Nature of methyl cation bonding to dihydroxybenzenes elucidated using an ion trap mass spectrometer

Methylation of the isomeric dihydroxybenzeres using the dimethylfluoronium ion ([CH₃FCH₃]⁺) was studied in a quadrupole ion trap mass spectrometer. The products were characterized by tandem mass spectrometry using collision-activated dissociation. A comparison of the daughter ion spectra of the methylated products with those of model ions, generated by protonation of substituent- and ring-methylated analogs, demonstrates that a mixture of methylated products is generated. Included are structures in which the methyl is σ-bonded to the ring and others with σ-bonds to the heteroatom, the latter being favored in catechol and hydroquinone. The energy-resolved daughter ion spectra for the methylated isomers, acquired by varying the amplitude of the a.c. voltage used to excite resonantly the mass-selected ions, support these conclusions regarding the site of methylation.     

Analysis of Mixtures by Ion Kinetic Energy Spectrometry

Abstract

A mass spectrometric method in which separation and compound identification are accomplished in a mass-analyzed ion kinetic energy spectrometer (MIKES) is described. This procedure is possible in a reversed sector (source-magnet-energy analyzer-detector) mass spectrometer when ions characteristic of each mixture component are caused to fragment after mass analysis. For each mass-selected ion, a scan of the ion kinetic energy spectrum identifies the daughter ions arising from unimolecular and/or collision-induced dissociations. Straightforward application of this method to isomeric C₅H₁₀0 ketone mixtures allows separation, identification, and quantitative analysis which is easy, rapid, and unambiguous.     

Unimolecular decomposition of ethoxytrimethylsilane

A study of the metastable spectra from ethoxytrimethylsilane and the mass shifts of the deuterium-labeled species permitted the rationalization of the fragmentation mechanism for forming all major ions in the mass spectrum. A new mechanistic pathway for the formation of [Si(CH₃)₃]⁺ (m/z 73) is demonstrated. A strong metastable ion for elimination of neutral acetaldehyde from the parent ion was observed despite the absence of a detectable daughter ion.     

An attempt to distinguish between the components of a mixture of isomeric fragment ions

The [C₇H₁₀NO₃]⁺ ion in the normal mass spectrum of 2-ethoxycarbonyl-5-oxo-2-pyrrolidinepropanoic acid was shown to correspond to a mixture of two isomeric structures. By decreasing the ionizing electron energy, the one containing a CH₂CH₂COOH group becomes dominant. The proportion of the concentrations of the two isomeric ions was calculated by comparing their daughter ion spectra (obtained by linked scan at constant B/E) with the analogous spectra of a derivative partially labelled by deuterium at the carboxyl and amide groups.     

Elemental compositions from daughter ion spectra of m1+ and [m1 + 1]+: Some applications of the method

The elemental compositions of ions can be determined in tandem mass spectrometry by comparing the daughter ion spectra of the m₁⁺ and [m₁ + 1]⁺ ions. The method is demonstrated for mass-analyzed ion kinetic energy spectra but is applicable to all types of daughter ion spectra, including complex collisionally activated dissociation spectra. In this work, the method is applied to compounds that produce daughter ions of known elemental compositions, and the errors and limitations are evaluated. Following that test, the procedure is applied to a compound that may produce daughters of more than one possible elemental composition. The method is sometimes useful even if the formula of the parent is not known; that is, the formulae of unknown parent and daughter ions may be found. Locating a specific atom in an isotopically labeled molecule is another capability of the method. The basic equation of the method was generalized and incorporated into a computer program for performing the calculations.     

The ion kinetic energy and mass spectra of isomeric methyl indoles

The mass spectra of the seven isomeric methylindoles were recorded and the [metastable ion]/[daughter ion] ratios for the reactions m/e 130 → m/e 103 and m/e 103 → m/e 77 have been obtained. The ratios indicate that the decomposing [M — 1] ions (m/e 130) from the 4, 5, 6 and 7 isomers are energetically similar as are the [M — 1] ions from the 2 and 3 isomers. The results observed for the m/e → 103 m/e 77 reaction showed that the decomposing m/e 103 ions from the 2, 3, 4, 5, 6 and 7 isomers all have the same energy distribution. N-Methylindole gave ratios which were similar to the 4 to 7 isomers at 70eV but different at 20 eV. The ion kinetic energy (IKE) spectra of all the isomeric methylindoles were also obtained and the results compared with the data obtained from the [metastable ion]/[daughter ion] approach. The results from the IKE spectra indicated that the energy distributions of the [M — 1] and [(M — 1) — HCN] ions from 1-methylindole and the [(M — 1) — HCN] ions from 2-methylindole could readily be distinguished from other isomers whose [metastable ion]/[daughter ion] ratios were similar. Thus by using both techniques certain ambiguities can be resolved.     

Reactions within and between adjacent dimethylamino groups: Collision-induced dissociation tandem mass spectrometric study of the 1,9-bis(dimethylamino)phenalenium cation

The electron impact (EI) ionization-induced fragmentation pathways of the new 1,9-bis(dimethylamino) phenalenium cation [1]⁺ were investigated. The peri-dimethylamino substituents of [1]⁺ are incorporated in a trimethine cyanine substructure and show strong steric interactions. A mechanism is proposed for the unusual elimination of CH₃N?CH₂, HN(CH₃)₂ and (CH₃)₃N from [1]⁺ and for the accompanying cyclizations to heterocyclic ions: prior to fragmentation, the intact cation [1]⁺ rearranges, by reciprocal CH₃ and H transfers, to new isomeric cations which decompose subsequently in a characteristic way. A wealth of consistent information on dissociation pathways and fragment structures is provided by collision-induced dissociation tandem mass spectra, collision-induced dissociation mass-analysed ion kinetic energy spectra and exact mass measurements of the salt cation and of its primary fragment ions. The liquid secondary ion mass spectrum of [1]⁺ is very similar to its EI mass spectrum.     

The gas-phase amino–claisen rearrangement of protonated N-allylaniline

Metastable molecular protonated ions of N-allylaniline dissociate with significant losses of ethene and ammonia in the flight path of a mass spectrometer. The structures of the daughter ions formed on the loss of ethene have been elucidated using collision-induced dissociation and it is postulated that two isomeric structures are formed, one corresponding to molecular protonated ions which have undergone an amino–Claisen rearrangement. The relative proportion of this rearranged species is dependent on the exothermicity of the proton-transfer reaction between the sample molecule and the chemical ionization reagent gas ion. It is proposed that the two isomeric parent species differ in the site of protonation.     

Chemical ionization mass spectrometry studies—VII: Deuterium labeled decanes

CI mass spectra of the five isomeric vicinal d₂-decanes have been recorded using methane and d₄-methane as reagent gases. In contrast to earlier suggestions, we find that a large fraction of the alkyl fragment ions from n-decane are formed by elimination of olefins from the abundant [M – 1] ion. Only the C₉ and C₈ fragment ions are produced completely by a one-step reaction between the decanes and the methane reagent ions. Isotope exchange does not occur between the hydrocarbon and the reagent ions derived from d₄-methane but extensive scrambling of the deuterium label in the d₂-decanes does take place in the [M – 1] ion.     

Mass spectrometric studies on 17β-estradiol-17-fatty acid esters: Evidence for the formation of anion-dipole intermediates

The behaviour towards low collision energy processes (eV range) of [M  H]⁻ prepared under negative ion chemical ionization (NICI) ammonia conditions from 17β-estradiol-17-fatty acid esters has been investigated. From such bifunctional compounds containing two acidic sites (i.e. phenol and ester groups), two isomeric forms (i.e. phenoxide and enolate forms) characterize the [M  H]⁻ ion structures, whose distribution depends on the ion preparation mode. Here NICI (ammonia) provides both phenoxide and enolate forms as the [M  H]⁻ species. This behaviour contrasts with the regioselectivity observed for proton abstraction from phenol under NICI (N₂O) and fast atom bombardment conditions. Production of both phenoxide and enolate forms in NICI (ammonia) is demonstrated under NICI (ND₃) conditions in which DO-labelled [M_d  H]⁻ enolate ions are produced in a similar yield to unlabelled [M_d  D]⁻ phenoxide ions. Collisionally activated dissociation (CAD) spectra of both isomeric deprotonated molecules differ strongly by the presence of two different pairs of complementary daughter ions, suggesting that these ionic species are unconvertible. This is due to a steric hindrance effect on the long-distance proton transfer. A mechanistic investigation on the formation of fragment ion pairs produced under CAD was performed with various deuterium-labelled molecules. From these experiments, evidence is provided for molecular isomerizations into ion-dipole complexes (prior to dissociation) which are structurally dependent on the initial charge location. Direct dissociation of these intermediates competes with the occurrence of exothermic proton transfer(s) yielding the formation of other isomeric intermediate forms. The orientation of these proton transfers is dictated by the relative acidities of both moieties of the complex.     

Collisionally activated decomposition of poly(ethylene glycol)s: An investigation of high-mass ion abundances in the collisional activation technique with large molecules

The utility of the collisional activation technique in structure determination of ions is limited as parent ion mass increases. Optimum collisionally activated dissociation yield is often obtained at parent masses of 1000–2000 u, after which daughter ion yield decreases. The apparent decrease in the efficiency of the collisional activation process has been thought of as a degree-of-freedom effect: as new rotational-vibrational modes are added to the parent ion, its lifetime with respect to dissociation increases. We have investigated this effect using an easily characterized system of several poly(ethylene glycol) homologs from the 15-mer to the 35-mer. Observed trends in the collisional activation spectra as parent mass increases support the postulated ‘degree-of-freedom’ effect in general. The loss of C₂H₄O from the [M ? H]^? parents, a fragmentation which has a high activation barrier, however, actually becomes more favored as the parent ion becomes larger. This effect is explained in terms of statistical rate theory.     

Ozone-induced dissociation on a modified tandem linear ion-trap: Observations of different reactivity for isomeric lipids

Ozone-induced dissociation (OzID) exploits the gas-phase reaction between mass-selected lipid ions and ozone vapor to determine the position(s) of unsaturation. In this contribution, we describe the modification of a tandem linear ion-trap mass spectrometer specifically for OzID analyses wherein ozone vapor is supplied to the collision cell. This instrumental configuration provides spatial separation between mass-selection, the ozonolysis reaction, and mass-analysis steps in the OzID process and thus delivers significant enhancements in speed and sensitivity (ca. 30-fold). These improvements allow spectra revealing the double-bond position(s) within unsaturated lipids to be acquired within 1 s: significantly enhancing the utility of OzID in high-throughput lipidomic protocols. The stable ozone concentration afforded by this modified instrument also allows direct comparison of relative reactivity of isomeric lipids and reveals reactivity trends related to (1) double-bond position, (2) substitution position on the glycerol backbone, and (3) stereochemistry. For cis- and trans-isomers, differences were also observed in the branching ratio of product ions arising from the gas-phase ozonolysis reaction, suggesting that relative ion abundances could be exploited as markers for double-bond geometry. Additional activation energy applied to mass-selected lipid ions during injection into the collision cell (with ozone present) was found to yield spectra containing both OzID and classical-CID fragment ions. This combination CID-OzID acquisition on an ostensibly simple monounsaturated phosphatidylcholine within a cow brain lipid extract provided evidence for up to four structurally distinct phospholipids differing in both double-bond position and sn-substitution.     

Intramolecular aromatic substitution reactions in substituted N,N-dimethylthiobenzamide ions

The molecular ions of N,N-dimethylthiobenzamide and its ortho substituted derivatives (substituents CH₃, Cl, Br, I) lose a hydrogen atom and/or the ortho substituent. The mechanism of this process has been studied by measurements of the ionization energies, appearance energies of the product ions m/z 164 and the kinetic energy release during this process. The structure of the product ions m/z 164 and relevant reference ions have been investigated by mass analysed ion kinetic energy spectra, B/E linked scan spectra and collision induced decompositions. The results show clearly the formation of two different kinds of product ions m/z 164 depending on the substituent lost. Type a ions are formed by loss of a H atom or the CH₃ substituent and correspond to protonated 3,4-benzo-N-methylpyroline-2-thione. The formation of these ions occurs by a hydrogen rearrangement followed by an intramolecular substitution via a 5-membered cyclic intermediate and is associated with a large release of kinetic energy. In contrast, the loss of the halogeno substituents to give type b ions probably occurs via a direct displacement reaction by the sulfur atom of the thioamide group giving rise to Gaussian shaped peaks mass analysed ion kinetic energy spectra.     

Determination of the dissociation kinetics of a transient intermediate

Tandem mass spectrometry provides information on the dissociation pathways of gas-phase ions by providing a link between product ions and parent ions. However, there exists a distinct possibility that a parent ion does not dissociate directly to the observed product ion, but that the reaction proceeds through unobserved reaction intermediates. This work describes the discovery and kinetic analysis of an unobserved reaction intermediate with a quadrupole ion trap. [a ₄−NH₃] ions formed from [YGβFL+H] ions dissociate to [(F*YG−NH₃)−CO] ions. It is expected, however, from previous results, that [F*YG−NH₃] ions should form prior to [(F*YG−NH₃)−CO] ions. Double-resonance experiments are used to demonstrate the existence of intermediate [F*YG − NH₃] ions. Various kinetic analyses are then performed using traditional collision-induced dissociation kinetics and double-resonance experiments. The phenomenological rates of formation and decay of peptide rearrangement ion dissociation products are determined by curve fitting decay and formation data generated with the kinetics experiments. The data generated predict an observable level of the intermediate in a time frame accessible but previously not monitored. By examining early product-ion formation, the intermediate ions, [F*YG−NH₃]⁺, are observed.     

Mass spectral fragmentation pathways in phthalate esters. A tandem mass spectrometric collision-induced dissociation study

A collision-induced dissociation study of a series of phthalate esters was carried out using a tandem BB mass spectrometer. Fragmentation pathways of the phthalates were determined in the electron impact mode. Two major daughter ions are formed, one by a McLafferty rearrangement and hydrogen transfer and the other by loss of an alkoxy radical Another major daughter ion, at m/z 149—which is the base peak in the electron impact mass spectra of most phthalate esters—is being formed through four alternate pathways.     

Mass spectral behavior of n-alkynes

The 70 e V-electron impact mass spectra of the C₇–C₁₀ n-alkynes have been determined as well as the metastable ion spectra of the molecular ions and the [CS₂]⁺ and [N₂O]⁺ charge exchange mass spectra of the C₇-C₉ n-alkynes. The metastable ion mass spectra provide only a limited opportunity to distinguish between isomers; however, the 70-eV EI mass spectra of isomeric compounds permit a ready distinction between isomers. The [CS₂]⁺ charge exchange mass spectra of isomeric compounds also show substantial differences. The [N₂O]⁺ charge exchange mass spectra do not show the enhancement of β-fission fragments observed in field ionization experiments, despite representing ions of similar internal energy, and it is concluded that field dissociation is responsible for the β-fission fragments in the field ionization experiments.     

The structure of [C3H4N2]+· and [C5H5N2]+ ions formed from vinylimidazoles,studied by collisionally activated dissociation mass spectrometry

Mass spectra of the three isomeric vinylimidazoles have been compared and the structures of the fragment ions [C₃H₄N₂]^+· and [C₅H₅N₂]⁺ have been investigated by collisionally activated dissociation mass spectrometry. The greater part of the non-decomposing ions m/z 68 from 2-vinylimidazole and from 2-imidazolecarboxylic acid methyl ester, and a minor part of this ion formed from the free acid, all have the same structure: the imidazole ring system, with hydrogens at both nitrogen atoms but none at C(2). An analogous structure, with an ethyl group at C(2), is proposed for the m/z 93 ion from 2-vinylimidazole.     

Ion-dipole complex formation from deprotonated phenol fatty acid esters evidenced by using gas-phase labeling combined with tandem mass spectrometry

The behavior of para-hydroxy-benzyl and hydroxy-phenylethyl fatty acid esters and methoxy derivatives toward the NH₃/NH₂ ^? system was investigated. Under these negative ion chemical ionization (NICI) conditions, proton abstraction takes place mainly at the more acidic site (i.e., phenol); however, this reaction is not entirely regioselective. Using NICI-ND₃ conditions, both isomeric phenoxide and enolate molecular species are produced in competition from these phenol esters. Their respective low-energy collision-activated dissociation spectra are studied, and they strongly differ, showing that these molecular species are not convertible to a common structure. Analysis of specific fragmentations of the OD-enolate parent species labeled by ND₃ in the gas phase, indicates that by charge-promoted cleavage, isomerization into an ion-dipole intermediate takes place prior to dissociation. This complex, containing a ketene moiety, isomerizes into different isomeric forms via two consecutive proton transfers: the first, which is very exothermic, is irreversible in contrast to the second, less exothermic reaction, which occurs via a reversible process. It is evidenced by the loss of labeling at phenol or enolizable sites in the fragment ions. Such a stepwise process does not take place from the phenoxide parent ion, which preferentially yields a very stable carboxylate ion. A thermochemical approach, using estimated acidity values, yields a rationalization of the observed reactivities of the various substrates studied.     

Determination of polychlorodibenzo-p-dioxins and polychlorodibenzofurans by gas chromatography/tandem mass spectrometry and gas chromatography/triple mass spectrometry in a quadrupole ion trap

The determination of tetra- to octachlorodibenzo-p-dioxins and tetra- to octachlorodibenzofurans (PCCD/Fs) by high-resolution gas chromatography/tandem mass spectrometry (HRGC/MS/MS) and high-resolution gas chromatography/triple mass spectrometry (HRGC/MS(3)) in a quadrupole ion trap, equipped with an external ion source, is presented. MS/MS involves a typical four-step process, namely ionization, parent ion isolation, collision-induced dissociation (CID) and mass analysis of the daughter ions. For the MS(3) experiment, the MS/MS scan function is used with the addition of selected daughter ion isolation, their CID and the mass analysis of second-generation product ions called 'grand-daughter ions.' For both methods, the energies necessary for the CID of the 17 PCDD/Fs were determined and optimized using multiple scan functions with different CID amplitudes. The CID efficiency, defined as the signal ratio of fragment ions detected from the major dissociation channels to molecular ions isolated, was 1.15-2.40 V for parent ion dissociation (MS/MS) and 1.05-1.50 V for daughter ion dissociation (MS(3)) and for all the chloro congeners. The same sensitivity (1 pg microl(-1)) can be reached with both the MS/MS and MS(3) methods and linear responses were obtained between 1 and 100 pg microl(-1) injected.     

Determination of daughter ion formulas by multiple stages of mass spectrometry

The ability to obtain daughter ion formulas via comparison of MSⁿ spectra of parent ions containing only ¹²C with those of parent ions with one ¹³C (from the natural ¹³C abundance) is shown for cases in which isobaric interferences with the ¹³C-containing ion preclude the use of the conventional tandem mass spectrometric approach. This method allows the presence of isobaric daughter ions to be ascertained, and unexpected, complex dissociation pathways to be identified. A three-dimensional quadrupole ion trap is used for these experiments. Its high tandem mass spectrometry efficiency makes possible this type of experiment.