Unexpected Reduction of Iminoquinone and Quinone Derivatives in Positive Electrospray Ionization Mass Spectrometry and Possible Mechanism Exploration

  • Jiying Pei
  • Cheng-Chih Hsu
  • Ruijie Zhang
  • Yinghui Wang
  • Kefu Yu
  • Guangming Huang
Research Article


Unexpected reduction of iminoquinone (IQ) and quinone derivatives was first reported during positive electrospray ionization mass spectrometry. Upon increasing spray voltage, the intensities of IQ and quinone derivatives decreased drastically, accompanying the increase of the intensities of the reduction products, amodiaquine (AQ) and phenol derivatives. To gain more insight into the mechanism of such reduction, we explored the experimental factors that are influential to corona discharge (CD). The results show that experimental parameters that favor severe CD, including metal spray emitter, using water as spray solvent, sheath gas with low dielectric strength (e.g., nitrogen), and shorter spray tip-to-mass spectrometer inlet distance, facilitated the reduction of IQ and quinone derivatives, implying that the reduction should be closely related to CD in the gas phase.

Graphical Abstract


Iminoquinone Quinone Reduction Corona discharge Electrospray ionization 


Redox modifications of analytes generally occur during electrospray ionization mass spectrometry (ESI MS). Solution-phase electrochemical reaction and gas-phase corona discharge (CD) are commonly deemed as “culprits” for the phenomena. An ESI source can be viewed as an electrolytic cell [1, 2, 3], in which oxidation reaction occurs in the positive mode and reduction reaction occurs in the negative mode. Van Berkel’s group has done extensive investigations over the past decades [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. In addition, analyte redox can also be induced by the massive oxidative or reductive species, e.g., OH, O, H, N, HO2, N2 +, N+, O3, which are generated via CD in the gas phase [15, 16, 17, 18]. Since CD occurs in both positive and negative polarities, oxidation or reduction of analytes will also occur in both ionization modes for ESI.

In conventional ESI MS, there exists a striking asymmetry between the incidence of oxidation and reduction reaction [19]. Oxidation reactions occur more frequently than reduction reactions. Electrochemistry-induced oxidations include electrolysis of analyte molecules (peptide [20, 21], metallo porphyrins [4, 14], reserpine [5], amodiaquine (AQ) [7], steroid sulfates [22], hydroquinones [13], and isochromene [23]), solvent molecules [20, 24] as well as ESI electrodes [7, 8, 21, 25, 26]. CD-induced oxidations include oxygenation of hydroquinone [27], stearic acid [19], phosphorothioate oligonucleotides [28], peptide [29], and protein [30]. However, only a few reduction reactions were reported during ESI process. Gianelli et al. reported Cu (II) reduction during positive ESI mode, which was attributed to charge transfer between Cu (II) complexes and the solvent molecules in the gas phase, and electrochemical reaction during ESI process was excluded by deuterated methanol [31]. Gu et al. reported the reduction of CH3CN to CH3CH2NH2 in positive ESI [32], and electrolysis of water was believed to be responsible for that reduction. However, there was no sufficient evidence to illustrate that the water electrolysis occurred at the electrode/solution interface rather than in the gas phase. Furthermore, the reduction of diquat and paraquat dication to monovalent cation [33] and 1,6-dichloro-1,4-benzoquinone to phenol were also reported during ESI MS [34]. However, the underlying mechanism remained unexplored.

Phenolic compounds are readily oxidized during ESI, and therefore are commonly used to investigate the electrolysis performance of ESI [7, 35, 36] or as the redox buffer [13] during ESI MS. However, unexpected reduction of iminoquinone (IQ, oxidative product of AQ) and quinone derivatives was recently observed in positive ESI MS in our study. Though quinone reduction was reported in some traditional ionization sources, such as electron ionization (EI) [37], fast atom bombardment (FAB) [38], secondary ion mass spectroscopy (SIMS) [39], and atmosphere pressure chemical ionization (APCI) [40], it was for the first time reported in positive ESI MS. To gain more insight into the mechanism of this unusual reduction in positive ESI MS, we investigated the effect of experimental parameters that relate to CD in the gas phase on that reduction, including solvent composition, sheath gas, spray emitter material, and spray tip-to-mass spectrometer inlet distance.


Materials and Reagents

HPLC grade methanol (CH3OH) was purchased from Honeywell Burdick & Jackson Inc. (Morristown, NJ, USA). AQ, 1,4-benzoquinone (1,4-BQ), methyl-p-benzoquinone (MBQ), 1,4-naphthoquinone (1,4-NQ), and 1,4-anthraquinone (1,4-AQ) were obtained from Sigma-Aldrich Chemical Co. Ltd. (St. Louis, Missouri, USA). Glutathione (GSH) was purchased from Sangon Biological Engineering Technology & Services Co. Ltd. (Shanghai, China). Ammonium acetate (NH4Ac) and hydrofluoric acid (HF) were obtained from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). All these reagents were used without any further purification. Distilled water (18.2 MΩ) was produced by Milli-Q system (Millipore Inc., Bedford, MA, USA).

ESI Setup

The home-built ESI setup in Figure 1 was used throughout this study unless otherwise stated. The spray voltage was applied on the syringe needle. The sample solution loaded in a syringe (1700 series; Hamilton, NV, USA) was delivered through a ~50 cm-long fused silica capillary (100-μm-i.d., 365-μm-o.d.), and the sample solution was sprayed from the etched capillary tip. The distance between the spray tip and the mass spectrometer inlet was 1 cm. Nitrogen (N2) and sulfur hexafluoride (SF6) were used as the sheath gas. When investigating the effect of spray emitter material on quinone reduction, commercial ESI source was used.
Figure 1

Home-built ESI setup used in the experiment. There is a ~50 cm-long fused silica capillary between the electrode where the spray voltage is applied and the spray emitter tip. Electrochemical reactions occur at the solution/electrode interface, and CD reactions occur near the Taylor cone in the gas phase

To achieve stable spray with the home-built ESI source, the top 5 mm of the fused silica capillary was etched using the method introduced by Kelly [41].

Mass Spectrometry

All MS experiments were carried out using a Thermo LTQ mass spectrometer (Thermo Fisher Scientific, San Jose, CA, USA). The mass spectrometer conditions were as follows: S lens voltage, 42% (positive mode); capillary temperature, 275 °C. Ion injection time was set as 10 ms and all signals were averaged by three microscans.

Results and Discussion

Incidental Discovery of Iminoquinone Reduction in Positive ESI MS

AQ, an easily oxidized phenolic compound, is commonly used to investigate the electrolysis performance of ESI source [7]. AQ is generally oxidized to semi-iminoquinone free radical (SIQ) and IQ in liquid phase (Scheme 1) [42]. With the setup in Figure 1, we attempted to investigate AQ oxidation induced by ESI electrolysis. After a 10-min uninterrupted electrospray, the spray voltage was increased from 3 to 5 kV. It was surprisingly observed that the signal intensity of IQ (m/z 354) decreased drastically. Meanwhile, the signal intensities of AQ (m/z 356) and SIQ (m/z 355) (Figure 2a) rose significantly. When the spray voltage was reduced to 3 kV, the signal recovered. IQ and SIQ were identified via their MS/MS spectra (Supplementary Figure S1). Though the isotopic peak of IQ (the abundance of the first isotope at m/z 355 is ~22% to that of the monoisotopic ion at m/z 354) might partly contribute to m/z 355, its contribution could be negligible with the spray voltage of 5 kV because the peak intensity of m/z 355 was much higher than that of m/z 354. With the spray voltage of 3 kV, the peak intensity of m/z 355 was mostly contributed by the isotopic peak of IQ. Since the ratio of the signal intensities of IQ to IQ + SIQ + AQ changed during the spray process due to electrochemical reaction (the oxidation extent of the analyte is proportional to the solution/electrode contact time) and the adjustment of the spray voltage, we fitted a curve between the value of I354/(I354 + I355 + I356) and the spray time. The results showed that the oxidation ratio kept constant in the first 2 min, and then gradually rose in the next 8 min. When the spray voltage was increased from 3 to 5 kV, the oxidation ratio decreased promptly (Figure 2b). It could be concluded that the oxidation of AQ from 2 to 8 min was mainly due to the electrochemical reaction at the solution/electrode interface. Since the fused silica capillary (i.d. 50 μm) between the electrode and the spray emitter tip was ~50 cm long, it should take ~2 min (with a flow rate of 2 μL/min) for sample solution with electrode contact to pass through it. With longer contact time, AQ would be oxidized more severely.
Scheme 1

Oxidation pathway of AQ

Figure 2

(a) TIC (total ion chromatogram), SICs (selected ion chromatograms), and mass spectra of AQ solution upon adjusting spray voltage during the electrospray process, and (b) the corresponding oxidation curve. Conditions: CAQ = 0.25 μg/mL, CNH4Ac = 5 mM, flow rate = 2 μL/min, solvent, H2O

However, what was the “culprit” that led to the unexpected reduction of IQ when the spray voltage was raised from 3 to 5 kV (10–11 min)? It is well known that ionization efficiency affects the signal intensities of analytes in the mass spectrometer. An intuitive speculation was that the signal variations of AQ, SIQ, and IQ were attributable to their ionization efficiencies during the electrospray process. This possibility was excluded by the following experiment. Before the AQ solution was electrochemically oxidized, the spray voltage was adjusted from 3 to 5 kV. It showed that the signal intensity of AQ barely changed (Supplementary Figure S2), stating that the signal variations of AQ, SIQ, and IQ were not induced by their ionization efficiencies at the given spray voltages.

Electrochemical reaction in the liquid phase and CD reaction in the gas phase are known as two common reasons for analyte redox during ESI MS. However, the possibility of electrochemistry-induced reduction could be excluded from two aspects. First, electrochemical reduction reaction is well recognized to occur only in negative ESI mode. Second, electrochemical reaction occurs at the solution/electrode interface, which requires time for sample solution with electrode contact to pass through the fused silica capillary. Therefore, electrochemistry-induced redox should be observed with a time delay, as discussed already in this article.

Effect of Mass Spectrometer Parameters on Iminoquinone Reduction

Gianelli et al. reported the reduction of Cu (II) to Cu (I) in positive ESI mode and attributed it to CD [31]. To explore whether CD is responsible to the IQ reduction in positive ESI mode, we investigated the effect of the following experimental parameters related to CD on that reduction, including spray solvent composition, sheath gas, spray emitter material, and the spray tip-to-mass spectrometer inlet distance.

Solvent composition affects the discharge extent of electrospray, which could be characterized by the light emission [43]. To investigate the effect of solvent composition on IQ reduction in positive ESI MS, we tried three spray solvents (H2O, CH3OH/H2O (v/v, 1:1), CH3OH). When pure H2O was used as the solvent, ~61.3% of IQ was reduced upon increasing the spray voltage from 3 kV to 4 or 5 kV. When CH3OH/H2O (v/v, 1:1) and CH3OH were used as the solvents, IQ reduction could hardly be observed with spray voltages of 4 or 5 kV (Supplementary Figure S3), and only 2%–7% of IQ was reduced with spray voltages further tuned to 6 and 7 kV (Figure 3). The initial oxidation ratios of AQ were ~70%, ~52%, and ~24% in the solvents of H2O, CH3OH/H2O (v/v, 1:1), and CH3OH, respectively, which was due to electrochemistry-induced oxidation related to the solvent composition [20]. The discharge extents for the three solvents during the electrospray process are as follows: H2O > CH3OH/H2O (v/v, 1:1) > CH3OH. The consistency of the extents of CD with IQ reduction implied that severer discharge facilitated IQ reduction. However, it is noteworthy that the discharge extent in our experiment was not severe enough to prevent the formation of stable Taylor cone during the electrospray process, as CD could occur even at low spray current [43].
Figure 3

Effect of solvent composition on IQ reduction. (a) H2O, (b) CH3OH/H2O (v/v, 1:1), (c) CH3OH. Conditions: CAQ = 0.25 μg/mL, CNH4Ac = 5 mM, flow rate = 2 μL/min

Sheath gas affects the surrounding environment where discharge occurs. When no sheath gas was used, the value of I354/(I354 + I355 + I356) was ~50% (Figure 4). However, when N2 was used as sheath gas, IQ was dramatically reduced (I354/(I354 + I355 + I356), ~5%). With N2 turned off afterwards, the value of I354/(I354 + I355 + I356) recovered to ~50% instantaneously. This is not surprising as the surrounding environment of electrospray is air (78% nitrogen and 21% oxygen) without the use of the sheath gas. Molecular nitrogen has weaker electron scavenging activity than molecular oxygen; therefore an ESI source with pure nitrogen gas is more prone to discharge than ambient air during electrospray process [15]. Conversely, SF6 is a typical electron capturing gas that is generally used as the discharge-suppression gas [44]. This is in agreement with our result that IQ reduction was suppressed when using SF6 as the sheath gas (Supplementary Figure S4), implying that IQ reduction was related with CD.
Figure 4

Effect of sheath gas (N2) on IQ reduction. (a) TICs and mass spectra of AQ solution with and without N2, and (b) the corresponding oxidation curve. Conditions: CAQ = 0.25 μg/mL, CNH4Ac = 5 mM, flow rate = 2 μL/min, spray voltage = 3 kV, solvent, H2O. Note: (+) and (–) indicate use and no use of sheath gas, respectively, which is suitable for the following figures

The third parameter that potentially affects discharge extent is the material of the spray emitter. In this study, fused silica capillary and metal capillary were investigated. As depicted in Figure 5, ~51% of IQ was reduced to AQ by metal capillary with the spray voltage of 5.5 kV, whereas only ~5.1% of IQ was reduced by fused silica capillary at even higher spray voltage of 6 kV (Figure 3b). Electrospray by metal capillary is known to discharge more dramatically than fused silica capillary. This is because CD occurs on the tip of the liquid cone in the nonconductive fused silica capillary [43], whereas it occurs on the needle of the metal capillary [45], during which process metal more readily releases electrons than liquid. Therefore, the above results indicate that the more severe discharge with metal capillary facilitates the reduction of IQ more efficiently.
Figure 5

Reduction of IQ under different spray voltages with metal capillary. Conditions: CAQ = 0.25 μg/mL, CNH4Ac = 5 mM, flow rate = 2 μL/min, solvent, CH3OH/H2O (v/v, 1:1)

In addition, we investigated the effect of spray tip-to-mass spectrometer inlet distance on IQ reduction. The distances were set as 5, 10, 15, 20, 25, 30, 35, and 40 mm. Before each adjustment of the distance, the spray voltage was lowered to 3 kV. The signal intensity of IQ decreased and those of SIQ and AQ increased when the spray voltage was raised from 3 to 5 kV with the distances of 5–25 mm. With longer distance (30–40 mm), the signal intensity of IQ did not change significantly upon increasing spray voltage, though the signal intensities of SIQ and AQ increased (Figure 6a). This might be because the longer distance enabled fewer ions to enter the mass spectrometer, which might further affect the ionization efficiencies. It could be observed that longer distance resulted in less IQ reduction (Figure 6b, Supplementary Figure S5). Since CD weakened with longer distance from the spray tip to the mass spectrometer, it indicated that less severe discharge suppressed the reduction of IQ.
Figure 6

Effect of spray tip-to-mass spectrometer inlet distance on IQ reduction. Conditions: CAQ = 0.25 μg/mL, CNH4Ac = 5 mM, flow rate = 2 μL/min, solvent, H2O.

Reduction of Quinone Derivatives in Positive ESI MS

The above results indicate the effect of CD on IQ reduction during ESI MS. In addition, we tested whether other quinonoid compounds could experience similar reduction during ESI MS. BQ, MBQ, 1,4-NQ, and 1,4-AQ, which are well-defined reversible redox species, were chosen as the test compounds.

To improve the ionization efficiencies, quinones were derived by GSH. Similar to the method used to investigate IQ reduction, the derived products (QH2-GSH) were first electrochemically oxidized by the inherent electrolysis of ESI (the oxidation pathways of BQH2-GSH and MBQH2-GSH are shown in Supplementary Figure S6). Then, the effect of experimental parameters, including spray voltage, solvent composition, and sheath gas on the quinone derivatives reduction, was investigated. As shown in Figure 7 and Supplementary Figure S7, higher spray voltage and use of nitrogen as sheath gas facilitated the reduction of BQ-GSH and MBQ-GSH. BQ-GSH (m/z 414) and BQH2-GSH (m/z 416) were identified by the MS/MS spectra (Supplementary Figure S8). The reduction reaction also occurred for the sodium and potassium adducts of MBQ-GSH (Supplementary Figure S9). Upon increasing spray voltage from 3 to 6.5 kV, [MBQ-GSH + Na] (m/z 450) and [MBQ-GSH + K] (m/z 466) were reduced to [MBQH2-GSH + Na] (m/z 452) and [MBQH2-GSH + K] (m/z 468), respectively. In addition, H2O induced more dramatic reduction of BQ-GSH than CH3OH/H2O (v/v, 1:1) (Supplementary Figure S10).
Figure 7

Effect of spray voltage and sheath gas (N2) on BQ-GSH reduction. (a) TICs and mass spectra of BQH2-GSH solution upon adjusting spray voltage and sheath gas, and (b) the corresponding oxidation curve. Conditions: CBQ = 2 μg/mL, CGSH = 2.5 μg/mL, CNH4Ac = 5 mM, flow rate = 2 μL/min, solvent, H2O

However, not all quinonoid compounds experienced such reduction during positive ESI MS. 1,4-NQ-GSH and 1,4-AQ-GSH could not be reduced no matter what experimental parameters were adjusted (Supplementary Figure S11). This might be related with the reduction potential of different quinone species. The species with higher reduction potential may be more readily reduced during electrospray process, as implied by the reduction potentials of BQ, MBQ, 1,4-NQ, and AQ being –0.851, –0.928, –1.029, and –1.259 V, respectively [46].


Unexpected reductions of IQ and quinone derivatives during ESI MS were reported for the first time. The reductions were further investigated and the experimental results suggested that it was closely related with CD in the gas phase. Through adjusting experimental parameters that strengthened CD, e.g., improving spray voltage, using water as spray solvent, using metal spray emitter, or shortening the spray tip-to-mass spectrometer distance, IQ and quinone derivatives were more readily reduced. This finding implies that greater attention should be paid during analyzing readily reductive species with ESI MS, such as quinonoid compounds, as well as readily oxidative species.



The authors are grateful for financial supports from the National Natural Science Foundation of China (21475121, 21665003), the Guangxi Natural Science Fund Project (no. 2016GXNSFBA380140).

Supplementary material

13361_2017_1770_MOESM1_ESM.docx (469 kb)
ESM 1 (DOCX 468 kb)


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Copyright information

© American Society for Mass Spectrometry 2017

Authors and Affiliations

  1. 1.School of Marine SciencesGuangxi UniversityNanningPeople’s Republic of China
  2. 2.Department of Chemistry, School of Chemistry and Materials ScienceUniversity of Science and Technology of China (USTC)HefeiPeople’s Republic of China
  3. 3.Department of ChemistryNational Taiwan UniversityTaipeiTaiwan

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