Multiply Charged Redox-Active Oligomers in the Gas Phase: Electrolytic Electrospray Ionization Mass Spectrometry of Metallocenes

Assigning the ESI mass spectra of organometallic and coordination compounds

Electrospray ionization mass spectrometry (ESI-MS) is a useful technique for solving organometallic and coordination chemistry characterization problems that are difficult to address using traditional methods. However, assigning the ESI mass spectra of such compounds can be challenging, and the considerations involved in doing so are quite different from assigning the mass spectra of purely organic samples. This is a tutorial article for organometallic/coordination chemists using ESI-MS to analyze pure compounds or reaction mixtures. The fundamentals of assigning ESI mass spectra are discussed within the context of organometallic and coordination systems. The types of ions commonly observed by ESI-MS are categorized and described. Finally, a step-by-step guide for the assignment of organometallic and coordination chemistry ESI mass spectra is provided along with two case studies.

Making the invisible visible: improved electrospray ion formation of metalloporphyrins/-phthalocyanines by attachment of the formate anion (HCOO(-))

A protocol is developed for the coordination of the formate anion (HCOO(-)) to neutral metalloporphyrins (Pors) and -phthalocyanines (Pcs) containing divalent metals as a means to improve their ion formation in electrospray ionization (ESI). This method is particularly useful when the oxidation of the neutral metallomacrocycle fails. While focusing on Zn(II)Pors and Zn(II)Pcs, we show that formate is also readily attached to Mn(II), Mg(II) and Co(II)Pcs. However, for the Co(II)Pc secondary reactions can be observed. Upon collision-induced dissociation (CID), Zn(II)Por/Pc·formate supramolecular complexes can undergo the loss of CO2 in combination with transfer of a hydride anion (H(-)) to the zinc metal center. Further dissociation leads to electron transfer and hydrogen atom loss, generating a route to the radical anion of the Zn(II)Por/Pc without the need for electrochemical reduction, although the Zn(II)Por/Pc may have a too low electron affinity to allow electron transfer directly from the formate anion. In addition to single Por molecules, multi Por arrays were successfully analyzed by this method. In this case, multiple addition of formate occurs, giving rise to multiply charged species. In these multi Por arrays, complexation of the formate anion occurs by two surrounding Por units (sandwich). Therefore, the maximum attainment of formate anions in these arrays corresponds to the number of such sandwich complexes rather than the number of porphyrin moieties. The same bonding motif leads to dimers of the composition [(Zn(II)Por/Pc)2·HCOO](-). In these, the formate anion can act as a structural probe, allowing the distinction of isomeric ions with the formate bridging two macrocycles or being attached to a dimer of directly connected macrocycles.

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.

Letter: Influence of experimental conditions on electrospray ionization mass spectrometry of ferrocenylalkylazoles

Influence of experimental conditions on electrospray/ionization (ESI) mass spectra of ferrocene derivatives FcCHRAz (Fc = eta5-C5H5-Fe-eta5- C5H4; R = H, Az = benzimidazole; R = Ph, Az = 2-methylimidazole) has been investigated. The spectra of all the compounds revealed [M]+*, product of its fragmentation [FcCHR]+ as well as products of ion/molecular interactions (protonated molecule [MH]+, binuclear ion [(FcCHR)2 Az]+, dimeric ion [M2]+* and its protonated form [M2H]+). Relative abundances of these ions appreciably (more than one order) depend on experimental conditions: analyte concentration, temperature of heated capillary, spray voltage, flow rate of mobile phase and polarity of solvents. Established correlations allow the selection of optimum experimental conditions for registration of ESI mass spectra, as required by the application. If an unknown compound is to be identified, it is necessary to operate by using polar solvents, small concentration, low temperature of heated capillary, high spray voltage and flow rates. There are high-intensity binuclear and protonated dimeric ions in mass spectra under other conditions. It can give rise to wrong interpretation of the structure of investigated compound. At the same time, for study of ion-molecular processes by ESI-MS it is necessary to use concentrated samples in non-polar solvents. In this case the dependence of reaction products yields on temperature and flow rate of mobile phase must be investigated.

Expanded Electrochemical Capabilities of the Electrospray Ion Source Using Porous Flow-Through Electrodes as the Upstream Ground and Emitter High-Voltage Contact

Use of a porous flow-through electrode at the upstream ground contact or at both the upstream ground contact and the high-voltage emitter contact in an electrospray ion source was shown to provide for new types of electrochemical experiments utilizing only the electrochemistry inherent to electrospray. The normal stainless steel bore-through union serving as the upstream grounding point in a floated electrospray emitter system was replaced with a high surface area porous flow-through electrode assembly to achieve effective electrochemical reduction of analytes at this point in positive ion mode, and effective electrochemical oxidation of analytes in negative ion mode. This was demonstrated by the oxidation of 3,4-dihydroxybenzoic acid and reserpine in negative ion mode and by the reduction of thionine in positive ion mode. In the case of reversible oxidation (3,4-dihydroxybenzoic acid) and reduction (thionine) processes, partial rereduction and reoxidation of the products due to reaction with products generated by cathodic and anodic processes at the emitter were observed, respectively. By implementing two high surface area porous flow-through electrodes in the system, one as the upstream grounding point and the other as the emitter electrode, a multiple-step reaction scheme was achieved that included consecutive electrochemical reduction and oxidation reactions and a following chemical reaction as demonstrated by the hydroquinone tagging of an initially disulfide-linked peptide.

Electrolytic electrospray ionization mass spectrometry of quaterthiophene-bridged bisporphyrins: beyond the identification tool

Several quaterthiophene-bridged bisporphyrins were analyzed by electrospray ionization mass spectrometry (ESI-MS). The active centers of these molecular assemblies are two porphyrins moieties complexed (Z) or not (H) with a metal ion, typically Zn(2+), and the spacer is a quaterthiophene. The two end-groups were chemically linked to the quaterthiophene spacers by (i) a C--C single bond, (ii) a trans double bond or (iii) a triple bond. The formation of charged species either by protonation ([M + H](+) and [M + 2H](2+)) or electron(s) loss (M(+) and M(2+)), account for the occurrence of electrochemical processes in the basic operation of an electrospray source acting in a non-aqueous solvent. The nature of the observed charged species is correlated with the electro-oxidation properties and proton production by electro-oxidation of residual water. The occurence of these electrochemical reaction is proposed when the electroactivity of the electrosprayed substrates is not sufficient to support the current demand of the ESI source. In this way, the results obtained from the analysed series suggest the occurrence of such a process when the interfacial potential of the metal capillary reaches a value of 0.75 V vs Ag/AgCl. The results of theoretical calculations confirm the importance of the ionization energy with regard to the protonation energy in the course of the ionization reaction. The structural differences at the porphyrin-linker junctions lead to significantly smaller ionization energy in the case of the trans double bond. The MS observation of discharged dimers from molecular assemblies, including two complexed porphyrins ZZ or two free bases HH as end-group and a triple bond as the quaterthiophene-bisporphyrin junction, indicates together with molecular modelling (carried out at the semi-empirical PM3 level), that the planar and symmetric structures favour stacking.

Negative ion mode evolution of potential buildup and mapping of potential gradients within the electrospray emitter

Differential electrospray emitter potential (DEEP) maps, displaying variations in potential in the electrospray (ES) capillary and in the Taylor cone, have been generated in the negative ion mode of ES operation. In all examples, measured potential was found to be the highest at the points furthest into the Taylor cone, and values descended to zero at distances beyond approximately 15 mm within the ES capillary. In agreement with results obtained previously in the positive ion mode, negative mode data show a strong influence of electrolyte concentration on measured potentials. Weakly conductive solutions exhibited the highest values, and the steepest gradients, at points furthest into the Taylor cone. However, these same low conductivity solutions did not yield nonzero measured potentials to as deep a distance into the ES capillary as was possible from their higher conductivity counterparts. Addition of a readily reducible compound lowered measured potentials at all points near the ES capillary exit, in accordance with the description of the ES device as a controlled-current electrolytic cell. The development of potential inside the ES capillary upon the onset of ES was also studied, and initial results are presented. Potential waves are observed that can require 15 min or longer, to stabilize. The slow drift to steady potentials is evidence of upstream movement of electrochemically-produced species and follow-up reaction products; low conductivity solutions require longer intervals to reach a steady state. Potentials measured along the central ES axis reflect those at the ES capillary surface, although equipotential lines can be considered to be more compressed at the latter surface.

Effects of ground loop currents on signal intensities in electrospray mass spectrometry

The occurrence of electrochemical processes during the operation of an electrospray ionization (ESI) source is well established. In the positive ion mode, electrons are drawn from the ESI metal capillary to a high voltage power supply. These electrons are the product of charge-balancing oxidation reactions taking place at the liquid/metal interface of the ion source. In a recent study, (Anal. Chem.2001, 73, 4836-4844), our group has shown that the introduction of a ground loop can dramatically enhance the rate of these oxidation processes. Such a ground loop can be introduced by connecting the sample infusion syringe (or the liquid chromatography column, in the case of LC-MS studies) to ground. The magnitude of the ground loop current can be controlled by the electrolyte concentration in the analyte solution, and by the dimensions of the capillary connecting the syringe needle and the ESI source. Using ferrocene as a model system, it is demonstrated that the introduction of such a ground loop can significantly enhance the signal intensity of analytes that form electrochemically ionized species during ESI. However, analytes that form protonated molecular ions, such as reserpine, also show higher signal intensities when a ground loop is introduced into the system. This latter observation is attributed to the occurrence of electrolytic solvent (acetonitrile and/or water) oxidation processes. These reactions generate protons within the ion source, and thus facilitate the formation of [M + nH](n+) ions. Overall, this work provides an example of how the careful control of electrochemical parameters can be exploited to optimize signal intensities in ESI-MS.

Efficient analyte oxidation in an electrospray ion source using a porous flow-through electrode emitter

This article describes the components, operation, and use of a porous flow-through electrode emitter in an electrospray ion source. This emitter electrode geometry provided enhanced mass transport to the electrode surface to exploit the inherent electrochemistry of the electrospray process for efficient analyte oxidation at flow rates up to 800 microL/min. An upstream current loop in the electrospray source circuit, formed by a grounded contact to solution upstream of the emitter electrode, was utilized to increase the magnitude of the total current at the emitter electrode to overcome current limits to efficient oxidation. The resistance in this upstream current loop was altered to control the current and "dial-in" the extent of analyte oxidation, and thus, the abundance and nature of the oxidized analyte ions observed in the mass spectrum. The oxidation of reserpine to form a variety of products by multiple electron transfer reactions and oxidation of the ferroceneboronate derivative of pinacol to form the ES active radical cation were used to study and to illustrate the performance of this new emitter electrode design. Flow injection, continuous infusion, and on-line HPLC experiments were performed.

Strategies for the liquid chromatographic-mass spectrometric analysis of non-polar compounds

Electrospray ionization and atmospheric pressure chemical ionization (APCI) have evolved recently as very useful tools for the liquid chromatographic-mass spectrometric (LC-MS) analysis of polar substances. Non-polar compounds, however, are difficult to analyze with these atmospheric pressure ionization techniques due to their soft ionization mechanism. Recently, new approaches have been introduced which are likely to overcome this obstacle, at least partly. On-line electrochemical conversion of the analytes to more polar reaction products, atmospheric pressure photoionization, atmospheric pressure electron capture negativeion-MS and coordination ionspray-MS are four techniques which are presented in detail compared and discussed critically with respect to their current status and future perspectives. Particular focus is directed from a chemical viewpoint on the substance groups which are accessible by each of the new approaches.

Electrochemically induced pH changes resulting in protein unfolding in the ion source of an electrospray mass spectrometer

The operation of an electrospray ion source in the positive ion mode involves charge-balancing oxidation reactions at the liquid/metal interface of the sprayer capillary. One of these reactions is the electrolytic oxidation of water. The protons generated in this process acidify the analyte solution within the electrospray capillary. This work explores the effects of this acidification on the electrospray ionization (ESI) mass spectrum of the protein cytochrome c (cyt c). In aqueous solution containing 40% propanol, cyt c unfolds around pH 5.6. Mass spectra recorded under these conditions, using a simple ESI series circuit, display a bimodal charge-state distribution that reflects an equilibrium mixture of folded and unfolded protein in solution. These spectra are not strongly affected by electrochemical acidification. An "external loop" is added to the ESI circuit when the metal needle of the sample injection syringe is connected to ground. The resulting circuit represents two coupled electrolytic cells that share the ESI capillary as a common anode. Under these conditions, the rate of charge-balancing oxidation reactions is dramatically increased because the ion source has to supply electrons for both, the external circuit and the ESI circuit. The analytical implications of this effect are briefly discussed. Mass spectra of cyt c recorded with the syringe needle grounded are shifted to higher charge states, indicating that electrochemical acidification has caused the protein to unfold in the ion source. The acidification can be suppressed by increasing the flow rate and lowering the electrolyte concentration of the solution and by using an electrolyte that acts as redox buffer. The observed acidification is similar for sprayer capillaries made of platinum and stainless steel. Removal of the protective oxide layer on the stainless steel surface results in effective redox buffering for a few minutes.

Redox buffering in an electrospray ion source using a copper capillary emitter

An electrospray ion source used in electrospray mass spectrometry is a two-electrode, controlled-current electrochemical flow cell. Electrochemical reactions at the emitter electrode (oxidation and reduction in positive and negative ion modes respectively) provide the excess charge necessary for the quasi-continuous production of charged droplets and ultimately gas-phase ions with this device. We demonstrate here that a copper capillary emitter, in place of the more commonly used stainless-steel capillary emitter, can be utilized as a redox buffer in positive ion mode. Anodic corrosion of the copper capillary during normal operation liberates copper ions to solution and in so doing maintains the interfacial potential at this electrode near the equilibrium potential for the copper corrosion process [E degrees = 0.34 V versus standard hydrogen electrode (SHE)]. Fixing the interfacial potential at the emitter electrode provides control over the electrochemical reactions that take place at this electrode. It is shown that the oxidation of N-phenyl-1,4-phenylenediamine to N-phenyl-1,4-phenylenediimine (E(p/2) = 0.48 V versus SHE) can be completely avoided using the copper emitter, whereas this analyte is completely oxidized with a stainless-steel capillary emitter under the same conditions. Moreover, using N-phenyl-1,4-phenylenediimine, we demonstrate that reduction reactions can occur at the copper emitter electrode in positive ion mode. Emitter corrosion, in addition to redox buffering, provides a convenient means to introduce metal ions into solution for analytical use in electrospray mass spectrometry.

Minimizing analyte electrolysis in an electrospray emitter

The electrospray (ES) ion source is a controlled-current electrolytic flow cell. Electrolytic reactions in the ES emitter capillary are continually ongoing to sustain the production of charged droplets and ultimately gas-phase ions from this device. Under certain circumstances, the analytes under study may be directly involved in these electrolytic processes. It is demonstrated that a simple means to minimize analyte electrolysis is to exchange the normal metal emitter capillary of commercial ES sources with one made of fused silica. This change is shown to provide an ES mass spectrometric system of similar performance in terms of gas-phase ion signal generated for non-electroactive analytes and also assures minimal oxidation of electroactive analytes even at low (2.0 microl x min(-1)) solution flow-rates and high (millimolar) solution electrolyte concentrations.

Insights into analyte electrolysis in an electrospray emitter from chronopotentiometry experiments and mass transport calculations

Insights into the electrolysis of analytes at the electrode surface of an electrospray (ES) emitter capillary are realized through an examination of the results from off-line chronopotentiometry experiments and from mass transport calculations for flow through tubular electrodes. The expected magnitudes and trends in the interfacial potential in an ES emitter under different solution conditions and current densities, using different metal electrodes, are revealed by the chronopotentiometry data. The mass transport calculations reveal the electrode area required for complete analyte electrolysis at a given volumetric flow rate. On the basis of these two pieces of information, the design of ES emitters that may maximize and those that may minimize analyte electrolysis during ES mass spectrometry are discussed.

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this month's Special Feature. We particularly hope that younger scientists will find value in this month's Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this month's Special Feature. We particularly hope that younger scientists will find value in this month's Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.

Electrolytic deposition of metals on to the high-voltage contact in an electrospray emitter: implications for gas-phase ion formation

The electrospray ion source is an electrolytic flow cell. Electrolytic reactions in the electrospray emitter maintain the production of charged droplets by this ion source that contain an excess of ions of one polarity. These redox reactions necessarily change the composition of the solution that initially enters the emitter. As a result, the ions ultimately observed in the gas phase by electrospray mass spectrometry (ESMS) may be substantially influenced by both the nature and extent of these electrochemical reactions. It is demonstrated in this paper that Ag(+), Cu(2+) and Hg(2+) ions in solution can be electrolytically reduced and deposited as the respective metals on to the surface of the high-voltage contact in the electrospray emitter in negative ion mode electrospray. The deposited metals are shown to be liberated from the surface by switching the electrospray high-voltage polarity to operate in the positive ion mode. The deposited metals are oxidized in positive ion mode, releasing the metal ions back into solution where they are detected in the electrospray mass spectrum. In a semi-quantitative analysis, it was found that up to 50% of the Ag(+) in a 2.5 microM solution was deposited on the high-voltage contact of the emitter as the solution flowed through the emitter. Deposition of Cu(2+) and Hg(2+) was less efficient. These data illustrate that in the analysis of metals by ESMS, one must be aware that both the concentration and form of the metals may be altered by electrochemical processes in the emitter. Hence reduction or oxidation of metals in the electrospray emitter, which may remove ions from solution, or change metal valence, would be expected to impact both quantitative metal determinations and metal speciation attempts using ESMS.

Electrical equivalence of electrospray ionization with conducting and nonconducting needles

An electrical equivalent circuit is derived for the electrospray process. It is a series circuit which consists of the power supply, the electrochemical contact to the solution, the solution resistance (R(s)), a constant-current regulator which represents the processes of charge separation and charge transport in the gap between the spray needle aperture and the counter electrode, and charge neutralization at the counter electrode. A current i, established by the constant-current regulator flows throughout the entire circuit. Current-voltage curves are developed for each element in the circuit. From these it is shown that in the case where R(s) is negligible (the power supply is connected directly to a conducting needle) the shape of the current-voltage curve is dictated by the constant-current regulator established by the charge separation process, the gap, and the counter electrode. The solution resistance may be significant if a nonconducting needle is used so that the electrochemical contact to the solution is remote from the tip. Experiments with a nonconducting spray needle quantify the effect of the solution resistance on the current-voltage curve. Subtracting the iRs voltage from Vapp (power supply voltage) yields the current-voltage curve for the constant-current regulator. When iRs drop is a significant fraction of Vapp, the current-voltage curve of the constant-current regulator is changed substantially from the case when the solution resistance is negligible.