Parameters affecting analyte signal enhancement in anodic stripping voltammetry-inductively coupled plasma mass spectrometry (ASV-ICP-MS), using a thin-layer ASV cell and microconcentric nebulization (MCN), have been examined. Silver was used as a test analyte and was deposited at a glassy carbon working electrode. The MCN allowed use of solution flow rates that were beneficial to optimum electrolytic performance of the thin-layer cell. High analyte deposition efficiencies obtained with the thin-layer cell, combined with minimal sample consumption of the MCN, allowed substantial signal enhancement (>400 times higher than continuous nebulization level) to be obtained with 2-3 mL of sample and deposition times of less than 30 min. Signal enhancement was strongly influenced by the opposing effect of flow rate on the electrolytic deposition efficiency (deposition efficiency decreases with increasing flow rate) and on the quantity of analyte delivered to the cell (analyte mass throughput increases with increasing flow rate). Excellent linearity for stripping peak heights was demonstrated for a wide range of analyte deposition times and for peak heights and peak areas (r > 0.999) over a wide concentration range (25 ng/L-20 μg/L). Precision was good (RSD typically <3% for n = 3-6) except for a high Ag blank contributed to by corrosion of the counter electrode and by Ag diffusion from the reference electrode into the cell. Details of the flow manifold and ASV cells are discussed, along with relevant performance characteristics of the MCN.

The redox reactions that take place in the metal capillary of an electrospray ion source to maintain charge balance alter the composition of the initial solution entering the capillary. Data presented here demonstrate that under certain ES conditions, solution pH may be decreased significantly (by at least 4 pH units) as a result of the electrolytic oxidation of water in positive ion mode electrospray-mass spectrometry (ES-MS). Furthermore, it is shown that this pH change can have an affect on the appearance of the ES mass spectrum of an of analyte. An ES ion source in which the pH of an indicator solution exiting the capillary is monitored optically, before the spraying process, is used to demonstrate that electrolytic reactions in positive ion mode ES can decrease the pH of the initial solution. ES-MS studies using bovine heart cytochrome c are used to illustrate the influence of the electrolytically-induced pH change on the gas-phase ion signals for the multiply protonated protein. The magnitude of electrolytically-induced changes in solution pH will be most significant in non-buffered solutions near neutral pH when using metal spray capillaries or metal contacts to solution comprised of difficult to oxidize material (e.g. platinum or gold). Any pH changes will increase in magnitude as the flow rate decreases and/or ES current increases (all other ES parameters constant). Therefore, the potential impact of these redox reactions in ES-MS, if any, will probably be most important in the very low flow rate ES-MS systems (≤ 1.0 μL min−1).

In this paper an electrospray ion source is shown to be a controlled-current electrolytic now cell which, when operated so that three key requirements are met, can be used for efficient neutral analyte ionization (i,e,, complete analyte electrolysis) and sensitive gas-phase detection (i,e., minimized gas-phase signal suppression) in electrospray mass spectrometry (ES-MS), These three requirements are as follows: (1) the magnitude of the ES current, i(ES), must be sufficient for the oxidization of the molar equivalent of all species available for reaction in the ES capillary that are as easily or more easily oxidized than the targeted analyte, including all of the analyte; (2) the analyte must be available for reaction at the metal/solution interface in the ES capillary; and (3) the steps taken to ensure the first two requirements must not inhibit the formation of gas-phase ions from the ions generated electrolytically in solution. The means to meet these requirements are discussed, including the addition of an appropriate electrolyte to the electrosprayed solutions (e,g,, lithium triflate), the use of slower now rates (e.g., 5.0 vs 40 mu L/min), and the use of a platinum capillary in the ES device, rather than the more commonly used stainless steel capillary. Neutral metallocenes, metalloporphyrins, and polycyclic aromatic hydrocarbons are used as the model compounds, Operation of the ES ion source in the manner described expands the neutral compound types amenable to low level detection by ES-MS to include even those that are relatively difficult to oxidize (i,e,, E > 1.0 V vs SCE) and, therefore, also expands the universality of ES as an ionization source, From the electrochemical point of view, this operation of the ES ion source might be viewed as a means to provide molecular weight information, and possibly the structure, for the ionic products formed during a controlled-current electrolysis experiment.

An electrospray (ES) ion source is described as a constant or controlled-current device for which the magnitude of the ES current is controlled by the rate of charged droplet production. Thus, the nature of the electrolytic process that occurs in the metal ES capillary to charge balance the loss of one ion polarity in the charged droplets is shown to be analogous to that of a controlled-current electrolytic (CCE) cell and controlled-current electrolysis carried out in a now cell. That is, the potential at the metal/solution interface in the ES capillary, which ultimately determines whether or nota particular species will undergo a redox reaction in the capillary, is a function of both the ES current and the relative redox potentials and concentrations of the various species in the solvent system, including the metal capillary. Furthermore, the extent to which one or more reactions occur is limited both by the ES current and by the now rate of the solvent system through the ES capillary. Experimental confirmation of the ES ion source as a CCE cell is made through experiments employing a novel ES ion source in which the effluent from the ES capillary enters the detection cell of a UV/visible diode array spectrophotometer prior to the spraying process. This ES setup allowed for the first time the detection of the products of the redox reactions in the ES capillary, while they were still in solution, thereby avoiding experimental complications imposed by the spraying process or by the subsequent mass analysis of the gas-phase ions that might complicate data interpretation. The analytical implications of the operation of the ES ion source as a CCE cell for neutral compound ionization and detection in ES-MS are briefly discussed.

Analytes are typically detectable by electrospray ionization mass spectrometry (ES-MS) only if they are ionic in solution. Neutral, nonpolar analytes are not generally amenable to the technique. In this paper, neutral polycyclic aromatic hydrocarbons (PAHs), a heteroaromatic, a substituted aromatic, and the highly conjugated molecule buckminsterfullerene (C-60) are ionized (i.e., derivatized) in solution via reaction with the chemical electron-transfer reagents trifluoroacetic acid (TFA), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), or antimony pentafluoride and then detected in the gas phase as their respective radical cations by ES-MS. The nature of these electron-transfer reactions dictates selectivity for analytes of this type, i.e., analytes that are easy to oxidize. The oxidizing strength of the chemical electron-transfer reagents determines the degree of ionization/detection selectivity. Weak oxidants provide selectivity for the easiest to oxidize compounds while stronger oxidants provide for greater detectability (i.e., greater ionization efficiency) and more universal detection among analytes that undergo these reactions. For the ionization of a suite of PAHs, the relative oxidizing strength of the solvent/ oxidant systems investigated was methylene chloride/0.1% TFA (v/v) < methylene chloride/0.1% TFA/DDQ (v/v/60 mu M) < methylene chloride/0.1% TFA/0.5% antimony pentafluoride (v/v/v). The potential of this derivatization approach to be used on-line following a separation method for selective analyte ionization/detection in ES-MS is demonstrated using now injection experiments.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIon trap mass spectrometry of externally generated ionsScott A. McLuckey, Gary J. Van Berkel, Douglas E. Goeringer, and Gary L. GlishCite this: Anal. Chem. 1994, 66, 13, 689A–696APublication Date (Print):July 1, 1994Publication History Published online22 September 2008Published inissue 1 July 1994https://pubs.acs.org/doi/10.1021/ac00085a001https://doi.org/10.1021/ac00085a001research-articleACS PublicationsRequest reuse permissionsArticle Views461Altmetric-Citations94LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Cyclic voltammetry (CV), UV/visible absorption spectroscopy, and electrospray mass spectrometry (ES-MS) are used in conjunction to study the mono- and/or dications produced in solution from the reaction of three model compounds (beta-carotene, cobalt(II) octaethylporphyrin (Co(II)OEP), nickel(II) octaethylporphyrin (Ni(II)OEP)), in three different solvent/electron-transfer reagent systems (methylene chloride/0.1% trifluoroacetic acid (TFA) (v/v), methylene chloride/0.1% TFA/2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (v/v/200 mu M), methylene chloride/0.1% TFA/0.1% antimony pentafluoride (SbF5) (v/v/v)). The reactions were carried out on-line with ES-MS by means of flow injection. Correlation of the CV data for these analytes with the ionic species determined to be in the solution on the basis of UV/visible absorption spectra and/or on the basis of the ionic species observed in the gas phase by ES-MS, along with our previously published data on these solvent/reagent systems, allowed an effective oxidation potential range, E, to be assigned to these solvent/reagent systems: methylene chloride/0.1% TFA (v/v), 0.6 V less than or equal to E(TFA) < 0.7 V; methylene chloride/0.1% TFA/ DDQ (v/v/200 mu M), 0.8 V less than or equal to E(TFA/DDQ) < 1.0 V; methylene chloride/0.1% TFA/0.1% SbF5 (v/v/v), 1.3 less than or equal to E(TFA/SbF5) < 1.5. Knowledge of the solvent/reagent oxidation potentials and the electrochemical redox potentials of a particular analyte of interest allows a solvent/reagent system to be chosen to selectively ionize an analyte, or a range of analytes, to a desired ionic state for subsequent analysis by ES-MS. The ability to detect, in the gas phase, an ionic species produced in solution by chemical electron transfer will depend, however, on the stability of the ion in the given solvent/reagent system and on whether the ion can survive intact the transfer to the gas phase via the ES process.

Derivatization strategies and specific derivatization reactions for conversion of simple alkyl halides, alcohols, phenols, thiols, and amines to ionic or solution-ionizable derivatives, that is ''electrospray active'' (ES-active) forms of the analyte, are presented. Use of these reactions allows detection of analytes among those listed that are not normally amenable to analysis by electrospray ionization mass spectrometry (ES-MS). In addition, these reactions provide for analysis specificity and flexibility through functional group specific derivatization and through the formation of derivatives that can be detected in positive ion or in negative ion mode. For a few of the functional groups, amphoteric derivatives are formed that can be analyzed in either positive or negative ion modes. General synthetic strategies for transformation of members of these five compound classes to ES-active species are presented along with illustrative examples of suitable derivatives. Selected derivatives were prepared using model compounds and the ES mass spectra obtained for these derivatives are discussed. The analytical utility of derivatization for ES-MS analysis is illustrated in three experiments: (1) specific detection of the major secondary alcohol in oil of peppermint, (2) selective detection of phenols within a synthetic mixture of phenols, and (3) identification of the medicinal amines within a commercially available cold medication as primary, secondary or tertiary.

The utility of electrospray ionization combined with mass spectrometry (ES-MS) for the analysis of free-base, nickel and vanadyl geoporphyrins is demonstrated. In positive ion mode, free-base alkyl-substituted porphyrins are detected as the protonated molecule, (M + H)+, while the nickel and vanadyl chelates of these same porphyrins are observed as their radical cations, M.+, as is chlorophyll a. Detection of free-base octaethylporphyrin (OEP) using continuous infusion at 1-5 muL/min required sampling as little as 1 fmol of material, while at these same flow rates 18 fmol of OEP could be detected via flow injection. Detection of 500 fmol of mesoporphyrin IX dimethyl ester injected is demonstrated using on-line reverse-phase microbore-HPLC at a solvent flow rate of 40 muL/min. ES-MS is shown to be well-suited for geoporphyrin molecular weight and carbon number determination since no fragment ions are produced by this ionization process. Accuracy in determining the relative abundances of porphyrins within geoporphyrin mixtures is demonstrated using standard solutions containing known molar ratios of OEP and free-base etioporphyrin-III (etio-III) and a total free-base porphyrin mixture isolated from Gilsonite bitumen. On-line separation/mass spectrometric detection of free-base and nickel geoporphyrin mixtures using reverse-phase microbore-HPLC/ES/MS is demonstrated with Gilsonite porphyrins.

A straightforward approach for determining the cyclobutane pyrimidine dimers, thymine-thymine, uracil-thymine, and uracil-uracil, based upon electrospray ionization/ion trap mass spectrometry is described. These compounds are detected as the sodiated molecules, [M + Na]+, the response for which is maximized by adding low concentrations of sodium salts to the electrospray solutions. Under these conditions subnanogram levels of the dimers are determined by flow injection analysis. Structural information is obtained by collision-induced dissociation of the lithium adducts, [M + Li]+, which are formed by adding lithium salts to the solvents.

A variety of experimental data is presented that implicates electrochemical oxidation of analytes in the electrospray (ES) needle as the mechanism for formation of molecular radical cations observed in the ES ionization mass spectra of alkyl-substituted metalloporphyrins, polycyclic aromatic hydrocarbons (PAH's), and other compound types. Analyte structural characteristics and solution-phase half-wave oxidation potentials (which correlate with gas-phase ionization energies) can be used to evaluate the likelihood of forming and observing a particular compound as a radical cation. Use of an appropriate solvent is critical in the observation of radical cations generated by the ES process. In addition to dissolving the analyte and providing a stable electrospray, the solvent(s) must "stabillize" or otherwise protect the radical cation from reactions in solution. Appropriate solvent systems (e.g., methylene chloride/0.1 % trifluoroacetic acid) are much the same as used in traditional studies of electrochemically generated radical cations. The ability to produce radical cations in the ES process expands the utility of ES ionization mass spectrometry to include compound classes not normally amenable to the technique (e.g., neutral, nonpolar compounds such as PAH's) and provides for generation of a different type of molecular species than normally produced in positive-ion ES ionization (i.e., M.+ versus (M + H)+, (M + Na)+, etc.).

The number of stages of mass spectrometry that can be performed with a quadrupole ion trap is discussed. The conditions necessary, both instrumental and chemical, to maximize the number of stages are delineated. The overall efficiency of the MSn experiment is broken down into the isolation efficiency of the initial parent ion, the conversion efficiencies associated with the conversion of first generation parents to second generation parents, and so on, and the efficiency of the final step which yields the final product ions. The conversion efficiencies are further broken down into component terms that reflect both instrumental and chemical factors. Both collision-induced dissociation reactions and low energy ion/molecule reactions are discussed. These considerations are illustrated with MSn experiments involving ions derived from aniline, N,N-dimethylaniline, and several porphyrins.

Exact masses (±0.0001 u) are tabulated (Cmin− C50) for free-base, Mn, Fe, Ni, Cu, Zn, vanadyl (VO), and Ga complexes of the major geoporphyrin skeletal types and several commercially available porphyrin complexes. The data in these tables are intended as a resource to aid in geoporphyrin identification (i.e. determination of carbon number, skeletal type and metal chelated) by low-resolution electron ionization-mass spectrometry (EI-MS) and by high-resolution EI-MS in cases where exact mass determination is performed. A comparison is made of the isotope patterns calculated for the molecular ions of the various porphyrin skeletal types as free-base species and as the seven metal complexes mentioned above. The isotope pattern of a metalloporphyrin molecular ion is shown to be a useful aid in identifying the metal complexed. Correction of molecular ion abundances (i.e. porphyrin abundances) for overlap of molecular ion isotope peaks and for overlap of fragment ion peaks with molecular ion peaks, both of which can occur when analyzing complex geoporphyrin mixtures, is discussed.