For the detection of unknown organic bromine compounds, a liquid chromatography–mass spectrometry (LC-MS) method with negative-ion electrospray ionization (NI-ESI) and induced in-source fragmentation (IISF) was established. After LC separation, the molecules are fragmentized in the source, and bromide is detected via m/z 79 and m/z 81 based on the isotopic occurrence of bromine. In this way, the retention times of the unknown organobromine compounds are determined, and this can be used to extract additional structural information (number of bound bromine atoms, molecular mass and fragmentation scheme) from measurements in the commonly used but less sensitive scan mode. The analysis of known organobromine compounds shows that LC/NI-ESI-IISF mass spectrometry with detection of m/z 79 and 81 is more sensitive than the detection of daughter ions (LC/ESI/MS-MS). Therefore, we present a method not only for the detection of unknown organic bromine compounds, but also for the selective and sensitive detection and quantification of known organobromine compounds. 相似文献
Decarboxylation is known to be the major fragmentation pathway for the deprotonated carboxylic acids in collision-induced dissociation (CID). However, in the CID mass spectrum of deprotonated benzoic acid (m/z 121) recorded on a Q-orbitrap mass spectrometer, the dominant peak was found to be m/z 93 instead of the anticipated m/z 77. Based on theoretical calculations, 18O-isotope labeling and MS3 experiments, we demonstrated that the fragmentation of benzoate anion begins with decarboxylation, but the initial phenide anion (m/z 77) can react with trace O2 in the mass analyzer to produce phenolate anion (m/z 93) and other oxygen-containing ions. Thus oxygen adducts should be considered when annotating the MS/MS spectra of benzoic acids. 相似文献
Tabun (ethyl N,N‐dimethylphosphoramidocyanidate), or GA, is a chemical warfare nerve agent produced during the World War II. The synthesis of its analogs is rather simple; thus, it is a significant threat. Furthermore, experiments with tabun and other nerve agents are greatly limited by the involved life risks and the severe restrictions imposed by the Chemical Weapons Convention. For these reasons, accurate theoretical assignment of fragmentation pathways can be especially important. In this work, we employ the Quantum Chemistry Electron Ionization Mass Spectra method, which combines molecular dynamics, quantum chemistry methods, and stochastic approaches, to accurately investigate the electron ionization/mass spectrometry (EI/MS) fragmentation spectrum and pathways of the tabun molecule. We found that different rearrangement reactions occur including a McLafferty involving the nitrile group. An essential and characteristic pathway for identification of tabun and analogs, a two‐step fragmentation producing the m/z 70 ion, was confirmed. The present results will be also useful to predict EI/MS spectrum and fragmentation pathways of other members of the tabun family, namely, the O‐alkyl/cycloalkyl N,N‐dialkyl (methyl, ethyl, isopropyl, or propyl) phosphoramidocyanidates. 相似文献
The chemical ionization mass spectrometric (CIMS) analysis of doxylamine, N,N-dimethyl-2-[1-phenyl-1-(2-pyridinyl)ethoxy]ethanamine, and related compounds, using both ammonia and methane as reagent gases, is discussed. The two reagent gases did not produce the same major fragment ion for doxylamine. Mechanisms for the fragmentation of doxylamine under either ammonia or methane CIMS conditions are proposed. The mechanisms explain the observation of an m/z 182 fragment ion for doxylamine analyzed under methane CIMS conditions and an m/z 184 product ion detected under ammonia CIMS conditions. 相似文献
Intense rearrangement processes involving migrations of hydrogen atoms and the phenyl group were observed in the electron impact induced fragmentation of 1-benzyl-3,3-dimethyldiaziridine. The following ions are observed: (i) m/z 146: a two-step fragmentation involving hydrogen transfer followed by loss of NH2; (ii) m/z 119: C—N1 bond fission followed by a 1–4 phenyl shift and loss of CH3N2; (iii) m/z 106: a process involving reciprocal hydrogen migration between the methyl and benzylic methylene groups; (iv) m/z 58: hydrogen transfer from benzylic methylene and subsequent loss of PhCHN. The origin of these ions has been confirmed by measurements of metastable transitions in 1-benzyl-3,3-dimethyldiaziridine, and on specifically deuterated and substituted diaziridines. The structure of the ions at m/z 119 and m/z 106 has been deduced by means of collisional activation spectrometry. 相似文献
The thermal degradation of an amphiphilic block copolymer poly(ethylene)-b-poly(ethylene oxide)-carboxylic acid terminated
(PE-b-80%PEO–CH2COOH) and its salt obtained as intermediary product from chemical oxidation of the end group of poly(ethylene)-b-poly(ethylene
oxide) (PE-b-80%PEO) has been studied using a thermogravimetric mass spectrometry (TG/MS) coupled system. The isothermal fragmentation
of PE-b-80%PEO–CH2COOH showed a more complex fragmentation pattern than PE-b-80%PEO owing to the simultaneous occurrence of the polyether block
and the carboxylic end group fragmentations. This led to the appearance of four overlapping ion current peaks of fragments
with m/z 44 and two peaks relative to m/z 18 at different times by acid-terminated copolymer. For the PE-b-80%PEO copolymer, two ion current peaks associated to m/z 44 and one large peak relative to m/z 18 fragments were detected. The intermediary product (PE-b-80%PEO–CH2COO− K+) showed differences related to the fragmentation behavior. It has more defined ion current signals and presented characteristic
peaks attributed to m/z 43 fragment at the very beginning of the thermal degradation process, which it not detected in the acid copolymer. 相似文献
The solution conformation of alpha-conotoxin GI and its two single disulfide analogues are simulated using a polarizable force field in combination with the molecular fragmentation quantum chemical calculation. The polarizability is explicitly described by allowing the partial charges and fragment dipole moments to be variables, with values coming from the linear-scaling energy-based molecular fragmentation calculations at the B3LYP/6-31G(d) level. In comparison with the full quantum chemical calculations, the fragmentation approaches can yield precise ground-state energies, dipole moments, and static polarizabilities for peptides. The B3LYP/6-31G(d) charges and fragment-centered dipole moments are introduced in calculations of electrostatic terms in both AmberFF03 and OPLS force fields. Our test calculations on the gas-phase glucagon (PDB code: 1gcn) and solvated alpha-conotoxin GI (PDB code: 1not) demonstrate that the present polarization model is capable of describing the structural properties (such as the relative conformational energies, intramolecular hydrogen bonds, and disulfide bonds) with accuracy comparable to some other polarizable force fields (ABEEM/MM and OPLS-PFF) and the quantum mechanics/molecular mechanics (QM/MM) hybrid model. The employment of fragment-centered dipole moments in calculations of dipole-dipole interactions can save computational time in comparison with those polarization models using atom-centered dipole moments without much loss of accuracy. The molecular dynamics simulations using the polarizable force field demonstrate that two single disulfide GI analogues are more flexible and less structured than the native alpha-conotoxin GI, in agreement with NMR experiments. The polarization effect is important in simulations of the folding/unfolding process of solvated proteins. 相似文献
This is the first ab initio study of the energetics of the fragmentation mechanisms of phthalate, by mass spectrometry, leading
to protonated phthalic anhydride (m/z 149). Phthalates fragment by two major pathways; namely, the McLafferty + 1 rearrangement and the loss of alkoxy. Both pathways
involve a carbonyl oxygen attack to the ortho-carbonyl carbon leading to structures with tetrahedral carbon intermediates that eventually give m/z 149. These pathways were studied by collision induced dissociation (CID) using triple quadrupole mass spectrometry. The proposed
McLafferty + 1 pathway proceeds through a distonic M•+, leading to the loss of an allylic-stabilized alkene radical. The McLafferty rearrangement step proceeds through a six-membered
ring transition state with a small activation energy ranging 0.4–6.2 kcal/mol; the transfer of a second H from the distonic
ion of the rearrangement step proceeds through a radical cation molecule complex. Based on quantum chemical modeling of the
cation molecule complexes, two kinds of cation molecule complexes were identified as radical cation molecule complex and hyperconjugated
cation molecule complex. This distinction is based on the cation and simplifies future modeling of similar complexes. Optimization
of important fragments in these pathways showed cyclized and hydrogen-bonded structures to be favored. An exception was the
optimized structure of the protonated phthalic anhydride (m/z 149) that showed a structure with an open anhydride ring. 相似文献
Molecular fragmentation quantum mechanics (QM) calculations have been combined with molecular mechanics (MM) to construct the fragmentation QM/MM method for simulations of dilute solutions of macromolecules. We adopt the electrostatics embedding QM/MM model, where the low-cost generalized energy-based fragmentation calculations are employed for the QM part. Conformation energy calculations, geometry optimizations, and Born-Oppenheimer molecular dynamics simulations of poly(ethylene oxide), PEO(n) (n = 6-20), and polyethylene, PE(n) ( n = 9-30), in aqueous solution have been performed within the framework of both fragmentation and conventional QM/MM methods. The intermolecular hydrogen bonding and chain configurations obtained from the fragmentation QM/MM simulations are consistent with the conventional QM/MM method. The length dependence of chain conformations and dynamics of PEO and PE oligomers in aqueous solutions is also investigated through the fragmentation QM/MM molecular dynamics simulations. 相似文献
The electron impact-induced fragmentation patterns of 3,5-diphenyl-lH-1,2,4-[4-15N]triazole, 4-ethyl-3,5-diphenyl-4H-1,2,4-[4-15N]triazole, l-ethyl-3,5-diphenyl-lH-1,2,4-[4-15N]triazole and the corresponding unlabelled compounds were established from exact mass measurements and from metastable ion evidence. Evidence for two fragmentation mechanisms was established for the formation of the m/z 104 peak in the spectrum of the 4-ethyl compound. Only the 1-alkylated triazole exhibited an intense peak at m/z 131. Major peaks at m/z 221 (222) and 118 are characteristic of the investigated triazoles. 相似文献
Derivatization of peptides as quaternary ammonium salts (QAS) is a promising method for sensitive detection by electrospray
ionization tandem mass spectrometry (Cydzik et al. J. Pept. Sci.
2011, 17, 445–453). The peptides derivatized by QAS at their N-termini undergo fragmentation according to the two competing mechanisms –
charge remote (ChR) and charge directed (ChD). The absence of mobile proton in the quaternary salt ion results in ChR dissociation
of a peptide bond. However, Hofmann elimination of quaternary salt creates an ion with one mobile proton leading to the ChD
fragmentation. The experiments on the quaternary ammonium salts with deuterated N-alkyl groups or amide NH bonds revealed
that QAS derivatized peptides dissociate according to the mixed ChR-ChD mechanism. The isotopic labeling allows differentiation
of fragments formed according to ChR and ChD mechanisms. 相似文献
The synthesis of a novel electrophoric derivatisation reagent, o-(pentafluorobenzyloxycarbonyl)-2,3,4,5-tetrafluorobenzoyl chloride, is described. The reagent was tested against selected
primary and secondary amino compounds, as well as phenolic and aliphatic hydroxyl compounds as analytical targets. The derivatives
exhibit excellent mass spectral properties under negative ion chemical ionisation, i.e. reduced fragmentation and thus high
ion current for the targeted m/z during analysis. Since the reagent bears a pentafluorobenzyl ester group, resulting negative ion chemical ionisation mass
spectra were expectedly dominated by dissociative resonance electron capture typically observed with these compounds, additionally
showing neutral loss of carbon dioxide and ammonia (in the case of primary amines). The reagent is suitable for detecting
the target compounds with high sensitivity, as exemplified for the analysis of amphetamine and methylphenidate from human
plasma where chromatographic background is drastically reduced by a shift in detected m/z and retention time and lower limits of quantification at 7.8 pg/mL (amphetamine) and 4.5 pg/mL (methylphenidate) can be obtained.
The choice of two or three target quantification masses allows selective detection and adjustment of lowest background interference.
No carryover effect was observed for the derivatives of amphetamine and methylphenidate. 相似文献
The electron impact (EI) and chemical ionization (CI) spectra of 2,2-diphenyl-3-aryl cyclobutanone oximes (1–5) are reported. Formation of diphenylmethyl cation at m/z 167 is a major fragmentation process in both EI and CI spectra. Labelling studies established that the hydrogen involved in this rearrangement transfers from the NOH group and not from cyclobutane ring positions. The [M + 3]+ ions are formed under CI conditions as a result of C?N double bond reduction. An interesting secondary kinetic isotope effect is observed in the formation of ion e at m/z 183 in both EI and CI spectra. Other characteristic fragmentation pathways occurring in the EI and CI spectra of these compounds are outlined. 相似文献