Configurational assignment of fragment ions by CID. Stereochemistry and mechanism of retro-Diels–Alder fragmentation accompanied by hydrogen transfer in gas-phase cations

Low energy CID mass spectra of m/z 173, C₈H₁₃O₄⁺, obtained from the diethyl ester of cis,syn,cis-l,2,3,3a,4,5,5a,6,7,8-decahydroindacene-4,5-dicarboxylic acid and cis,syn,cis-l,2,3,4,5,6,7,8,8a,9,10,10a-dodecahydrophenanthrene-9,10-dicarboxylic acid indicate they have the structure of protonated diethyl maleate. This finding together with previous deuterium labelling results suggest that the formation of this ion from both precursors takes place by migration of a hydrogen atom from an allylic δ-position followed by the concerted cleavage of two allylic C? C bonds in analogy to the ground state retro-Diels–Alder fragmentation.     

Mass spectrometric monitoring of oxidation of aliphatic C6–C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas

Experimental and theoretical studies on the oxidation of saturated hydrocarbons (n‐hexane, cyclohexane, n‐heptane, n‐octane and isooctane) and ethanol in 28 Torr O₂ or air plasma generated by a hollow cathode discharge ion source were made. Ions corresponding to [M + 15]⁺ and [M + 13]⁺ in addition to [M ? H]⁺ and [M ? 3H]⁺ were detected as major ions where M is the sample molecule. The ions [M + 15]⁺ and [M + 13]⁺ were assigned as oxidation products, [M ? H + O]⁺ and [M ? 3H + O]⁺, respectively. By the tandem mass spectrometry analysis of [M ? H + O]⁺ and [M ? 3H + O]⁺, H₂O, olefins (and/or cycloalkanes) and oxygen‐containing compounds were eliminated from these ions. Ozone as one of the terminal products in the O₂ plasma was postulated as the oxidizing reagent. As an example, the reactions of C₆H₁₄^+? with O₂ and of C₆H₁₃⁺ (CH₃CH₂CH⁺CH₂CH₂CH₃) with ozone were examined by density functional theory calculations. Nucleophilic interaction of ozone with C₆H₁₃⁺ leads to the formation of protonated ketone, CH₃CH₂C(=OH⁺)CH₂CH₂CH₃. In air plasma, [M ? H + O]⁺ became predominant over carbocations, [M ? H]⁺ and [M ? 3H]⁺. For ethanol, the protonated acetic acid CH₃C(OH)₂⁺ (m/z 61.03) was formed as the oxidation product. The peaks at m/z 75.04 and 75.08 are assigned as protonated ethyl formate and protonated diethyl ether, respectively, and that at m/z 89.06 as protonated ethyl acetate. Copyright © 2016 John Wiley & Sons, Ltd.     

CID of singly charged antioxidants applied in lubricants by means of a 3D ion trap and a linear ion trap-Orbitrap mass spectrometer

The aim of this study was to investigate the fragmentation behavior induced by low‐energy collision‐induced dissociation (LE‐CID) of four selected antioxidants applied in lubricants, by two different types of ion trap mass spectrometers: a three‐dimensional ion trap (3D‐IT) and a linear IT (LIT) Orbitrap MS. Two sterically hindered phenols and two aromatic amines were selected as model compounds representing different antioxidant classes and were characterized by positive‐ion electrospray ionization (ESI) and LE‐CID. Various types of molecular ions (e.g. [M]^+?, [M + H]⁺, [M + NH₄]⁺ or [M + Na]⁺) were used as precursor ions generating a significant number of structurally relevant product ions. Furthermore, the phenolic compounds were analyzed by negative‐ion ESI. For both IT types applied for fragmentation, the antioxidants exhibited the same unusual LE‐CID behavior: (1) they formed stable radical product ions and (2) C? C bond cleavages of aliphatic substituents were observed and their respective cleavage sites depended on the precursor ion selected. This fragmentation provided information on the type of structural isomer usually not obtainable for branched aliphatic substituents utilizing LE‐CID. Comparing the two instruments, the main benefit of applying the LIT‐Orbitrap was direct access to elemental composition of product ions enabling unambiguous interpretation of fragmentation trees not obtainable by the 3D‐IT device (e.g. loss of isobaric neutrals). It should be emphasized that the types of product ions formed do not depend on the type of IT analyzer applied. For characterizing degradation products of antioxidants, the LIT‐Orbitrap hybrid system, allowing the determination of accurate m/z values for product ions, is the method of choice. Copyright © 2011 John Wiley & Sons, Ltd.     

Differentiation of lisinopril and its RSS diastereomer by liquid chromatography combined with collision‐induced dissociation mass spectrometry

A simple and sensitive liquid chromatography tandem multiple‐stage mass spectrometry (HPLC/MS/MS) method suitable for bulk lisinopril analysis was developed, by which lisinopril and its RSS isomer were separated and differentiated. In the collision‐induced dissociation (CID) mass spectra of the [M + H]⁺ ions, the abundance of the fragment ion of m/z 246 for lisinopril was about two times higher than the ion of m/z 245; however, the former fragment ion was noted to be a little lower than the latter for RSS isomer at all collision energies. In the CID mass spectra of the [M + Li]⁺ ion, the abundance of the rearrangement ion of m/z 315 for the RSS isomer was about three times higher than that for lisinopril. Furthermore, the difference was supported by the results of energy‐resolved mass spectrometry (ERMS) in the test range of collision energies. Similar differences were also observed between the CID mass spectra of lisinopril and RSS isomer methylester, which indicated that the RSS isomer could be rapidly characterized by the CID mass spectra of both the protonated and lithium adduct ion. Elemental compositions of all the ions were confirmed by Fourier Transform ion cyclotron resonance ESI mass spectrometry (FT‐ICR‐ESI/MS). In addition, theoretical computations were carried out to support the experimental results. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of configuration of gas phase protonated ethenedicarboxylates on their low energy collision induced dissociation behaviour

Low energy collision induced dissociation (CID) spectra were measured by a triple stage quadrupole mass spectrometer for the [MH]⁺ ions of diethyl and dimethyl esters of maleic, fumaric, citraconic and mesaconic acids. A very high degree of stereospecificity was observed for the geometrically isomeric diethyl esters. The cis esters give rise to very abundant [MH? EtOH]⁺ and [MH? EtOH? C₂H₄]⁺ ions, while the trans isomers exhibit very abundant [MH? C₂H₄]⁺ and [MH? 2 C₂H₄]⁺ ions. The highly stereospecific processes indicate that the double bond configuration is retained in the protonated species under the conditions of the experiment.     

Some PhCNO-containing molecules; mass spectral similarity and molecular diversity

Some diverse compounds possessing a PhC?NO unit cleave the N? O link upon electron impact to give [PhCN]⁺˙. Different and especially significant modes of N? O scission occur in metastable processes when the oxygen atom of the PhCNO group is exocyclic to a heterocyclic nitrogen atom. Upon electron impact, rupture of the N? O bond in PhCNO-containing molecules generally dominates over 1,3-dipolar cycloreversion, which generates the radical cation of benzonitrile oxide. Stable PhCNO-containing molecules survive competing fragmentations in the ion source to produce [M+H]⁺ ions of moderate relative intensity. Other ions, which are larger than [M]⁺ are implicated where M is PhCH?NOR. Several values of m/z for metastable ions are common to compounds which have a PhCNO moiety. These m/z values generally derive both from a parent ion with m/z?105 and from a PhC?NO moiety.     

Substituent effects in the mass spectra of diethyl phenyl phosphates

The mass spectra of diethyl phenyl phosphates show substituent effects with electron-donating groups favouring the molecular ion M⁺˙, and the [M? C₂H₄]⁺˙, [M – 2C₂H₄]⁺˙ and [XPhOH]⁺˙ ions. The [PO₃C₂H₆]⁺ (m/z 109) and [PO₃H₂]⁺ (m/z 81) ions are favoured by electron-withdrawing groups. Results suggest that the formation of the [XPhC₂H₃]⁺˙ ion involves rearrangement of C₂H₃ to the position ortho to the phosphate group. Ortho effects are also observed.     

Charge reversal of the conjugate base of formamide

Charge reversal collisional activation mass spectremetry of negative ions has been used in conjunction with positive ion collisional activation to investigate several isomeric [H₂, C, N, O]⁺ ions. Generation of m/z 44 ions from formamide, acetamide, JV-methylformamide, acetaldoxime and by charge reversal of the [M–1]^? ion formed from formamide yields several different isomeric structures. Charge reversal of the conjugate base of formamide appears to yield a mixture of singlet and triplet H₂NC?O⁺ ions; experiments with deuterium-labeled compounds have been used to support this. Ab initio molecular orbital calculations indicate that the triplet ion is a stable structure, existing in a potential minimum 390.6 kJ mol^?1 above the ground state singlet.     

Structural determination of glucosylceramides isolated from marine sponge by fast atom bombardment collision‐induced dissociation linked scan at constant B/E

Five glucosylceramides (GlcCers) were isolated by reversed phase high‐performance liquid chromatography from the MeOH extracts of a marine sponge, Haliclona (Reniera) sp., collected from the coast of Ulleung Island, Korea, and analyzed by fast atom bombardment mass spectrometry (FAB–MS) in positive‐ion mode. FAB‐mass spectra of these compounds included protonated molecules [M + H]⁺ and abundant sodiated molecules [M + Na]⁺ from a mixture of m‐NBA and NaI. The structures of these GlcCers, which were similar, were elucidated by FAB‐linked scan at constant B/E. To find diagnostic ions for their characterization, the GlcCers were analyzed by collision‐induced dissociation (CID) linked scan at constant B/E. The CID‐linked scan at constant B/E of [M + H]⁺ and [M + Na]⁺ precursor ions resulted in the formation of numerous characteristic product ions via a series of dissociative processes. The product ions formed by charge‐remote fragmentation provided important information for the characterization of the fatty N‐acyl chain moiety and the sphingoid base, commonly referred to as the long‐chain base. The product ions at m/z 203 and 502 were diagnostic for the presence of a sodiated sugar ring and β‐D ‐glucosylsphinganine, respectively. For further confirmation of the structure of the fatty N‐acyl chain moiety in each GlcCer, fatty acid methyl esters were obtained from the five GlcCers by methanolysis and analyzed by FAB–MS in positive‐ion mode. On the basis of these dissociation patterns, the structures of the five GlcCers from marine sponge were elucidated. In addition, the accurate mass measurement was performed to obtain the elemental composition of the GlcCers isolated from marine sponge. Copyright © 2009 John Wiley & Sons, Ltd.     

The ortho effect in the mass spectra of ortho/para isomers of bisphenol a derivatives and related compounds

New examples of the ortho effect in bisphenol A derivatives including interaction of the hydrogen of the ortho-hydroxy group with the neighbouring aromatic ring have been observed. The characteristic ions [M ? PhOH]^+middot; (m/z = 134) and [M ? CH₃ ? PhOH]⁺ (m/z = 119) were shown to form through the hydrogen transfer from hydroxy and isopropyl groups, respectively. The spectra of cyclic derivatives having ortho-hydroxy functions show [M ? 43]⁺, [M ? C₈H₉O]⁺, m/z = 147, m/z = 135 and [M ? C₉H₁₀O]⁺ ions. The proposed mechanims of the corresponding transformations were supported by mass spectra of deuterated analogues, methyl and trimethyl silyl ethers.     

Gas-Phase Fragmentation of [M + nH + OH]<Superscript>n•+</Superscript> Ions Formed from Peptides Containing Intra-Molecular Disulfide Bonds

In this study, we systematically investigated gas-phase fragmentation behavior of [M + nH + OH]^n•+ ions formed from peptides containing intra-molecular disulfide bond. Backbone fragmentation and radical initiated neutral losses were observed as the two competing processes upon low energy collision-induced dissociation (CID). Their relative contribution was found to be affected by the charge state (n) of [M + nH + OH]^n•+ ions and the means for activation, i.e., beam-type CID or ion trap CID. Radical initiated neutral losses were promoted in ion-trap CID and for lower charge states where mobile protons were limited. Beam-type CID and dissociation of higher charge states of [M + nH + OH]^n•+ ions generally gave abundant backbone fragmentation, which was highly desirable for characterizing peptides containing disulfide bonds. The amount of sequence information obtained from CID of [M + nH + OH]^n•+ ions was compared with that from CID of disulfide bond reduced peptides. For the 11 peptides studied herein, similar extent of sequence information was obtained from these two methods.     

The [M?OH]+ ions of m- and p-ethylnitrobenzene: A common structure?

The structures of the [M? OH]⁺ ions of m- and pethylnitrobenzene have been compared by measurements of metastable ion spectra, collisional activation spectra, kinetic energy releases and critical energies for the formation of these ions and their subsequent decomposition. Normalized rates of fragmentation of metastable molecular ions and metastable [M? OH]⁺ ions have been compared for ion lifetimes up to 30 μs. The energy measurements fail to distinguish between the structures of the [M? OH]⁺ ions, but the normalized fragmentation rates and the collisional activation spectra show their structures to be different.     

The renaissance of high-energy CID for structural elucidation of complex lipids: MALDI-TOF/RTOF-MS of alkali cationized triacylglycerols

Triacylglycerols were analyzed as cationized species (Li⁺, Na⁺, K⁺) by high-energy CID at 20 keV collisions utilizing MALDI-TOF/RTOF mass spectrometry. Precursor ions, based on [M+Li]⁺-adduct ions exhibited incomplete fragmentation in the high and low m/z region whereas [M+K]⁺-adducts did not show useful fragmentation. Only sodiated precursor ions yielded product ion spectra with structurally diagnostic product ions across the whole m/z range. The high m/z region of the CID spectra is dominated by abundant charge-remote fragmentation of the fatty acid substituents. In favorable cases also positions of double bonds or of hydroxy groups of the fatty acid alkyl chains could be determined. A-type product ions represent the end products of these charge-remote fragmentations. B- and C-type product ions yield the fatty acid composition of individual triacylglycerol species based on loss of either one neutral fatty acid or one sodium carboxylate residue, respectively. Product ions allowing fatty acid substituent positional determination were present in the low m/z range enabling identification of either the sn-1/sn-3 substituents (E-, F-, and G-type ions) or the sn-2 substituent (J-type ion). These findings were demonstrated with synthetic triacylglycerols and plant oils such as cocoa butter, olive oil, and castor bean oil. Typical features of 20 keV CID spectra of sodiated triacylglycerols obtained by MALDI-TOF/RTOF MS were an even distribution of product ions over the entire m/z range and a mass accuracy of ±0.1 to 0.2 u. One limitation of the application of this technique is mainly the insufficient precursor ion gating after MS1 (gating window at 4 u) of species separated by 2 u.     

Reactivity of gaseous sodiated ions derived from benzene dicarboxylate salts toward residual water in the collision gas

The sodium adduct of disodium salts of benzene dicarboxylic acids (m/z 233), when subjected to collision‐induced dissociation (CID), undergoes a facile loss of CO₂ to produce an ion of m/z 189, which retains all the three sodium atoms of the precursor. The CID spectrum of this unusual m/z 189 ion shows significant peaks at m/z 167, 63 and 85. The enigmatic m/z 167 ion, which appeared to represent a loss of a 22‐Da neutral fragment from the precursor ion is in fact a fragment produced by the interaction of the m/z 189 ion with traces of water present in the collision gas. The change of the m/z 167 peak to 168, when D₂O vapor was introduced to the collision gas of a Q‐ToF instrument, proved that such an intervention of water could occur even in collision cells of tandem‐in‐space mass spectrometers. The m/z 189 ion has such high affinity for water; it forms an ion/molecule complex even during the brief residence time of ions in collision cells of triple quadrupole instruments. The complex formed in this way then eliminates elements of NaOH to produce the ion observed at m/z 167. In an ion trap, the relative intensity of the m/z 167 peak increases with longer activation time even at the lowest possible collision energy setting. Similarly, the m/z 145 ion (which represents the sodium adduct of phenelenedisodium, formed by two consecutive losses of CO₂ from the m/z 233 ion of meta‐ and para‐isomers) interacts with water to produce a fragment ion at m/z 123 for the sodium adduct of phenylsodium. Other uncommon ions that originate also from water/ion interactions are observed at m/z 85 and 63 for [Na₃O]⁺ and [Na₂OH]⁺, respectively. Tandem mass spectrometric experiments conducted with appropriately deuterium‐labeled compounds confirmed that the proton required for the formation of the [Na₂OH]⁺ ion originates from traces of water present in the collision gas and not from the ring protons of the aromatic moiety. Copyright © 2010 John Wiley & Sons, Ltd.     

Participation of halogen atoms in hydrogen transfer between remote positions in gas-phase ionized isomeric haloresorcinols

Mono, di- and trihaloresorcinols substituted by a halogen atom at position 2 exhibit a highly specific elimination of H₂O on electron impact ionization and under conditions of collisionally induced dissociation (CID). The high isomer specificity suggests the intermediacy of a hydrogen transfer from one of the hydroxy groups to the adjacent halogen atom. A subsequent hydrogen migration to the other hydroxy group readily explains the facile elimination of H₂O from the M^+· ions of these particular isomers. An analogous three-step hydrogen transfer has not been observed in 2,3-dihalo-l,4-hydroquinones. 4-Bromo- and 4-icdoresorcinol undergo elimination of the halogen atom followed by a very fast loss of CO under CID conditions, affording [M ? Hal]⁺ ions of low abundance and highly abundant[M ? Hal ? CO]⁺ ions. The elimination of CO is suppressed in the isomeric 5-haloresorcinols, resulting in very highly abundant [M ? Hal]⁺ ions. This behavior suggests that a ‘hidden hydrogen transfer’ accompanies the elimination of the halogen atom from the molecular ions of 4-haloresorcinols.     

Fragmentation of protonated O,O-diethyl O-aryl phosphorothionates in tandem mass spectral analysis

The gas-phase ion chemistry of protonated O,O-diethyl O-aryl phosphorothionates was studied with tandem mass spectrometric and ab initio theoretical methods. Collision-activated dissociation (CAD) experiments were performed for the [M+H]⁺ ions on a triple quadrupole mass spectrometer. Various amounts of internal energy were deposited into the ions upon CAD by variation of the collision energy and collision gas pressure. In addition to isobutane, deuterated isobutane C₄D₁₀ also was used as reagent gas in chemical ionization. The daughter ions [M+H?C₂H₄]⁺ and [M+H?2C₂H₄]⁺ dominate the CAD spectra. These fragments arise via various pathways, each of which involves γ-proton migration. Formation of the terminal ions [M+H?2C₂H₄?H₂O]⁺, [M+H?2C₂H₄?H₂S]⁺, [ZPhOH₂]⁺, [ZPhSH₂]⁺, and [ZPhS]⁺ [Z = substituent(s) on the benzene ring] suggests that (1) the fragmenting [M+H]⁺ ions of O,O-diethyl O-aryl phosphorothionates have protons attached on the oxygen of an ethoxy group and on the oxygen of the phenoxy group; (2) thiono-thiolo rearrangement by aryl migration to sulfur occurs; (3) the fragmenting rear-ranged [M+H]⁺ ions have protons attached on the oxygen of an ethoxy group and on the sulfur of the thiophenoxy group. To get additional support for our interpretation of the mass spectrometric results, some characteristics of three protomers of O,O-diethyl O-phenyl phosphorothionate were investigated by carrying out ab initio molecular orbital calculations at the RHF/3–21G* level of theory.     

Site of alkylation of N-methyl- and N-ethylaniline in the gas phase: a tandem mass spectrometric study

N-Methylaniline (NMA) was ethylated and N-ethylaniline (NEA) was methylated under chemical ionization conditions using C₂H₅I and CH₃I, respectively, as reagent gases. The structures of the resulting m/z 136 adduct ions have been probed using metastable ion and collision-induced dissociation (CID) methods. From the similarity of the spectra obtained and from the presence of structure-diagnostic ions at m/z 59 (CH₃NHC₂H₅^+•) and m/z 44 (CH₃NHCH₂⁺), it is concluded that predominantly N-alkylation occurs in both systems. This interpretation was aided by the use of C₂D₅I and CD₃I as reagents. Adduct ions of m/z 136 were also formed by ethylation of the isomeric toluidines and by methylation of the ring-ethylanilines. The resulting CID mass spectra were distinctly different from those obtained for the m/z 136 ions obtained by alkylation of NMA and NEA. Protonation of N-ethyl-N-methylaniline using CH₃C(O)CH₃ as Brønsted acid reagent produced an m/z 136 species whose CID mass spectrum also featured intense ion signals at m/z 59 and 44. This observation led to the conclusion that protonation with acetone as reagent results, in this case, in dominant N-protonation. However, the CID mass spectrum of the m/z 136 ion formed when CH₃OH was the protonating agent featured a weak signal at m/z 44 and no signal at m/z 59. Hence it was concluded that the latter m/z 136 ion contains a larger contribution from the ring-protonated adduct. Copyright © 2004 John Wiley & Sons, Ltd.     

Electron ionization mass spectra of novel 2,3:4,5-bis-O-(1-methylethylidene)-β-D-fructopyranose derivatives and related sugar sulfamates

The electron-impact (EI) mass spectral fragmentation of ten bis-O- (1-methylethylidene)fructopyranose derivatives and three related sugar sulfamates were investigated. In particular, 2,3:4,5-bis-O - (1-methylethylidene)-β-D-fructopyranose sulfamate (topiramate), a potent anticonvulsant, was examined in greater detail. The fragmentation of the 2,3:4,5-bis-O-(1-methylethylidene) fructopyranose derivatives in general was not very dependent on the nature of substitution; the mechanisms of the common and unique fragmentation patterns are presented. These compounds showed characteristic peaks at m/z [M – 15]⁺, [M – 15 – 58]⁺, [M – 15 – 58 – 60]⁺, [M ? CH₂X]⁺ and [M ? CH₂X – 58]⁺ where X = OSO₂NR₂ (R ? H, CH₃, and/or Ph), OC (O)NHR, NH₂, CI and OH. The fragmentation of isomeric bis-O-(1-methylethylidene) derivatives of aldopyranose, ketopyranose and ketofuranose sulfamates was also investigated. The results indicate that isomeric sugar sulfamates can be easily distinguished in the EI mode. Key fragmentation pathways are discussed for these compounds.     

Structural determination by atmospheric pressure photoionization tandem mass spectrometry of some compounds isolated from the SARA fractions obtained from bitumen

We have identified compounds obtained from the SARA fractions of bitumen by using atmospheric pressure photoionization mass spectrometry and low‐energy collision tandem mass spectrometric analyses with a QqToF‐MS/MS hybrid instrument. The identified compounds were isolated from the maltene saturated oil and the aromatic fractions of the SARA components of a bitumen. The QqToF instrument had sufficient mass resolution to provide accurate molecular weight information and to enhance the tandem mass spectrometry results. The APPI‐QqToF‐MS analysis of the separated compounds showed a series of protonated molecules [M + H]⁺ and molecular ions [M]^+? of the same mass but having different chemical structures, in the maltene saturated oil and the aromatic SARA fractions. These isobaric ions were a molecular ion [M₂]^+? at m/z 418.2787 and a protonated molecule [M₅ + H]⁺ at m/z 287.1625 in the saturated oil fraction, and molecular ions [M₆]^+? at m/z 418.1584 and [M₇]^+? at m/z 287.1285 in the aromatic fraction. The identification of this series of chemical compounds was achieved by performing CID‐MS/MS analyses of the molecular ions [M]^+? ([M₁]^+? at m/z 446. 2980, [M₂]^+? at m/z 418.2787, [M₃]^+? at m/z 360.3350 and [M₄]^+? at m/z 346.2095) in the saturated oil fraction and of the [M₅ + H]⁺ ion at m/z 287.1625 also in the saturated oil fraction. The observed CID‐MS/MS fragmentation differences were explained by proposed different breakdown processes of the precursor ions. The presented tandem mass spectrometric study shows the capability of MS/MS experiments to differentiate between different classes of chemical compounds of the SARA components of bitumen and to explain the reasons for the observed mass spectrometric differences. However, greater mass resolution than that provided by the QqToF‐MS/MS instrument would be required for the analysis of the asphaltene fraction of bitumen. Copyright © 2011 John Wiley & Sons, Ltd.     

Sensitivity of Positive Ion Mode Electrospray Ionization Mass Spectrometry in the Analysis of Thiol Metabolites

High signal intensities of glutathione (GSH), [GSH+H]⁺ (m/z 308), cysteine (CySH), [CySH+H]⁺ (m/z 122), and homocysteine (hCySH), [hCySH+H]⁺ (m/z 136), are observed in ESI MS with on‐line electrochemistry (EC). Dimers formed by H‐bonding, which are not electrochemical products, are detected as [2GSH+H]⁺ (m/z 615), [2CySH+H]⁺ (m/z 243) and [2hCySH+H]⁺ (m/z 271) together with disulfide dimers GSSG, CySSCy and hCySSCyh, [GSSG+H]⁺ (m/z 613), [CySSCy+H]⁺ (m/z 241) and [hCySSCyh+H]⁺ (m/z 269). When dopamine is present a thiol/dopamine quinone (DAQ) adduct is observed. Formation of this adduct is proposed to occur by an electrochemical mechanism during ESI. Catalysis of thiol oxidation and analysis of thiol mixtures is addressed.