Investigation of the electrochemical oxidation products of zotepine and their fragmentation using on-line electrochemistry/electrospray ionization mass spectrometry

Mass spectrometric methods for monitoring redox processes in electrochemical cells

Electrochemistry (EC) is a mature scientific discipline aimed to study the movement of electrons in an oxidation-reduction reaction. EC covers techniques that use a measurement of potential, charge, or current to determine the concentration or the chemical reactivity of analytes. The electrical signal is directly converted into chemical information. For in-depth characterization of complex electrochemical reactions involving the formation of diverse intermediates, products and byproducts, EC is usually combined with other analytical techniques, and particularly the hyphenation of EC with mass spectrometry (MS) has found broad applicability. The analysis of gases and volatile intermediates and products formed at electrode surfaces is enabled by differential electrochemical mass spectrometry (DEMS). In DEMS an electrochemical cell is sampled with a membrane interface for electron ionization (EI)-MS. The chemical space amenable to EC/MS (i.e., bioorganic molecules including proteins, peptides, nucleic acids, and drugs) was significantly increased by employing electrospray ionization (ESI)-MS. In the simplest setup, the EC of the ESI process is used to analytical advantage. A limitation of this approach is, however, its inability to precisely control the electrochemical potential at the emitter electrode. Thus, particularly for studying mechanistic aspects of electrochemical processes, the hyphenation of discrete electrochemical cells with ESI-MS was found to be more appropriate. The analytical power of EC/ESI-MS can further be increased by integrating liquid chromatography (LC) as an additional dimension of separation. Chromatographic separation was found to be particularly useful to reduce the complexity of the sample submitted either to the EC cell or to ESI-MS. Thus, both EC/LC/ESI-MS and LC/EC/ESI-MS are common.

Detection of oxygen addition peaks for terpendole E and related indole-diterpene alkaloids in a positive-mode ESI-MS

This report describes that a regular positive electrospray ionization mass spectrometry (MS) analysis of terpendoles often causes unexpected oxygen additions to form [M + H + O](+) and [M + H + 2O](+), which might be a troublesome in the characterization of new natural analogues. The intensities of [M + H + O](+) and [M + H + 2O](+) among terpendoles were unpredictable and fluctuated largely. Simple electrochemical oxidation in electrospray ionization was insufficient to explain the phenomenon. So we studied factors to form [M + H + O](+) and [M + H + 2O](+) using terpendole E and natural terpendoles together with some model indole alkaloids. Similar oxygen addition was observed for 1,2,3,4-tetrahydrocyclopent[b]indole, which is corresponding to the substructure of terpendole E. In tandem MS experiments, a major fragment ion at m/z 130 from protonated terpendole E was assigned to the substructure containing indole. When the [M + H + O](+) was selected as a precursor ion, the ion shifted to m/z 146. The same 16 Da shift of fragments was also observed for 1,2,3,4-tetrahydrocyclopent[b]indole, indicating that the oxygen addition of terpendole E took place at the indole portion. However, the oxygen addition was absent for some terpendoles, even whose structure resembles terpendole E. The breakdown curves characterized the tandem MS features of terpendoles. Preferential dissociation into m/z 130 suggested the protonation tendency at the indole site. Terpendoles that are preferentially protonated at indole tend to form oxygen addition peaks, suggesting that the protonation feature contributes to the oxygen additions in some degrees.

Selective separation, detection of zotepine and mass spectral characterization of degradants by LC-MS/MS/QTOF

A simple, precise, accurate stability-indicating gradient reversed-phase high-performance liquid chromatographic (RP–HPLC) method was developed for the quantitative determination of zotepine (ZTP) in bulk and pharmaceutical dosage forms in the presence of its degradation products (DPs). The method was developed using Phenomenex C₁₈ column (250 mm×4.6 mm i.d., 5 µm) with a mobile phase containing a gradient mixture of solvents, A (0.05% trifluoroacetic acid (TFA), pH=3.0) and B (acetonitrile). The eluted compounds were monitored at 254 nm; the run time was within 20.0 min, in which ZTP and its DPs were well separated, with a resolution of >1.5. The stress testing of ZTP was carried out under acidic, alkaline, neutral hydrolysis, oxidative, photolytic and thermal stress conditions. ZTP was found to degrade significantly in acidic, photolytic, thermal and oxidative stress conditions and remain stable in basic and neutral conditions. The developed method was validated with respect to specificity, linearity, limit of detection, limit of quantification, accuracy, precision and robustness as per ICH guidelines. This method was also suitable for the assay determination of ZTP in pharmaceutical dosage forms. The DPs were characterized by LC–MS/MS and their fragmentation pathways were proposed.

Investigation of some biologically relevant redox reactions using electrochemical mass spectrometry interfaced by desorption electrospray ionization

Recently we have shown that, as a versatile ionization technique, desorption electrospray ionization (DESI) can serve as a useful interface to combine electrochemistry (EC) with mass spectrometry (MS). In this study, the EC/DESI-MS method has been further applied to investigate some aqueous phase redox reactions of biological significance, including the reduction of peptide disulfide bonds and nitroaromatics as well as the oxidation of phenothiazines. It was found that knotted/enclosed disulfide bonds in the peptides apamin and endothelin could be electrochemically cleaved. Subsequent tandem MS analysis of the resulting reduced peptide ions using collision-induced dissociation (CID) and electron-capture dissociation (ECD) gave rise to extensive fragment ions, providing a fast protocol for sequencing peptides with complicated disulfide bond linkages. Flunitrazepam and clonazepam, a class of nitroaromatic drugs, are known to undergo reduction into amines which was proposed to involve nitroso and N-hydroxyl intermediates. Now in this study, these corresponding intermediate ions were successfully intercepted and their structures were confirmed by CID. This provides mass spectrometric evidence for the mechanism of the nitro to amine conversion process during nitroreduction, an important redox reaction involved in carcinogenesis. In addition, the well-known oxidation reaction of chlorpromazine was also examined. The putative transient one-electron transfer product, the chlorpromazine radical cation (m/z 318), was captured by MS, for the first time, and its structure was also verified by CID. In addition to these observations, some features of the DESI-interfaced electrochemical mass spectrometry were discussed, such as simple instrumentation and the lack of background signal. These results further demonstrate the feasibility of EC/DESI-MS for the study of the biology-relevant redox chemistry and would find applications in proteomics and drug development research.

Application of on-line electrochemistry/electrospray/tandem mass spectrometry to a quantification method for the antipsychotic drug zotepine in human serum

A simple, rapid, and sensitive on-line liquid chromatographic electrochemistry/electrospray/tandem mass spectrometry (LC-EC/ESI-MS/MS) method for the determination of zotepine in human serum was developed using a new generated-electrochemically fragment ion, and was validated. A recent novel technique of LC-EC/ESI-MS/MS that combines LC-MS/MS and the on-line EC reaction is potentially applicable to developing a quantification method for drugs in biological samples. Newly formed products generated by the on-line EC cell are expected to provide appropriate precursor and product ions for the MS/MS determination method. This technique was successfully applied to a drug assay in a biological matrix. After adding imipramine (IS) to a 30-microL aliquot of human serum, the resulting sample was simply deproteinated with acetonitrile for a measurement. The analytical run time was 5 min. The calibration curve was linear in the concentration range of 10-2000 ng/mL. The intra-assay precision and accuracy were in the range of 1.8-8.9 and 98.4-113%, respectively.