Electrospray ionization tandem mass spectrometry of 3-phenyl-N-(3-(4-phenylpiperazin-1-yl)propyl)-1H-pyrazole-5-carboxamide derivatives: unusual fragmentation involving loss of 11 u

Formation of Carbon Dioxide Attached Fragment Ions in the Fragmentation of Deprotonated Tolfenpyrad and Tebufenpyrad

The in-source collision-induced dissociation (CID) and MS/MS mass spectra of deprotonated tolfenpyrad and tebufenpyrad both showed an unusual fragment ion at m/z 187, but its fragmentation pattern and structure could not be explained by logical neutral losses. Accurate mass measurement indicated that the mass difference between this fragment ion and the dominant fragment ion at m/z 143 equaled to a carbon dioxide (CO₂) molecule. The isolation of the fragment ion m/z 143 in the mass analyzer could spontaneously give rise to the ion m/z 187. The Gibbs free energy of carbon dioxide addition to deprotonated pyrazole ion was significantly negative from the computational results. According to these results, we derived a proposal for the formation and structure of the ion m/z 187, which was an attachment of molecular carbon dioxide to the fragment ion m/z 143 to produce a carboxylate anion. The trace carbon dioxide was speculated to be derived from the residual atmosphere or collision gas in the instrument. This study is valuable for the qualitative and quantitative mass spectrometry analysis of pesticides containing the pyrazole functional group.

Intramolecular Halogen Atom Coordinated H Transfer via Ion-Neutral Complex in the Gas Phase Dissociation of Protonated Dichlorvos Derivatives

Intramolecular halogen atom coordinated H transfer reaction in the gas phase dissociation of protonated dichlorvos derivatives has been explored by electrospray ionization tandem mass spectrometry. Upon collisional activation, protonated dichlorvos underwent dissociation reaction via cleavage of the P-O bond to give reactive ion-neutral complex (INC) intermediate, [dimethoxylphosphinoylium + dichloroacetaldehyde]. Besides direct dissociation of the complex, intramolecular chlorine atom coordinated H transfer reaction within the complex takes place, leading to the formation of protonated dimethyl chlorophosphate. To investigate the fragmentation mechanism, deuterium-labeled experiments and several other halogen-substituted (Br and F) analogs of dichlorvos were prepared and evaluated, which display a similar intramolecular halogen transfer. Density functional theory (DFT)-based calculations were performed and the computational results also support the mechanism. Graphical Abstract ᅟ.

Apparent activation of H2O and elimination of H2 from gas-phase mixed-metal complexes containing silver, calcium and deprotonated glycine

RATIONALE

Ion trap mass spectrometry was used to study the reactivity of species derived from gas-phase, mixed-metal complexes, [Ag2 Xx(Gly-H)3 ](+) , where Xx = Ca, Mg, Sr and Ag, and in particular the apparent activation of an H2 O ligand added during an ion-molecule reaction.

METHODS

Precursor [Ag2 Xx(Gly-H)3 ](+) complexes were formed by electrospray ionization (ESI) using spray solutions in which AgNO3 , XxNO3 and glycine were mixed in a 1:1:3 molar ratio. Specific species for study of ion-molecule reactions were created in a "top down" fashion using collision-induced dissociation (CID). Ion-molecule reactions were performed by selective isolation and storage in a linear ion trap, where reactions with adventitious H2 O can occur.

RESULTS

Multiple stages of CID of [Ag2 Ca(Gly-H)3 ](+) resulted in the formation of [AgHCa(Gly-H)](+) . An ion-molecule reaction of this ion produced a peak 16 mass units higher which is hypothesized to be a result of addition of H2 O followed by loss of H2 . This reaction was studied further by replacing Ca with Mg, Sr and Ag; as well as by incorporating deuterium-labelled glycine into the complex.

CONCLUSIONS

The experimental results showed the following pattern for the apparent rates of reaction: Mg > Sr > Ca. When silver is the only metal present there is an addition of water but no loss of H2 . DFT and MP2 calculations help identify plausible pathways for decomposition of H2 O and formation of H2.

Ion-molecule adduct formation in tandem mass spectrometry

Nowadays most LC-MS methods rely on tandem mass spectrometry not only for quantitation and confirmation of compounds by multiple reaction monitoring (MRM), but also for the identification of unknowns from their product ion spectra. However, gas-phase reactions between charged and neutral species inside the mass analyzer can occur, yielding product ions at m/z values higher than that of the precursor ion, or at m/z values difficult to explain by logical losses, which complicate mass spectral interpretation. In this work, the formation of adduct ions in the mass analyzer was studied using several mass spectrometers with different mass analyzers (ion trap, triple quadrupole, and quadrupole-Orbitrap). Heterocyclic amines (AαC, MeAαC, Trp-P-1, and Trp-P-2), photo-initiators (BP and THBP), and pharmaceuticals (phenacetin and levamisole) were selected as model compounds and infused in LCQ Classic, TSQ Quantum Ultra AM, and Q-Exactive Orbitrap (ThermoFisher Scientific) mass spectrometers using electrospray as ionization method. The generation of ion-molecule adducts depended on the compound and also on the instrument employed. Adducts with neutral organic solvents (methanol and acetonitrile) were only observed in the ion trap instrument (LCQ Classic), because of the ionization source on-axis configuration and the lack of gas-phase barriers, which allowed inertial entrance of the neutrals into the analyzer. Adduct formation (only with water) in the triple quadrupole instruments was less abundant than in the ion trap and quadrupole-Orbitrap mass spectrometers, because of the lower residence time of the reactive product ions in the mass analyzer. The moisture level of the CID and/or damper gas had a great effect in beam-like mass analyzers such as triple quadrupole, but not in trap-like mass analyzers, probably because of the long residence time that allowed adduct formation even with very low concentrations of water inside the mass spectrometer.

Loss of atomic nitrogen from even-electron ions? A study on benzodiazepines

The fragment spectra of protonated nitro-substituted benzodiazepines show an unusual fragment [M + H - 14]⁺ , which is shown by accurate mass measurement to be due to the loss of a nitrogen atom. Our investigations show that this apparent loss of atomic nitrogen is rather an attachment of molecular oxygen to the [M + H - NO₂ ]^+• ion, which is the main fragment ion in these spectra. The oxygen attachment is exothermic, and rate constants have been derived. MSⁿ spectra show that it is not easily reversible upon fragmentation of the adduct ion and that it is also observed with some secondary and tertiary fragments, which allows to limit the attachment site to the aromatic ring annulated to the diazepine moiety. Fragments of the oxygen adduct ion indicate that the O₂ molecule dissociates in the adduct formation process, and the two oxygen atoms are bound to different sites of the ion. Comparison with radical cations generated by fragmentation of non-nitro-substituted benzodiazepines, none of which showed an oxygen attachment, and the fragmentation mechanisms involved in their formation indicates that the [M + H - NO₂ ]^+• ion is a distonic ion with the charge and radical site neighbored on the aromatic ring. From these results, we derive a proposal for the formation and structure of the [M + H - NO₂  + O₂ ]^+• ion, which explains the experimental observations. Copyright © 2015 John Wiley & Sons, Ltd.

Reaction of arylium ions with the collision gas N2 in electrospray ionization mass spectrometry

RATIONALE

The tandem mass spectra of many compounds contained peaks which could not have arisen from the precursor ion. Such peaks were found to be due to reaction of arylium ions with N2 in the collision cell. Therefore, this reaction was studied in detail with representative compounds.

METHODS

Various classes of compounds were dissolved in acetonitrile/water/formic acid and studied by electrospray ionization mass spectrometry to record their MS(2) and pseudo-MS(3) spectra in a QqQ mass spectrometer and their accurate m/z values in an Orbitrap Elite instrument. Arylium ions were found to react with N2 in the collision cell. The reaction was confirmed by pseudo-MS(3) studies, by comparison with authentic diazonium ions, and by the pressure dependence of the product ion survival yield.

RESULTS

Reactions of arylium ions with N2 were observed with p-toluenesulfonic acid, o-toluenesulfonamide, phenylphosphonic acid, phenol, aniline, aminonaphthalenes, benzoic acid, benzophenone, and other compounds. By using a QqQ mass spectrometer, we observed that the protonated compounds produce arylium ions, which then react with N2 to form diazonium ions. The diazonium ion was produced with N2 but not with Ar in the collision cell, and its abundance increased with increasing N2 pressure.

CONCLUSIONS

Arylium ions generated from a wide variety of compounds in electrospray ionization tandem mass spectrometry may react with N2 to form diazonium ions. The abundance of the diazonium ions is affected by collision energy and N2 pressure. This reaction should be considered when annotating peaks in MS/MS libraries. Published in 2015. This article is a U.S. Government work and is in the public domain in the USA.

Unexpected peaks in tandem mass spectra due to reaction of product ions with residual water in mass spectrometer collision cells

RATIONALE

Certain product ions in electrospray ionization tandem mass spectrometry are found to react with residual water in the collision cell. This reaction often leads to the formation of ions that cannot be formed directly from the precursor ions, and this complicates the mass spectra and may distort MRM (multiple reaction monitoring) results.

METHODS

Various drugs, pesticides, metabolites, and other compounds were dissolved in acetonitrile/water/formic acid and studied by electrospray ionization mass spectrometry to record their MS(2) and MS(n) spectra in several mass spectrometers (QqQ, QTOF, IT, and Orbitrap HCD). Certain product ions were found to react with residual water in collision cells. The reaction was confirmed by MS(n) studies and the rate of reaction was determined in the IT instrument using zero collision energy and variable activation times.

RESULTS

Examples of product ions reacting with water include phenyl and certain substituted phenyl cations, benzoyl-type cations formed from protonated folic acid and similar compounds by loss of the glutamate moiety, product ions formed from protonated cyclic siloxanes by loss of methane, product ions formed from organic phosphates, and certain negative ions. The reactions of product ions with residual water varied greatly in their rate constant and in the extent of reaction (due to isomerization).

CONCLUSIONS

Various types of product ions react with residual water in mass spectrometer collision cells. As a result, tandem mass spectra may contain unexplained peaks and MRM results may be distorted by the occurrence of such reactions. These often unavoidable reactions must be taken into account when annotating peaks in tandem mass spectra and when interpreting MRM results. Published in 2014. This article is a U.S. Government work and is in the public domain in the USA.

Fragmentation studies of SIRT1-activating drugs and their detection in human plasma for doping control purposes

RATIONALE

The efficiency of Sirtuin1, a major target for the treatment of various metabolic disorders such as inflammation and type 2 diabetes mellitus, can be modulated via low molecular mass SIRT1 activators (e.g. resveratrol, SRT1720, and SRT2104).The administration of such compounds results in increased deacetylation of substrates including p53, FOXO1, and PGC1alpha, potentially leading to an improved physical performance. Consequently, proactive and preventive anti-doping measures are required and an assay dedicated to serum and plasma was desirable.

METHODS

Model substances of emerging SIRT1 drug candidates were obtained and synthesized and their mass spectrometric behavior following positive or negative electrospray ionization and collision-induced dissociation was elucidated using low and high resolution/high accuracy (tandem) mass spectrometry. Subsequently, a screening and confirmation procedure necessitating 100 μL of plasma was established employing liquid chromatography/tandem mass spectrometry (LC/MS/MS) based on diagnostic ion transitions recorded in multiple reaction monitoring mode. Sample preparation consisted of the addition of two deuterated internal standards (D(8)-SRT1720 and D(4)-resveratrol) to the plasma specimen and subsequent protein precipitation.

RESULTS

Characteristic product ions indicative of the core structures of the model analytes were characterized and utilized for the development of a multi-analyte LC/MS/MS detection method applicable to sports drug testing programs. The doping control assay was validated with regard to specificity, limits of detection (0.1-1 ng/mL), recoveries (90-98%), intraday and interday precisions (2-18%), and ion suppression/enhancement effects.

CONCLUSIONS

The fragmentation pathways of SRT1720 and 4 SIRT1 activator models based on a common thiazole-imidazole nucleus as well as two different complementary activators (SIRT1 activator 3 and CAY10602), comprising a quinoxaline core, were studied. The resulting information was used to establish and validate a sports drug testing methodology relevant for an efficient and timely anti-doping procedure, targeting a new class of emerging therapeutics possessing significant potential for misuse in elite and amateur sport.