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indoles detection by electrospray ionization mass spectrometry

Indole skeletons are present in numerous alkaloid products, pharmaceuticals, and agrochemicals. Cyclopenta[b]indole skeletons can be found in a number of indole alkaloids, and compounds containing them show various biological activities. Such compounds include antidepressants, anti-implantation agents, and effective antagonists of the prostaglandin D2 receptor. The structure’s potential biological applicability has led to organic synthetic studies so as to facilitate its production. A reliable and rapid analysis of the compounds’ metabolites in complex matrices is also highly desirable, as is a method of screening their synthetic reactions. Mass spectrometry (MS), especially coupled with electrospray ionization (ESI), is an important physicochemical method for the analysis of organic compounds. The soft ionization of ESI does not destroy the analyte molecules, even thermally unstable intermediates. Protonated ions are usually generated by ESI–MS in positive-ion mode; other ionic adducts include [M+Na] and [M+K]. A few odd-electron ions have been observed as M ̇ or [M H+Na] ̇. Although anomalous [M H] ions for some cyclic acetals and derivatized monosaccharides have been observed in fast-atom bombardment/liquid-assisted secondary ion LSIMS)/LSIMS), only [M+H] ions have been observed in significant amounts from these compounds by ESI. Several [M H] ions in the chemical ionization mass spectra of some ring C-substituted methyl dehydroabietate derivates were reported. In the present study, a series of unexpected [M H] ions from cyclopenta[b]indoles were detected by ESI–MS. These ions were further studied using collision-induced dissociation (CID) MS/MS. High performance liquid chromatography grade methanol, formic acid, and sodium hydrate were purchased from Fisher Scientific (Pittsburgh, PA, USA), Fluka (Switzerland), and Alfa Aesar (Ward Hill, Ma, USA). The mixture of formic acid and sodium hydrate was used as the mass calibration standard. CD3OD was obtained from Cambridge Isotope Laboratories(Andover, MA, USA). Ultrapure water was prepared using an ultrapure water system from Aquapro (Chongqing, China). Compounds containing cyclopenta[b]indole skeletons (Fig. 1) were provided by professor Qixiang Guo. MS experiments were performed using a Bruker micrOTOF-Q MS (Bremen, Germany). Helium was used as the collision gas. Nitrogen was used as the drying and nebulizing