Electrochemical ionization

Evidence for the operation of cation radical mechanisms for cycloaddition has often been provided by means of a comparison of the results obtained for various methods of generating cation radicals. For example, in the Diels-Alder cycloaddition of phenyl vinyl sulfide to 1,3-cyclopentadiene (Scheme 36) the same adducts are formed whether the cation radicals are generated by chemical ionization (aminium salt), photochemical ionization (the PET method), or electrochemical ionization (anodic oxidation) [65]. 

When an electrocatalytic reaction involves a primary step of molecular dissociative chemisorption, for example, a c,e mechanism, then the electrocatalysis arises more directly, in the same way as for many regular catalytic processes that involve such a step of dissociative chemisorption. In this type of electrocatalytic reaction, the dissociated adsorbed fragments, for example, adsorbed H in H2 oxidation, become electrochemically ionized or oxidized in one or more charge-transfer steps following the initial dissociation. The rate... 

The group of Van Berkel [40-42] studied electrochemical processes in the ESI source, e.g., by demonstrating that the radical cations observed in the ESI mass spectra of metalloporphyrins and polycyclic aromatic hydrocarbons have an electrochemical origin [40], and by proposing strategies to convert analytes into electrochemically-ionizable derivatives [42]. In a review on analytical applications of on-line electrochemistry-ESl-MS, Diehl and Karst [43] discussed topics related to the electrochemical processes in ESI as well. 

Van Berkel, G.J., J.M.E. Quirke, R.A. Tigani, A.S. Dilley, and T.R. Covey. 1998. De-rivatization for electrospray ionization mass spectroscopy. 3. Electrochemically ionizable derivatives. Anal. Chem. 70 1544—1554. 

In contrast to the unwanted electrochemical alteration of the analyte just discussed, electrochemical ionization via oxidation or reduction of the analyte can be very advantageous. Using the ES ion source to electrochemically ionize an analyte provides the means to expand sensitive analysis/detection by ES-MS to include certain types of neutral analytes that otherwise are ES inactive, as well as expanding the overall universality of ES as an ionization source. Those analytes found to be most amenable to electrochemical ionization and detection in ES-MS form relatively stable ionic species upon electrochemical oxidation/reduction (Table 3.4). This is required, because the electrochemically... 

Table 3.4. Analytes reported to be electrochemically ionized in ES capillary during ES-MS...

PAHs, heteroaromatics, carotinoids, etc.) Ferrocene-based electrochemically ionizable 40, 52, 113-117, 124... 

The solvent system can be a very important factor in electrochemical ionization. Typical solvent systems for ES-MS are comprised of various combinations of methanol, acetonitrile, and/or water along with a small amount of acidic or basic additives. Such solvent systems are chosen because of the solubility characteristics of the more common analytes, because they produce a stable spray, and because they allow for solution phase ionization of the compounds (typically ionization via salt dissolution or acid/base chemistry ).Inthecase of electrochemical ionization, a more careful selection of the solvent may be required because particular ionic species produced may be consumed by several types of rapid reactions in solution, thus certain solvents/additives may have to be avoided. For example, particularly important with radical cations are reactions with nucleophilic solvents or solvent additives. 

Cole and co-workers study" of the electrochemical ionization of metallocenes in ES-MS cites several examples of the occurence of chemical follow-up reactions. For example, the electrochemical oxidation products of ruthenocene (Cp2Ru ) and osmocene (Cp20s ) are known to lack solution-phase stability and to be very susceptible to nucleophilic addition reactions. The ES mass spectra of these metallocenes sprayed at a relatively slow flow rate (1.6pL/min) from a methylene chloride/0.5-1.0% trifluoroacetic acid (TEA) solution include peaks corresponding to the chloride ion addition products ([Cp2RuCl]and [CP2OSCI] ) and, in the case of osmocene, the trifluoroacetate addition product ([CP2OS - - trifluoroacetate] ). 

Four major requirements are key to utilizing electrochemical ionization for efficient and low-level, gas-phase detection in ES-MS in addition to favorable redox properties of the... 

The ultimate detectability demonstrated so far in electrochemical ionization may be for ferrocene-based, electrochemically ionizable derivatives. Figure 3.19a shows the selected reaction monitoring (SRM, m/z 613 m/z 245) results for four replicate injections of a blank and derivative standards (ferrocenecarbamate esters of cholesterol) ranging... 

The chemical follow-up reactions or homogeneous reactions that can be detrimental to simple electrochemical ionization can be analytically useful in their own right. In such a case, the initial products of the electrolysis react with the analyte of interest and the product of this reaction provides an analytical advantage in the experiments. In some ways this mirrors the coulometric titrations used in classic electrochemistry. 

Van Berkel, G. J. Zhou, F. Electrospray as a controlled-current electrolytic cell electrochemical ionization of neutral analytes for detection by electrospray-mass spectrometry. Anal. Chem. 1995, 67, 3958-3964. 

Dupont, A. Gisselbrecht, J.-R Leize, E. Wagner, L. Van DratssELAER, A. Electrospray mass spectrometry of electrochemically ionized molecules Application to the smdy of fiillerenes. Tetrahedron. Lett. 1994, 35, 6083-6086. 

Term II represents the desorption of the adsorbed species from the reactive surface and decreases the surface coverage. Term III is the forward electrochemical ionization reaction responsible for current flow. From inspection of Eq. (4.62), in the case of an adsorption-limited reaction, the kinetic limiting current density should be the maximum possible adsorption rate where the surface coverage 6 becomes zero, or... 

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