Observation of the intermediates of in-source aldolization reaction in electrospray ionization mass spectrometry analysis of heteroaromatic aldehydes

Formation of the exceptional [M - H]+ cation in atmospheric pressure ionization mass spectrometry analysis of 2-(diphenylsilyl) cyclopropanecarboxylate esters

RATIONALE

In general, ionization of analytes in atmospheric pressure ionization mass spectrometry (API-MS) in positive ion mode results in the formation of protonated molecules ([M + H]⁺ ) and/or cationized molecules (e.g., [M + Na]⁺ ). The formation of specific [M - H]⁺ cations in the API process is of significant interest for further investigation.

METHODS

The ionization processes of 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate esters were investigated using electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization-MS in positive ion mode. Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level.

RESULTS

The anomalous [M - H]⁺ ion and the regular [M + Na]⁺ ion were both observed using ESI-MS. Interestingly, no [M + H]⁺ ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent accelerated the formation of [M - H]⁺ rather than [M + H]⁺ ion. DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate (1) indicated that the [1 + H]⁺ ion can thermodynamically and kinetically undergo facile H₂ elimination to generate [1 - H]⁺ .

CONCLUSIONS

The favorable formation of [M - H]⁺ ions in these compounds is attributed to the unique diphenylhydrosilyl group in their structure. The [M + H]⁺ ion formed easily underwent H₂ elimination to produce the [1 - H]⁺ ion in the API source, and thus, acidification of the ESI solvent apparently accelerates the formation of the [1 - H]⁺ ion.

Gas phase reaction between chromones and solvent in an electrospray ionization source

Chromones were measured by using electrospray ionization mass spectrometry in negative mode. Interestingly, in addition to the deprotonated ion ([M - H]^- ), unexpected [M + 17]^- and [M + 31]^- ions were observed in high intensity when water and methanol were used as the solvent. Chromones with different substitutes were tested. Compared with the deprotonated ion, [M + 17]^- and [M + 31]^- ions were observed with higher abundances when the C-3 site of chromones was substituted by electron withdrawing groups. Based on high performance liquid chromatography-mass spectrometry (LC-MS), deuterium-labeling and collisional-induced dissociation experiments, a covalent gas-phase nucleophilic addition reaction between chromone and water, and the formation of a noncovalent complex between chromone and methanol were proposed as the mechanism for the observed [M + 17]^- and [M + 31]^- ions, respectively. Understanding and using these unique gas phase reactions can avoid misannotation when analyzing chromones and their metabolites.

Two competing ionization processes in ESI-MS analysis of N-(1,3-diphenylallyl)benzenamines: formation of the unusual [M-H]+ ion versus the regular [M+H]+ ion

A series of N-(1,3-diphenylallyl)benzenamine derivatives (M) were investigated by electrospray ionization mass spectrometry in the positive-ion mode. Both the anomalous [M-H]⁺ and the regular [M+H]⁺ were observed in the ESI mass spectra. The occurrence of [M-H]⁺ has been supported by accurate mass spectrometry, liquid chromatography mass spectrometry, and tandem mass spectrometry analysis. Calculation results indicated that formation of [M-H]⁺ is attributed to the ion-molecule reaction of M with the protonated ESI solvent molecule (e.g. CH₃OH₂⁺) via hydride abstraction from a tertiary C_sp3-H. The competing ionization processes leading to [M-H]⁺ or [M+H]⁺ were significantly affected by the concentration of formic acid in the electrospray ionization solvent and the proton affinity of the N atom.

Degradation of the Neonicotinoid Pesticides in the Atmospheric Pressure Ionization Source

During the analysis of neonicotinoid pesticide standards (thiamethoxam, clothianidin, imidacloprid, acetamiprid, and thiacloprid) by mass spectrometry, the degradation of these pesticides (M-C=N-R is degraded into M-C=O, M is the skeleton moiety, and R is NO₂ or CN) was observed in the atmospheric pressure ionization interfaces (ESI and APCI). In APCI, the degradation of all the five neonicotinoid pesticides studied took place, and the primary mechanism was in-source ion/molecule reaction, in which a molecule of water (confirmed by use of H₂¹⁸O) attacked the carbon of the imine group accompanying with loss of NH₂R (R=NO₂, CN). For the nitroguanidine neonicotinoid pesticides (R=NO₂, including thiamethoxam, clothianidin, and imidacloprid), higher auxiliary gas heater temperature also contributed to their degradation in APCI due to in-source pyrolysis. The degradation of the five neonicotinoid pesticides studied in ESI was not significant. In ESI, only the nitroguanidine neonicotinoid pesticides could generate the degradation products through in-source fragmentation mechanism. The degradation of cyanoamidine neonicotinoid pesticides (R=CN, including acetamiprid and thiacloprid) in ESI was not observed. The degradation of neonicotinoid pesticides in the ion source of mass spectrometer renders some adverse consequences, such as difficulty interpreting the full-scan mass spectrum, reducing the sensitivity and accuracy of quantitative analysis, and misleading whether these pesticides have degraded in the real samples. Therefore, a clear understanding of these unusual degradation reactions should facilitate the analysis of neonicotinoid pesticides by atmospheric pressure ionization mass spectrometry. Graphical Abstract.