A non-covalent dimer formed in electrospray ionisation mass spectrometry behaving as a precursor for fragmentations

A non-covalent-bonded dimer was detected in the positive ion electrospray ionisation (ESI) mass spectra of a synthetic impurity. In tandem mass spectrometry (MS/MS) experiments using collision-induced dissociation (CID), the ion was found to behave as a [M+H]+-type precursor ion for fragmentation until MS5. The dimer was probably formed through multi-hydrogen bonds over a proton bridge. When the fragmentation occurred at the center of the bridge, the dimer was broken apart to give monomer fragments at MS6. However, no corresponding deprotonated dimer [2M-H]- was found in the negative ion ESI spectra. The dimer was extremely stable, and it could still be observed when a fragmentation voltage of up to 50 V was applied in the ionisation source. The formation of the non-covalent dimer was also found to be instrument-dependent, but independent of sample concentration. Accurate mass measurements of the [2M+H]+ and [M+H]+ ions, and their MSn product ions, provided the basis for assessing the fragmentation mechanism proposed for [2M+H]+. The fragmentation pathway was also illustrated for the deprotonated molecule [M-H]-.     

Electrospray ionisation and ion-trap fragmentation of substituted 3,4-dihydro-2(1H)-pyridin-2-ones

A variety of 5-alkoxycarbonyl-4-aryl-6-methyl-3,4-dihydro-2(1H)-pyridones and hexahydrofuro[3,4-b]-2(1H)-pyridones have been investigated by electron impact (EI) and electrospray ionisation (ESI) techniques. Sequential product ion fragmentation (MS(n)) was performed to elucidate the degradation pathways for these compounds. Comparisons are made between positive and negative even-electron ions from ESI spectra and the molecular radical cations obtained under EI conditions. The data collected in this paper provide information on the strong impact that different substituents have on the ion fragmentation process.     

A study of the electrospray ionisation of selected coumarin derivatives and their subsequent fragmentation using an ion trap mass spectrometer

The electrospray ionisation (ESI) of selected coumarin derivatives and their subsequent fragmentation using an ion trap mass spectrometer have been investigated. Sequential product ion fragmentation experiments (MS(n)) were performed in order to elucidate the degradation pathways for these compounds. A comparison was also made between these ESI spectra and those obtained under electron impact (EI) conditions. The data presented in this paper provides useful information on the effect of different substituents on the ionisation/fragmentation processes and can be used in the characterisation of these compounds.     

A study of the electrospray ionisation of pharmacologically significant 1,4-benzodiazepines and their subsequent fragmentation using an ion-trap mass spectrometer

The electrospray ionisation (ESI) of sixteen pharmacologically significant 1,4-benzodiazepines and their subsequent fragmentation using an ion-trap mass spectrometer have been investigated. Sequential product ion fragmentation experiments (MSn) were performed in order to elucidate the degradation pathways for these compounds. Comparisons were also made between these ESI spectra and those obtained under electron impact (EI) conditions. The data presented in this paper provide useful information on the effect of different substituents on the ionisation/fragmentation processes and can be used in the characterisation of this important class of drugs and their metabolites.     

A study of the electrospray ionisation and ion-trap fragmentation of [M - H](-) ions of new 3,5-disubstituted tetrahydro-2H-1,3,5-thiadiazin-2-thiones

The electrospray ionisation (ESI) in negative mode of the pharmacologically significant 3,5-disubstituted tetrahydro-2H-1,3,5-thiadiazin-2-thiones, and their subsequent fragmentations using an ion-trap mass spectrometer, have been investigated. Experiments on sequential product ion fragmentations (MS(n)) were performed in order to elucidate the degradation pathways for these compounds. The data presented show that the fragmentation of the even-electron [M - H](-) ions could proceed through an internal nucleophilic substitution displacement. Decarboxylation and extrusion of carbon disulfide are other fragmentations observed.     

Characterisation of anthocyanidins by electrospray ionisation and collision-induced dissociation tandem mass spectrometry.

The present study describes the use of electrospray ionisation mass spectrometry, in combination with collision-induced dissociation (CID) and tandem mass spectrometry, for the structural characterisation of anthocyanidins and their O-glycosides. The high-energy CID spectra of [M-Cl](+) ions of the free aglycones show characteristic fragmentation pathways, which provide useful information about the substitution pattern in the A- and B-rings of each compound. The major fragmentation observed in the high-energy CID spectra of [M-Cl](+) ions of anthocyanins involves loss of the mono- or disaccharide units resulting in ions containing only the aglycone moiety. From the spectral data, the identity of the aglycone can be established as well as the number and the class of monosaccharide units in the O-glycosides.     

A study of the electrospray ionisation and ion trap fragmentation of hemiterpenoid and dimeric coumarin derivatives

The electrospray ionisation (ESI) of selected hemiterpenoid and dimeric coumarin derivatives and their subsequent fragmentation using an ion trap mass spectrometer are reported and discussed. Sequential product ion fragmentation experiments (MS(n)) were performed in order to elucidate the degradation pathways for these compounds. The results illustrate that the observed characteristic fragmentation patterns are of considerable utility in the application of ESI mass spectrometry to the characterisation of this class of compounds.     

Collision-induced fragmentation pathways including odd-electron ion formation from desorption electrospray ionisation generated protonated and deprotonated drugs derived from tandem accurate mass spectrometry

The rapid desorption electrospray ionisation (DESI) of some small molecules and their fragmentation using a triple-quadrupole and a hybrid quadrupole time-of-flight mass spectrometer (Q-ToF) have been investigated. Various scanning modes have been employed using the triple-quadrupole instrument to elucidate fragmentation pathways for the product ions observed in the collision-induced dissociation (CID) spectra. Together with accurate mass tandem mass spectrometry (MS/MS) measurements performed on the hybrid Q-ToF mass spectrometer, unequivocal product ion identification and fragmentation pathways were determined for deprotonated metoclopramide and protonated aspirin, caffeine and nicotine. Ion structures and fragmentation pathway mechanisms have been proposed and compared with previously published data. The necessity for elevated resolution for the differentiation of isobaric ions are discussed.     

The fragmentation pathways of protonated Amiton in the gas phase: towards the structural characterisation of organophosphorus chemical warfare agents by electrospray ionisation tandem mass spectrometry

Amiton (O,O-diethyl-S-[2-(diethylamino)ethyl] phosphorothiolate), otherwise known as VG, is listed in schedule 2 of the Chemical Weapons Convention (CWC) and has a structure closely related to VX (O-ethyl-S-(2-diisopropylamino)ethylmethylphosphonothiolate). Fragmentation of protonated VG in the gas phase was performed using electrospray ionisation ion trap mass spectrometry (ESI-ITMS) and revealed several characteristic product ions. Quantum chemical calculations provide the most probable structures for these ions as well as the likely unimolecular mechanisms by which they are formed. The decomposition pathways predicted by computation are consistent with deuterium-labeling studies. The combination of experimental and theoretical data suggests that the fragmentation pathways of VG and analogous organophosphorus nerve agents, such as VX and Russian VX, are predictable and thus ESI tandem mass spectrometry is a powerful tool for the verification of unknown compounds listed in the CWC.     

Elucidation of the fragmentation pathways of azaspiracids, using electrospray ionisation, hydrogen/deuterium exchange, and multiple-stage mass spectrometry

Collision-induced dissociation (CID) mass spectra were generated for azaspiracids using electrospray ionisation (ESI), and hydrogen/deuterium (H/D) exchange was used to ascertain the number and type of replaceable hydrogens in the three predominant azaspiracid toxins. H/D exchange was conveniently achieved using deuterated solvents for liquid chromatography (LC). Using ion-trap mass spectrometry, multiple-stage CID experiments (MS(n)) on the protonated and fully exchanged ions were performed to decipher characteristic fragmentation pathways. The precursor and product ions from azaspiracids lost up to five water molecules from different regions during MS(n) experiments and it was possible to distinguish between the water losses from different molecular regions. These studies confirmed that the first water-loss ion in the spectra of azaspiracids resulted from dehydration at the vicinal diol at C20-C21. Five MS dissociation pathways were identified that resulted from fragmentation of the carbon skeleton of azaspiracids producing nitrogen-containing ions. Two pathways, involving cleavage of the E-ring and C27-C28, gave ions that were found in all azaspiracids. Three pathways, A-ring, C-ring and C19-C20 cleavages, were useful for distinguishing between azaspiracid analogues. The same product ions from backbone fragmentation were also observed using hybrid quadrupole time-of-flight mass spectrometry (QqTOFMS). The fragmentation of the A-ring was the most facile and was exploited in the development of LC/MS(n) methods for the analysis of azaspiracids.     

Electrospray ionisation mass spectral studies on hydrolysed products of sulfur mustards

Off-site detection of the hydrolysed products of sulfur mustards in aqueous samples is an important task in the verification of Chemical Weapons Convention (CWC)-related chemicals. The hydrolysed products of sulfur mustards are studied under positive and negative electrospray ionisation (ESI) conditions using an additive with a view to detecting them at trace levels. In the presence of cations (Li(+), Na(+), K(+) and NH(4) (+)), the positive ion ESI mass spectra of all the compounds include the corresponding cationised species; however, only the [M+NH(4)](+) ions form [M+H](+) ions upon decomposition. The tandem mass (MS/MS) spectra of [M+H](+) ions from all the hydrolysed products of the sulfur mustard homologues were distinct and allowed these compounds to be characterised unambiguously. Similarly, the negative ion ESI mass spectra of all the compounds show prominent adducts with added anions (F(-), Cl(-), Br(-), and I(-)), but the [M-H](-) ion can only be generated by decomposition of an [M+F](-) ion. The MS/MS spectra of the [M-H](-) ions from all the compounds result in a common product ion at m/z 77. A precursor ion scan of m/z 77 is shown to be useful in the rapid screening of these compounds in aqueous samples at trace levels, even in the presence of complex masking agents, without the use of time-consuming sample preparation and chromatography steps. An MS/MS method developed to measure the detection limits of the hydrolysed products of sulfur mustards found these to be in the range of 10-500 ppb.     

Determination of phenolic compounds in rose hip (Rosa canina) using liquid chromatography coupled to electrospray ionisation tandem mass spectrometry and diode-array detection

Liquid chromatography coupled with negative and positive electrospray ionisation (ESI) tandem mass spectrometry (MS/MS) and diode-array detection (DAD) was used for determination of phenols in rose hip (Rosa canina) extract. ESI mass spectra of the chromatographically separated phenols gave the molecular weight of the compounds through prominent [M - H](-) ions for most of the compounds and M(+) ions for the anthocyanins. Collision induced dissociation (CID) of the [M - H](-) (or M(+)) precursor ions yielded product ions which determined the molecular weight of the aglycones. In-source fragmentation followed by CID of the resulting deprotonated aglycone ([A - H](-)) provided product ions for the identification of the unconjugated phenols. The identification was based on comparison with product ion spectra of commercial standards. UV-diode-array spectra were used for identity confirmation. This combined approach allowed the identification in rose hip extract of an anthocyanin, i.e. cyanidin-3-O-glucoside, several glycosides of quercetin and glycosides of taxifolin and eriodictyol. Phloridzin was identified, and several conjugates of methyl gallate were also found, one of which was tentatively identified as methyl gallate-rutinoside. Catechin and quercetin were found as the aglycones in the extract.     

Mass spectrometric behaviour of carboxylated polyethylene glycols and carboxylated octylphenol ethoxylates

Mass spectrometric behaviour of mono- and di-carboxylated polyethylene glycols (PEGCs and CPEGCs) and carboxylated octylphenol ethoxylates (OPECs) are discussed. The tendency for ionisation (deprotonation, protonation and cationisation by alkali metal cations) of carboxylated PEGs was compared with that of non-carboxylated correspondents by using both secondary ion mass spectrometry (SIMS) and electrospray ionisation (ESI). The fragmentation of the PEGCs and CPEGCs is discussed and also compared with their neutral correspondents, PEGs. The B/E mass spectra were recorded, using secondary ion mass spectrometry as a method for generation, for deprotonated and protonated molecules and molecules cationised by alkali metal cations. The fragmentation behaviour of PEGs is found to be different from that of CPEGCs, The presence of carboxylic groups may be confirmed not only by the determination of molecular weights of the ethoxylates studied, but also on the basis of the fragment ions formed. The metastable decomposition of the [OPEC-H](-) ions proceed through the cleavage of the bond between the octylphenol moiety and the ethoxylene chain leading to the octylphenoxy anions. It permits determination of the mass of the hydrophobic moiety of the studied carboxylated alkylphenol ethoxylate. ESI mass spectra recorded in the negative ion mode were found to be more suitable for the determination of the average molecular weight of carboxylated ethoxylates than SI mass spectra.     

Capillary column supercritical fluid chromatography-atmospheric pressure ionisation mass spectrometry interface performance of atmospheric pressure chemical ionisation and electrospray ionisation.

A supercritical fluid chromatography interface probe for atmospheric pressure ionisation mass spectrometry (API-MS) with the advantage of convenient switch between ionisation modes [atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI)] has recently been reported [P.J.R. Sj?berg, K.E. Markides, J. Chromatogr. A, 785 (1997) 101]. In order to obtain a stable ion signal and a low minimum detectable quantity, the design of the spray devise has to be optimised. For easy optimisation in the APCI mode, the corona needle was mounted directly on the interface probe. To compensate for the adiabatic cooling of the expanding mobile phase in the APCI mode, a heated region around the restrictor tip was used. In comparison, ESI required no additional heat, which might also prevent fragmentation for thermolabile compounds. As the mobile phase used was neat CO2, a low flow of make-up liquid was utilised in the ESI mode for transfer of the analytes from the expanding CO2 gas to the liquid phase before ionisation. The low make-up liquid flow in the ESI mode was sufficient for preventing the restrictor from becoming blocked. Factors that influence the ion signal intensity and stability have been studied. In APCI mode, corona needle position, nebuliser gas flow and gas additives were studied and in ESI mode, spray capillary assembly dimension and position, liquid flow-rate and composition were studied. The achievable detection limits were in the 50-0.1 pg (i.e., 290 fmol-140 amol) range. The detection limit in APCI mode was improved by a factor of about 20-25 compared to an earlier design [L.N. Tyrefors, R.X. Moulder, K.E. Markides, Anal. Chem. 65 (1993) 2835].     

The characterisation of selected antidepressant drugs using electrospray ionisation with ion trap mass spectrometry and with quadrupole time-of-flight mass spectrometry and their determination by high-performance liquid chromatography/electrospray ionisation tandem mass spectrometry

The electrospray ionisation ion trap tandem mass spectrometry (ESI-MS(n)) of selected antidepressant drugs, i.e., citalopram, fluoxetine, mirtazapine, paroxetine, sertraline, and venlafaxine, has been investigated. Sequential product ion fragmentation experiments (MS(n)) have been performed in order to elucidate the degradation pathways for the [M+H](+) ions and their predominant product ions. These MS(n) experiments show certain characteristic fragmentations in that functional groups are generally cleaved from the ring systems as molecules such as H(2)O, amines and phenols. When an aromatic entity is present in a drug molecule together with a nitrogen-containing saturated ring structure as with mirtazapine, fragmentation initially occurs at the latter ring with the former being predictably resistant to fragmentation. Also, when an amine-containing drug molecule such as fluoxetine also contains a functional group, which liberates a phenol with a significantly lower DeltaH(f) (0) value than that of the corresponding amine, the phenol is preferentially liberated. The structures of product ions proposed for ESI-MS(n) can be supported by electrospray ionisation quadrupole-time-of-flight tandem mass spectrometry (ESI-QToF-MS/MS). These molecules can be identified and determined in mixtures at low ng/mL concentrations by the application of high-performance liquid chromatography/electrospray ionisation tandem mass spectrometry (HPLC/ESI-MS(2)), which can also be used for their analysis in hair samples.     

Characterisation of humic substances using atmospheric pressure chemical ionisation and electrospray ionisation mass spectrometry combined with size-exclusion chromatography

Humic substances were analysed by atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) mass spectrometry in positive and negative modes. Using APCI the average m/z range of humic substances was reduced 5-fold compared to ESI. High-resolution time-of-flight mass spectrometry revealed the formation of multiply charged molecules in the ESI mode. Moreover, it was possible to obtain daughter ion mass spectra of humic substances by nanospray tandem mass spectrometry. The size-exclusion chromatography elution profile of humic substances was highly influenced by the pH of the analyte solution. By contrast, the pH had no significant influence on the observed mass spectra of humic substances.     

Structural characterisation of underivatised olive pulp xylo-oligosaccharides by mass spectrometry using matrix-assisted laser desorption/ionisation and electrospray ionisation

Purified olive pulp glucuronoxylans, with a Xyl/GlcA ratio of 7:1, were subjected to mild acid hydrolysis and the mixture of oligosaccharides obtained was fractionated by size exclusion chromatography. One elution fraction representative of low molecular weight oligosaccharides was analysed by mass spectrometry using matrix-assisted laser desorption/ionisation (MALDI) and electrospray ionisation (ESI) as ionisation methods, in the positive mode. Both types of spectra showed cationised molecules [M + Na](+) of xylo-oligosaccharides in a range below m/z 1,000. The xylo-oligosaccharide structures identified were series of neutral oligosaccharides of xylose (Xyl(n), n = 3-7), of acidic oligosaccharides substituted by one glucuronic acid (Xyl(n)GlcA, n = 3-5) and by two glucuronic acid residues (Xyl(n)GlcA(2), n = 2 and 3), and also of acidic oligosaccharides substituted with one 4-O-methylglucuronic acid residue (Xyl(n)meGlcA, n = 2-4). The proposed structures were confirmed by tandem mass (MS/MS) spectra obtained using collision induced dissociation of the molecular ions. Fragmentation of cationised adducts of neutral Xyl(n) yielded C- and A-type fragments, while ammonium adducts mainly yielded B-type fragments. The fragmentation of the sodium adducts of acidic oligosaccharides (Xyl(n)meGlcA, Xyl(n)GlcA) resulted in the loss of the substituting residue (GlcA or meGlcA) as the predominant fragment, while the corresponding ammonium adducts yielded B-type fragments.     

Atmospheric pressure ionisation mass spectrometric fragmentation pathways of noscapine and papaverine revealed by multistage mass spectrometry and in-source deuterium labelling

Two opium alkaloids, noscapine and papaverine, show good response as [M+H]+ ions in positive ion electrospray mass spectrometry and atmospheric pressure chemical ionisation mass spectrometry. The two compounds exhibit markedly different fragmentation pathways and behaviour under multistage mass spectrometry (MSn), with papaverine displaying a wealth of ions in MS2 and noscapine providing a single dominant ion at each stage of MSn prior to MS4. Elucidation of the fragmentation pathways using the MSn capability of the ion trap was aided by spraying the analytes in 2H2O to incorporate an isotopic label. Simplex optimisation allowed optimum trapping and fragmentation parameters to be determined, leading to a six-fold improvement in response for one transition and a seven-fold improvement for one transition sequence.     

Evaluation of three ionisation modes for the analysis of chlorinated paraffins by gas chromatography/ion-trap mass spectrometry

The applicability of three ionisation modes to the analysis of short-chain chlorinated paraffins (SCCPs) using gas chromatography coupled with mass spectrometry (GC/MS) was evaluated. MS conditions for electron ionisation (EI), positive chemical ionisation (PCI) and electron-capture negative ionisation (ECNI) were optimised using commercially available individual tetra-, penta- and hexachlorodecanes. In addition, mass spectra were studied and fragmentation pathways were proposed for each individual congener. Different fragment ions were evaluated for quantification, and ECNI-MS using [HCl2](-) and [Cl2](-*) ions was selected for the determination of SCCPs. Quality parameters (repeatability, limits of detection and calibration range) were established for the proposed method, which was then applied to the analysis of SCCPs in river sediments.     

Studies on Fragmentation Pathways of N-Ethoxy （phenyl） phosphoryl Amino Acids by Electrospray Ionization Tandem Mass Spectrometry

IntroductionPhosphorylation often acts as a molecular switchcontrolling the protein activity in different pathways asin metabolism,signal transduction,cell division,andso on.Therefore,N-phosphoryl amino acids play aspecial and important role in biological…