•Creatine phosphate disodium salt was analyzed using ESI-QTOF.•A seven-centered rearrangement mechanism was proposed.•D-labeled experiment and DFT calculation support proposed mechanism.•The product ion formed by condensation reaction was detected.Creatine phosphate disodium salt was investigated by electrospray ionization quadrupole time-of-flight tandem mass spectrometry (ESI-QTOF-MS/MS) both in positive- and negative-ion modes. The results indicate that an unusual seven-centered rearrangement mechanism was involved in the whole fragmentation. The positive product ion at m/z 165 is formed through the seven-centered rearrangement from the precursor ions of [M+Na]+ at m/z 278 and further confirmed by D-labeled experiment and DFT calculation. The negative product ion at m/z 190 resulting from the precursor ion [M−Na]− at m/z 232 is formed through the seven-centered rearrangement, accompanied with internal residue loss of a methanediimine molecule. In addition, the processes producing the product ion at m/z 97 from [M−2Na+H]− at m/z 210 also involved a seven-centered rearrangement.

A series of diarylmethylamine compounds were analyzed using electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS). [M]+˙ and [M − H]+ were both observed, but showed different abundances. A possible mechanism for the formation of [M]+˙ and [M − H]+ was proposed to explicate the rule for the ratio change of I[M]+˙/I[M−H]+. The [M]+˙ has two structures, which can interconvert into each other in the gas phase. The substituted groups on the benzene rings play a crucial role in the transfer between the two structures. Electron withdrawing groups can prevent the formation of carbocations, thus nitro-containing diarylmethylamines remained mainly as structure I and were detected as [M]+˙. On the contrary, electron donating groups help to stabilize carbocations. This makes structure I transfer to structure II, and structure II prefers to further generate [M − H]+ by loss of an H radical. Nuclear magnetic resonance and D-labelled MS experiments indicate that the 1-C–H bond has strong activity.

•MS3 experiment was firstly used to reveal the H/D scrambling prior to fragmentation.•Protonation sites lead to the fragmentation difference of isomers.•Amino plays a key role for the H/D scrambling by firstly experiment of comparison.•The H/D scrambling involves alkyl hydrogen and the mechanism was firstly proposed.•The transfer of methoxyl was firstly reported using ESI-CID-MS/MS.Tryptophan and its analogues were analyzed using electrospray ionization quadrupole-time of flight (ESI-QTOF) combined with hydrogen–deuterium exchange. The similar profiles of MS3 spectra of the product ions at m/z 190, 191, and 192 from [M (D)+D]+ for tryptophan by the losses of NH(D)3 imply the same fragmentation patterns including the characteristic transfer of hydroxyl and very similar structures of these ions. This reveals the H/D scrambling prior to fragmentation. Compared the MS/MS spectrum of D-labelled compound 2 to the MS3 spectrum of m/z 191, it was found that amino group plays an important role for the H/D scrambling. The MS2 and MS3 spectra for D-labelled compound 4 indicate that the H/D scrambling involves the hydrogen on methyl linked to indole ring. In addition, the fragmentation of transfer of hydroxyl or methoxyl is confirmed by comparing the spectra of compounds 1–4.

•Three sesquiterpene pyridine alkaloids were analyzed by ESI–QTOF.•Product ion at m/z 310 formed by an ion–dipole complex was detected.•Processes forming m/z 310 was related to the position of substituted groups.•Close distance contributes to the addition reaction producing m/z 310.Sesquiterpene pyridine alkaloids are a large group of highly oxygenated sesquiterpenoids, attracting attention in the fields of medicine because of their significant biological activities. Three representative sesquiterpene pyridine alkaloids were analyzed by electrospray ionization collision-induced dissociation–tandem mass spectrometry (ESI–CID–MS/MS/MS). In the high mass range, the product ions were formed by the loss of side chains or H2O. In the low mass range, the high-abundance product ions at m/z 206 and 178 were the characteristic ions of the pyridine moiety. Interestingly, the characteristic product ion at m/z 310 was formed through an ion–dipole (ion–neutral) complex. Addition of oxo(phenyl)methylium formed by the loss of the side chain linked to C1 and pyridine moiety produces m/z 310. The product ions at m/z 188 and 105 from m/z 310 support the proposed processes. The product ions were supported by d-labeling experiment. It was found that the formation of the product ion at m/z 310 was related to the position of substituted groups.

•Two diarylmethylamines were analyzed using ESI-QTOF.•Anomalous [M]+ ions were detected.•The radical ions may be formed by losing an electron from CH bond.•Solvent is not involved in the formation of [M]+Two diarylmethylamines were analyzed using electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF). Anomalous [M]+ ions were detected. Interestingly, although 4-phenoxyphenol derivatives may produce radical ions by oxidation, the radical ions in diarylmethylamines detection may be formed by losing an electron from CH bond. It was found that both the electron-donating effect of 4-methoxyaniline group and conjugation effect of the two aryl groups play an important role in stabilizing the tertiary carbon radical. Tandem mass spectra further supported the proposed structure of [M] +. In addition, solvent is not involved in the formation of [M] +.

•Anomalous [M−H]+ ions for indole derivatives were detected in positive-ion mode.•Possible pathways forming [M−H]+ were proposed.•MS/MS spectra and radical trapping experiment support direct loss of hydride anion.•Positions losing the hydride anion were confirmed by D-labeled and MS/MS spectra.A series of six C-3 alkyl-substituted indole derivatives were investigated using electrospray, ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS). Anomalous [M−H]+ ions, were detected. Three possible pathways forming [M−H]+ were proposed. MS/MS spectra and radical, trapping experiment support direct loss of hydride anion. Notably, the positions losing the hydride, anion were different for these compounds, and were tentatively confirmed by D-labeled and MS/MS, spectra. Both steric hindrance and electronic effect might contribute the difference.

Paeoniflorin and albiflorin were investigated by electrospray quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS) in positive-ion mode, when ultrapure water was selected as solvent. In full-scan mode, the ions [M+H2O+Ca]2+, [M+2H2O+Ca]2+, [2M+Ca]2+, [3M+Ca]2+, and [4M+Ca]2+ as the primary ions were detected. Low-energy collision-induced experiments for the precursor [M+H2O+Ca]2+ and [2M+Ca]2+ ions provided many interesting product ions. Most product ions are formed by combinations of a paeoniflorin (or albiflorin) molecule, benzoic acid anion, aglycone moiety, glucosyl group, H2O, or Ca2+ ion. Strong noncovalent bond formed by Ca2+ and O atom of neutral molecules and proper coordination number might contribute to producing these complex ions. A labeling experiment involving H/D exchange of paeoniflorin indicates the H2O molecule did not come from solvent. Although the fragmentation patterns of the isomers of paeoniflorin and albiflorin are similar, they can be completely distinguished by the MS/MS spectra of either [M+H2O+Ca]2+ or [2M+Ca]2+.Graphical abstractHighlights► Calcium adduct molecules as the most abundant primary ions were detected in positive-ion mode. ► Many interesting complex product ions were detected in MS/MS mode. ► Complex product ions might contribute to understanding the unique character of calcium adduct molecules in the gas phase. ► Sufficient information obtained from calcium adduct molecules is especially valuable for rapid identification of the isomers.

•A series of six matrine-type alkaloids was investigated using ESI-MS/MS.•A pair of stereoisomers was unambiguously distinguished using CID and DFT calculations.•Classical fragmentation patterns and radical product ions are of scientific interest.•The protonation site was tentatively identified.A series of six matrine-type alkaloids, including a pair of stereoisomers, was investigated using electrospray ionization quadrupole time-of-flight tandem mass spectrometry (ESI-QTOF-MS/MS) in positive-ion mode. Protonated molecules were fragmented by collision-induced dissociation (CID) and the products identified. The identity of the major product ions was also supported by deuterium-labeling experiments. It was found that sequential four-centered H rearrangements play a significant role in the fragmentation of the piperidine-ring; the RDA reaction dominates the dissociations for sophocarpine and sophocarpidine; and the McLafferty-type rearrangement occurs with neosophoramine. Interestingly, a pair of stereoisomers matrine and sophoridine, can be unambiguously distinguished using CID, and this characterization was supported by the DFT calculations of the potential energy profiles for protonated matrine and sophoridine before dissociation to the product ion at m/z 152.

Iridoid glycosides are an important class of natural products and have many biological activities. Iridoid glucosides in an extract of the plant species Paederia scandens were investigated using reversed-phase high performance liquid chromatography and electrospray quadrupole time-of-flight-type tandem mass spectrometry. The elemental composition of most of the compounds was determined by accurate mass and relative isotopic abundance (RIA) measurements. In positive ion mode, the fragmentation of [M+NH4]+ precursor ions was carried out using low energy collision-induced electrospray ionization tandem spectrometry. The neutral losses of NH3, H2O, Glc, and the side chain of the iridoid moiety were the main fragmentation patterns observed. For simple iridoid glycosides, the main differences were related to the side chains. Fragmentation of the [M−H]−precursor ions was achieved for the compounds possibly having phenolic acid group. The connection order of the iridoid, sugar, and phenolic acid moieties, and the linkage of the 6-OH group of the sugar to the phenolic acid were unambiguously confirmed using a combination of MS/MS spectra in both positive and negative ion modes, and our previous work. For some trace dimeric iridoid glucosides, the connection order between the asperuloside and paederoside moieties was determined by the characteristic product ions; this was supported by D-labeling experiments. A total of 24 iridoid glucosides, including 14 new species, were identified or tentatively characterized based on exact mass, RIA values, tandem mass spectra, and D-labeling experiments.Highlights► A total of 24 iridoid glucosides (14 new ones) were characterized using HPLC–ESI-MS. ► The difference mainly focuses on the side chain of iridoid moiety for simple ones. ► The connection order of iridoid, sugar and phenolic acid moieties can be confirmed. ► The acylation of 6-OH of sugar by phenolic acid can be determinated. ► Connection order between asperuloside and paederoside moieties can be confirmed.