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Here we developed a novel strategy of isotope labeling in combination with high-performance liquid chromatography-double precursor ion scan mass spectrometry (IL-LC-DPIS-MS) analysis for nontargeted profiling of thiol-containing compounds. In this strategy, we synthesized a pair of isotope labeling reagents (ω-bromoacetonylquinolinium bromide, BQB; ω-bromoacetonylquinolinium-d7 bromide, BQB-d7) that contain a reactive group, an isotopically labeled moiety, and an ionizable group to selectively label thiol-containing compounds. The BQB and BQB-d7 labeled compounds can generate two characteristic product ions m/z 218 and 225, which contain an isotope tag and therefore were used for double precursor ion scans in mass spectrometry analysis. The peak pairs with characteristic mass differences can be readily extracted from the two precursor ion scan (PIS) spectra and assigned as potential thiol-containing candidates, which facilitates the identification of analytes. BQB and BQB-d7 labeled thiol-containing compounds can be clearly distinguished by generating two individual ion chromatograms. Thus, thiol-containing compounds from two samples labeled with different isotope reagents are ionized at the same time but recorded separately by mass spectrometry, offering good identification and accurate quantification by eliminating the MS response fluctuation and mutual interference from the two labeled samples. Using the IL-LC-DPIS-MS strategy, we profiled the thiol-containing compounds in beer and human urine, and 21 and 103 thiol candidates were discovered in beer and human urine, respectively. In addition, 9 and 17 thiol candidates in beer and human urine were successfully identified by further comparison with thiol standards or tandem mass spectrometry analysis. Taken together, the IL-LC-DPIS-MS method is demonstrated to be a promising strategy in the profiling of compounds with identical groups in metabolomics study.

Characterization of the melatonin (MLT) biosynthesis pathway in plants is still limited. Additionally, a metabolomic analysis of MLT biosynthesis in plants is still a challenge due to analyte structural and chemical diversity, low analyte abundances, and plant matrix complexities. Herein, a sensitive liquid chromatography‐mass spectrometry (LC‐MS) method enabling the simultaneous determination of seven plant MLT biosynthetic metabolites was developed. In the proposed strategy, the targeted metabolites, which included tryptophan (Trp), tryptamine (TAM), 5‐hydroxytryptophan (5HTP), serotonin (5HT), N‐acetylserotonin (NAS), 5‐methoxytryptamine (5MT), and MLT, were purified from plant extracts using a one‐step dispersive solid‐phase extraction (DSPE). The samples were then chemically labeled with dansyl chloride (DNS‐Cl), followed by analysis using LC‐MS. The limit of detection (LOD) values ranged from 0.03 to 1.36 pg/mL and presented a 22‐ to 469‐fold decrease when compared to the unlabeled metabolites. Due to the high sensitivity of the proposed method, the consumption of plant materials was reduced to 10 mg FW. Ultimately, the established method was utilized to examine the distributions of MLT and its intermediates in rice shoots and roots with or without cadmium (Cd) stress. The results suggested that under normal condition, MLT may also be generated via a Trp/TAM/5HT/5MT/MLT path (Pathway II) in addition to the previously reported Trp/TAM/5HT/NAS/MLT path (Pathway I), although Pathway I was shown to be dominant. During Cd stress, MLT was also shown to be produced through these two pathways, with Pathway II shown to be dominant in rice shoots and roots.

BackgroundPhytohormones play crucial roles in almost all stages of plant growth and development. Accurate and simultaneous determination of multiple phytohormones enabled us to better understand the physiological functions and the regulatory networks of phytohormones. However, simultaneous determination of multiple phytohormones in plant is still a challenge due to their low concentrations, structural and chemical diversity, and complex matrix of plant tissues. Therefore, development of a simple and selective method for the simultaneous determination of multiple phytohormones is highly needed.ResultsWe developed a clean-up strategy for profiling of multiple phytohormones, which can overcome the challenge of structural and chemical diversity. By using a one-step dispersive solid-phase extraction (DSPE) combined with UPLC–MS/MS, 54 phytohormones including auxins, ABA, SA, JA, GAs and CKs were simultaneously analyzed from a single rice sample extract. Using the developed method, we investigated the spatiotemporal distribution of phytohormones in rice. The profiling of various tissues of rice at different growth stages revealed the complexity of metabolic regulation and allocations of phytohormone species.ConclusionA rapid one-step method was developed for the simultaneous analysis of six groups of phytohormones, including cytokinins, auxins, salicylic acid, jasmonates, abscisic acid and gibberellins in a single run, using UPLC–ESI–MS/MS. The proposed method was successfully applied to investigate spatiotemporal distribution of multiple phytohormones in rice. The spatiotemporal information obtained may be helpful for better understanding of phytohormones functions throughout life cycle of rice when integrated into transcriptome and other omics data.Electronic supplementary materialThe online version of this article (doi:10.1186/s13007-016-0147-1) contains supplementary material, which is available to authorized users.

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