even-electron [M + H](+) and/or [M - H](-) ions of 121 model compounds (mainly small aromatic compounds with one to three functional groups) ionized by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) have been studied using an ion trap instrument, and the results are compared with the literature data. While some functional groups (such as COOH, COOCH(3), SO(3)H in the negative ion mode, or NO(2) in both the positive and negative ion modes) generally promote the loss of neutrals that are characteristic as well as specific, other functional groups (such as COOH in the positive ion mode) give rise to the loss of neutrals that are characteristic, but not specific. Finally, functional groups such as OH and NH(2) in aromatic compounds do not lead to the loss of a neutral that reflects the presence of these substituents. In general, the dissociation of [M + H](+) and [M - H](-) ions generated from aliphatic compounds or compounds containing an aliphatic moiety obeys the even-electron rule (loss of a molecule), but deviations from this rule (loss of a radical) are sometimes observed for aromatic compounds, in particular for nitroaromatic compounds. Thermochemical data and ab initio calculations at the CBS-QB3 level of theory provide an explanation for these exceptions. When comparing the dissociation behaviour of the even-electron [M + H](+) and/or [M - H](-) ions (generated by ESI or APCI) with that of the corresponding odd-electron [M](+) ions (generated by electron ionization, EI), three cases may be distinguished: (1) the dissociation of the two ionic species differs completely; (2) the dissociation involves the loss of a common neutral, yielding product ions differing in mass by one Da, or (3) the dissociations lead to a common product ion.     

Electron-impact induced fragmentation of some natural products containing bis-2,2-dimethylchromene and bis-2,2-dimethylchroman ring systems

Derivatives of eriostoic and eriostemoic acids (II and III) and their tetrahydro analogs (VI and IX) which contain the bis-2,2-dimethylchromene and the bis-2,2-dimethylchroman ring systems respectively, fragment subsequent to electron-impact in characteristic fashion. The former yield mass spectra dominated by the loss of a methyl radical from their molecular ions while the latter exhibit preferential fragmentation of the heterocyclic ring system. In particular the hydrogen transfer process accompanying elemination of C₄H₇ from the parent ion of bis-2,2-dimethylchromans was investigated by deuterium labeling studies. Fragment ions containing the bis-2,2-dimethylchroman ring undergo further fragmentation by the loss of C₄H₈ rather than C₄H₇. An unusual elimination of CH₅ (accompanied by the appropriate metastable ion) from the molecular ions of the 3,5-dinitrobenzoate esters (XIV and XV) was observed in their mass spectra.     

Electron capture and transfer dissociation: Peptide structure analysis at different ion internal energy levels

We decoupled electron-transfer dissociation (ETD) and collision-induced dissociation of charge-reduced species (CRCID) events to probe the lifetimes of intermediate radical species in ETD-based ion trap tandem mass spectrometry of peptides. Short-lived intermediates formed upon electron transfer require less energy for product ion formation and appear in regular ETD mass spectra, whereas long-lived intermediates require additional vibrational energy and yield product ions as a function of CRCID amplitude. The observed dependencies complement the results obtained by double-resonance electron-capture dissociation (ECD) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and ECD in a cryogenic ICR trap. Compared with ECD FT-ICR MS, ion trap MS offers lower precursor ion internal energy conditions, leading to more abundant charge-reduced radical intermediates and larger variation of product ion abundance as a function of vibrational post-activation amplitude. In many cases decoupled CRCID after ETD exhibits abundant radical c-type and even-electron z-type ions, in striking contrast to predominantly even-electron c-type and radical z-type ions in ECD FT-ICR MS and especially activated ion-ECD, thus providing a new insight into the fundamentals of ECD/ETD.     

Effect of substituents and protonation on the mechanism of 1,3-dipolar retro-cycloaddition reaction of pyrrolidino[60]- and [70]fullerenes

The mass spectra of new substituted pyrrolidino[60]- and [70]fullerenes have been obtained using electrospray ionization conditions in the positive and negative mode of detection with two different mass spectrometers, a quadrupole ion trap and a Fourier transform ion cyclotron resonance. Radical anions M(●-) and deprotonated molecules [M-H](-) are formed under negative electrospray ionization mass spectrometry conditions, and the collision-induced dissociations of both ionic species have been studied. Either negative odd-electron ions or negative even-electron ions undergo a retro-cycloaddition process forming the corresponding fullerene product ions C(60)(●-) and C(70)(●-). The generation of fullerene radical anions from deprotonated molecules is a new exception of the "even-electron rule." In contrast, the protonated molecules [M + H](+) obtained from the positive mode of detection do not undergo this cycloreversion reaction, and the MS(n) experiment reveals a variety of eliminations of neutral molecules involving different hydrogen shifts and multiple bond cleavages that lead eventually to substituted methanofullerene fragment ions. The observed fragmentations can be correlated with the electronic character of the substituents attached to the heterocyclic moiety. The results obtained from the thermal reactions of these compounds, carried out under different pH conditions, correlate well with those obtained in gas phase. The different behaviors between protonated and unprotonated molecules and ions can be explained assuming that the retro-cycloaddition reaction takes place only when the nitrogen atom of the pyrrolidine ring (the basic center of the molecule) is unprotonated both in gas and condensed phase. The protonation of the NH group inhibits the cycloreversion process, and therefore different fragmentations take place. The detailed mechanisms of the formation and evolution of the intermediate fragments are described.     

The mass spectra of some N-substituted barbitals and their trimethylsilyl derivatives

The mass spectra of N-propyl and N-butyl barbitals show the loss of olefin radical (C_nH_2n-1) in analogy to structurally similar molecules such as N-alkyl succinimides and 3-alkyl uracils. Trimethylsilylation of the N-substituted barbitals suppresses this fragmentation and loss of olefin via apparent McLafferty rearrangement from the even-electron ion, [M – 15]⁺, becomes significant. The trimethylsilyl derivatives of N-allyl barbital and N-phenyl barbital show an unusually facile elimination of the appropriate isocyanate from the molecular ion.     

Electron capture dissociation and infrared multiphoton dissociation of oligodeoxynucleotide dications

We report electron capture dissociation (ECD) and infrared multiphoton dissociation (IRMPD) of doubly protonated and protonated/alkali metal ionized oligodeoxynucleotides. Mass spectra following ECD of the homodeoxynucleotides polydC, polydG, and polydA contain w or d "sequence" ions. For polydC and polydA, the observed fragments are even-electron ions, whereas radical w/d ions are observed for polydG. Base loss is seen for polydG and polydA but is a minor fragmentation pathway in ECD of polydC. We also observe fragment ions corresponding to w/d plus water in the spectra of polydC and d(GCATGC). Although the structure of these ions is not clear, they are suggested to proceed through a pentavalent phosphorane intermediate. The major fragment in ECD of d(GCATGC) is a d ion. Radical a- or z-type fragment ions are observed in most cases. IRMPD primarily results in base loss, but backbone fragmentation is also observed. IRMPD provides more sequence information than ECD, but the spectra are more complex due to extensive base and water losses. It is proposed that the smaller degree of sequence coverage in ECD, with fragmentation mostly occurring close to the ends of the molecules, is a consequence of a mechanism in which the electron is captured at a P=O bond, resulting in a negatively charged phosphate group. Consequently, at least two protons (or alkali metal cations) must be present to observe a w or d fragment ion, a requirement that is less likely for small fragments.     

Speciation of nitrogen containing aromatics by atmospheric pressure photoionization or electrospray ionization fourier transform ion cyclotron resonance mass spectrometry

We determine the elemental compositions of aromatic nitrogen model compounds as well as a petroleum sample by atmospheric pressure photoionization (APPI) and electrospray Ionization (ESI) with a 9.4 Tesla Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. From the double-bond equivalents calculated for the nitrogen-containing ions from a petroleum sample, we can infer the aromatic core structure (pyridinic versus pyrrolic nitrogen heterocycle) based on the presence of M(+.) (odd-electron) versus [M+H](+) (even-electron) ions. Specifically, nitrogen speciation can be determined from either a single positive-ion APPI spectrum or two ESI (positive- and negative-ion) spectra. APPI operates at comparatively higher temperature than ESI and also produces radical cations that may fragment before detection. However, APPI fragmentation of aromatics can be eliminated by judicious choice of instrumental parameters.     

The mass spectra of diaziridinones

The electron-impact induced fragmentation of four N,N′-di-t-alkyl-substituted diaziridinones (I to IV) has been studied by both conventional and high resolution mass spectrometry. All diaziridinones exhibit weak molecular ions. Ejection of an alkyl isocyanate, corresponding to the N-alkyl substituent, from the molecular ion, is a dominant and general fragmentation process. Isocyanate-type odd-electron fragment ions occur only in III and IV (where at least one R group is phenyl) and are of low abundance. Elimination of a hydrocarbon radical from the tertiary alkyl substituents is observed in all compounds investigated. McLafferty rearrangement with elimination of a neutral alkene occurs in all compounds. Abundant even-electron hydrocarbon ions corresponding to the mass of the N-alkyl substituent are prevalent. The complete absence of elimination of carbon monoxide is noted. Loss of oxygen from the [M ? RCH²]+ species has been confirmed by accurate mass measurement. Several remarkable rearrangement reactions have been uncovered by high resolution studies and deuteration experiments.     

A mass spectrometric study of substituted furfuryl compounds

The mass spectra of six furfuryl compounds — namely, furfuryl alcohol, 5-furfuryl-furfuryl alcohol, difurfuryl ether, difurylmethane, 2,5-difurfurylfuran, and 4-furfuryl-2-pentenoic acid-γ-lactone — have been studied. Their fragmentation mechanisms are discussed in detail with particular emphasis on the modes that lead to the formation of aromatic fragments. The majority of the fragment ions are formed by elimination of CO and C₂H₂ from even-electron precursor ions and HCO from odd-electron precursor ions. Molecules containing two furan rings linked by a methylene group give mass spectra that exhibit large abundances of aromatic fragment ions.     

High resolution mass spectrometry—I. Some substituted δ-lactones

The mass spectra of some δ-lactones substituted in various positions by methyl groups have been studied both by accurate mass measurements at high resulution and by deuterium labelling. The loss of carbon dioxide has been shown to be important only for a mono-substituted lactone. A fragmentation mode common to all the lactones studied is the elimination of the ring oxygen atom, plus the adjacent carbon atom with its substituents, as a neutral carbonyl molecule such as formaldehyde, acetaldehyde or acetone. Deuterium labelling has uncovered the existence of a rearrangement involving hydrogen transfer in an even-electron ion, and a mechanism involving a six-membered transition state is proposed for this. The bond fissions, common to all the lactones, by which the principal ions in the mass spectra arise, are summarised.     

Decompositions of odd- and even-electron anions derived from deoxy-polyadenylates

Radical anions have been formed via electron transfer from multiply charged 5′-d(AAA)-3′ and 5′-d(AAAA)-3′ anions to CCl₃ ⁺. These ions have been isolated in a quadrupole ion trap operated with helium bath gas at a pressure of 1 mtorr and subjected to resonance excitation (i. e., conventional ion trap collisional activation). Collisional activation of the even-electron species of the same charge state formed directly via electrospray was also performed by using essentially identical conditions. The collisional activation data can be compared directly without ambiguity arising from differences in parent ion internal energies and/or dissociation time frames. Both the odd- and even-electron anions yield extensive sequence-informative fragmentation but show significant differences in the extent of nucleobase loss and in the relative contributions from the various sequence diagnostic dissociation channels. The results of this study indicate that radical anions derived from multiply deprotonated oligo-deoxynucleotides that survive the electron transfer process are stable with respect to fragmentation in the ion trap environment under normal storage conditions and that the unimolecular dissociation behavior of these ions differs from the even-electron anions of the same charge state. These findings suggest, therefore, that odd- and even-electron anions might be used to provide complementary sequence information in cases in which neither ion type provides the full sequence.     

Entropy effects in the fragmentation of 1,1-dimethylhydrazine ions

The 1,1-dimethylhydrazine ion ((CH3)2NNH2+*) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH3)(CH2)NNH2+, and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH3NNH2+. The dissociation of the 1,1-dimethylhydrazine ion has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, in the photon energy range 8.25-31 eV, and tandem mass spectrometry. Theoretical breakdown curves have been obtained from a variational transition state theory (VTST) modeling of the two reaction channels and compared to those obtained from experiment. Seven transition states have been found at the B3-LYP/6-31+G(d) level of theory for the methyl radical loss channel in the internal energy range of 2.32-3.56 eV. The methyl loss channel transition states are found at R(N-C) = 4.265, 4.065, 3.965, 3.165, 2.765, 2.665, and 2.565 A over this internal energy range. Three transition states have been found for the hydrogen atom loss channel: R(H-C) = 2.298, 2.198, and 2.098 A. The DeltaS++(45) value, at an internal energy of 2.32 eV and a bond distance of R(N-C) = 4.265 A, is 65 J K-1 mol-1. As the internal energy increases to 3.56 eV the variational transition state moves to lower R value so that at R(N-C) = 2.565 A, the DeltaS++ decreases to 29 J K-1 mol-1. For the hydrogen atom loss channel the variation in DeltaS++ is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, DeltaS++(59) = 26-16 J K-1 mol-1 over the studied internal energy range. The 0 K enthalpies of formation (DeltafH0) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E0) obtained from the fitting procedure: DeltafH0[CH3NNH2+] = 906 +/- 6 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 822 +/- 7 kJ mol-1. The calculated G3 values are DeltafH0[CH3NNH2+] = 911 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 825 kJ mol-1. In addition to the two low-energy dissociation products, 21 other fragment ions have been observed in the dissociation of the 1,1-dimethylhydrazine ion as the photon energy was increased. Their appearance energies are reported.     

Protonating polymer oligomers in the gas phase to change fragmentation pathways

Ionization of polymers in mass spectrometry is usually achieved by forming metal ion adducts. The metal ion has been shown by Wesdemiotis to often play a spectator role in the collision-induced dissociation (CID) chemistry of these species, wherein they fragment according to a free-radical mechanism similar to that found in their pyrolysis. The result is a predominance of low-mass ions in the CID mass spectrum. We have changed this behavior by generating protonated oligomers in the gas phase by first forming proton-bound complexes of the oligomers with amino acids or peptides by electrospray ionization. These complexes dissociate first by loss of the amino acid/peptide to form protonated oligomers, which then undergo a unique fragmentation chemistry. In this article we discuss the results for poly(methyl methacrylate) (PMMA) and poly(butyl acrylate) (PBA). Initially, protonated PMMA and PBA lose methanol and butanol, respectively, from the side chains of the respective monomers. The resulting PMMA-derived ion then undergoes a series of neutral losses corresponding to 32 and 28 Da, methanol and carbon monoxide. This continues as collision energy increases until a final, carbon-rich backbone ion is formed, which then undergoes a classic hydrocarbon fragmentation pattern. The PBA-derived ions are proposed to fragment by the loss of butylether molecules to form anhydride rings along the oligomer chain. The number of ether molecules lost corresponded to half the number of available side chains in the oligomer. The resulting poly-anhydride ion dissociates by small molecule loss. Mechanisms have been suggested for the fragmentation chemistry of these two classes of oligomers.     

Mass spectrometry of hop compounds—II: Mass spectra of related oxygen heterocyclic compounds

Mass spectra have been determined of a number of phloroglucinol derivatives and closely-related compounds encountered in chemical studies of hop components. Initial fission involving acyl, alkyl or alkenyl side-chains gives predominantly even-electron ions which then fragment their oxygen heterocyclic rings by elimination of neutral molecules, whilst gem-dialkenyl centres show preference for loss of an alkenyl group. The fragment ions induced by electron-impact may be correlated to a useful extent with molecular structure.