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.     

Double tracer whole body autoradiography using a short-lived positron emitter and a long-lived beta emitter

We investigated glucose and amino acid metabolism in tumors and other organs using whole body autoradiography with a short-lived positron emitter and a long-lived beta emitter. The radioactive compounds used were 2-deoxy-2-[18F]-fluoro-D-glucose (18F-FDG) with a half life of 109.8 min and L-[methyl-14C]-methionine (14C-MET) with a half life of 5,730 years. A Donryu rat weighing about 150 g was subcutaneously inoculated at the back with experimental tumors of AH109A and AH272. 74 MBq (2 mCi) of 18F-FDG and 740 kBq (20 microCi) of 14C-MET was administered and after 30 min, the rat was sacrificed. Whole body frozen sections were obtained using autocryotome. For the 18F-FDG autoradiogram, the frozen sections were exposed to an X-ray film for 6 h. After seven days, these frozen sections were again exposed to 14C-MET for a week. Cross-contamination was minimized by adjusting the exposure time, the interval of exposures and the administered dose. The accumulation of the tracers was represented as the optical density ratio of the tissue of interest to the muscle. The tumor ratios were 12.5 for 18F-FDG and 8.6 for 14C-MET showing the highest accumulation in the whole body autoradiogram. In contrast the inflammatory tissue ratios were 1.27 for 18F-FDG and 0.77 for 14C-MET showing very low amino acid metabolism. With the present double tracer whole body autoradiogram, 18F-FDG accumulation was seen in the brain and the heart but not to the liver as against 14C-MET accumulation which was seen to the liver but not to the brain and the heart.     

Electron transport in biological processes: Electrochemical behaviour of ubiquinone Q,10 adsorbed on a pyrolytic graphite electrode

The E-pH diagram has enabled us to obtain the acid-base constant of ubiquinone and the equilibrium potential at pH = 0 for the different couples participating in the process of quinone adsorption on a pyrolytic graphite electrode in aqueous buffered media. The electrochemical behaviour found for ubiquinone may explain in part the in vivo ubiquinone processes. The molecular contour of deoxyribonucleic acid changes the stability of the semiubiquinone intermediate, as shown by chemiluminescence and binding experiments.     

Electrochemical and spectroscopic characterization of surface sol-gel processes

(3-Mercaptopropyl)trimethoxysilane (MTS) forms a unique film on a platinum substrate by self-assembly and sol-gel cross-linking. The gelating and drying states of the self-assembled MTS sol-gel films were probed by use of electrochemical and spectroscopic methods. The thiol moiety was the only active group within the sol-gel network. Gold nanoparticles were employed to detect the availability of the thiol group and their interaction further indicated the physicochemical states of the sol-gel inner structure. It was found that the thiol groups in the open porous MTS aerogel matrix were accessible to the gold nanoparticles while thiol groups in the compact MTS xerogel network were not accessible to the gold nanoparticles. The characteristics of the sol-gel matrix change with time because of its own irreversible gelating and drying process. The present work provides direct evidence of gold nanoparticle binding with thiol groups within the sol-gel structures and explains the different permeability of "aerogel" and "xerogel" films of MTS on the basis of electrochemical and spectroscopic results. Two endogenous species, hydrogen peroxide and ascorbic acid, were used to test the permeability of the self-assembled sol-gel film in different states. The MTS xerogel film on the platinum electrode was extremely selective against ascorbic acid while maintaining high sensitivity to hydrogen peroxide in contrast to the relatively high permeability of ascorbic acid in the MTS aerogel film. This study showed the potential of the MTS sol-gel film as a nanoporous material in biosensor development.     

Electrochemical noise generated by anodic dissolution or diffusion processes

A model of the noise generated by electrochemical reactions and by diffusion is proposed. The elementary fluctuations are supposed to be the particle fluxes which are Poisson white noise. This model is successfully used to describe the experimental stochastic behaviour of two cases of non-equilibrium electrochemical interfaces: the noise generated by anodic dissolution of iron in acidic medium and that by diffusion of a reacting species in the bulk of the electrolyte.     

Accurate mass determination by multiple sprayers nano-electrospray mass spectrometry on a magnetic sector instrument

A new technique for accurate mass determination by using multiple sprayers nano-electrospray ionization mass spectrometry (nano-ESI-MS) on a magnetic sector instrument is described. Metal coated glass capillaries were used as nano-ESI sprayers. One of the sprayers was used for the reference compound solution, and others were used for the introduction of sample solutions. The spectra of the different compounds were obtained by shifting each sprayer's position relative to the sampling orifice. The accurate masses of several standard compounds were obtained with good accuracy, without problems arising from differences in ionization efficiency between the sample compounds and reference compound.     

Electrochemical processes on the copper electrode in water-ethanol solutions of copper sulfate

The system Cu/H₂O-C₂H₅OH-CuSO₄ was studied in a wide range of organic component concentrations by the impedance spectroscopy method. In the studied range of ethanol concentrations the diffusion of ions to an electrode is the limiting stage of the electrode process. An increase in the ethanol concentration results in a decrease in the double electrical layer capacity, which is caused by a change in the double electrical layer structure at the electrode-solution boundary.     

Electrochemical activation of chain radical processes by radical organophosphorus cations

It is found that electrochemically generated radical cations of organophosphorus compounds react with substrates that are capable of homolytic cleavage of the element-hydrogen bond via a radical detachment of the hydrogen atom, thus initiating the chain radical addition of the substrates over the double bond of alkenes. The presence of a strong base that is capable of deprotonating intermediate phosphonium salts in electrolyte allows one to set up an electrocatalytic cycle and use organophosphorus compounds in catalytic quantities. The main side reaction in the studied processes is the interaction between radical cations of organophosphorus compounds and olefin which leads to the formation of phosphorylated alkenes.     

Imaging G-SIMS: a novel bismuth-manganese source emitter

G-SIMS (gentle-SIMS) is a powerful method that considerably simplifies complex static secondary ion mass spectrometry (SSIMS) analysis of organics at surfaces. G-SIMS uses two primary ion beams that generate high and low fragmentation conditions at the surface. This allows an extrapolation to equivalent experimental conditions with very low fragmentation. Consequently, the spectra are less complex, contain more structural information and are simpler to interpret. In general, G-SIMS spectra more closely resemble electron ionisation mass spectra than SSIMS spectra. A barrier for the wider uptake of G-SIMS is the requirement for two ion beams producing suitably different fragmentation conditions and the need for their spatial registration (spatial alignment) at the surface, which is especially important for heterogeneous samples. The most popular source is the liquid metal ion source (LMIS), which is now sold with almost every new time-of-flight (TOF)-SIMS instrument. Here, we have developed a novel bismuth-manganese emitter (the 'G-tip') for the popular LMISs. This simplifies the alignment and gives excellent G-SIMS imaging and spectroscopy without significantly compromising the bismuth cluster ion beam performance.     

Development and evaluation of a nano-electrospray ionisation source for atmospheric pressure ion mobility spectrometry

A nano-electrospray ionisation source has been designed and constructed for a high temperature ion mobility spectrometer. The drift cell was modified by replacement of the 63Ni atmospheric pressure chemical ionisation source with a tube lens/desolvation region and operated using commercial nano-electrospray capillaries. Ions were introduced into the drift region via a Bradbury-Nielson gate (pulse width 50 micros, repetition period 20 ms). A unidirectional flow of nitrogen was used as the drift gas at temperatures in the range 100-150 degrees C to aid desolvation. The performance of the nano-electrospray ion source has been demonstrated for analytes including crown ethers, amino acids and peptides. Reduced mobilities determined by nano-ESI were consistent with those reported using a 63Ni ion source.     

Electrochemical oxygen transfer reactions: electrode materials,surface processes,kinetic models,linear free energy correlations,and perspectives

This work summarizes progresses achieved in electrochemical oxygen transfer reactions from water to organic pollutant molecules on metal oxide electrodes during the past two decades. Fundamental understanding of the dynamics of the electrochemical oxygen transfer reaction is of crucial importance for the development of key concepts of electrocatalytic processes, leading to the implementation of environmental electrochemistry wastewater treatment schemes with rational design of the suitable electrocatalytic systems. We discuss the current knowledge on the electrochemical oxygen transfer reaction, emphasizing the importance of surface processes in order to generalize mechanistically the experimental results obtained on different electrode materials, describing also the practical kinetic models developed and their implications. From the information gathered in this review, it is apparent that explanations for the kinetics of the reactions in relation to the structure of the organic compounds involved is lacking, hence that new information about structure-reactivity relationships is needed. We show in particular that the open circuit decay of the concentration of radical cations, obtained from spectroelectrochemical data, allows correlating the structure of adsorbed states with reactivity during oxygen transfer reactions, pointing as well to research efforts required to understand the catalytic performance of metal oxide electrodes in decomposing organic compounds strongly adsorbed on their surfaces. Finally, some perspectives for future research in this area are briefly commented.     

纳升级电喷雾离子源-四极杆-飞行时间质谱中磷酸化肽的离子抑制作用

目前磷酸化蛋白质组学研究中的主要技术是蛋白质酶解产生的磷酸化肽的质谱检测。但是实际样品中的磷酸化肽(特别是多磷酸化肽)很难被检测到。其原因普遍认为是由于质谱检测时,非磷酸化肽抑制磷酸化肽。但也有认为非磷酸化肽对磷酸化肽没有抑制作用。另外磷酸化肽之间是否存在离子抑制作用还没有报道。本文采用相同氨基酸序列的标准磷酸化肽和非磷酸化肽,将其单独和混合进行质谱检测,通过对比混合前后磷酸化肽的信号强度,证明了非磷酸化肽对磷酸化肽有离子抑制作用;单磷酸化肽对二磷酸化肽有一定的抑制作用,但不太显著;单磷酸化肽对三磷酸化肽、二磷酸化肽对三磷酸化肽均有显著的离子抑制作用。该研究为今后单磷酸化肽和多磷酸化肽的分段富集和检测提供了有力的证明。     

Fast profiling of anthocyanins in wine by desorption nano-electrospray ionization mass spectrometry

Desorption electrospray ionization (DESI) mass spectrometry appears to be a useful technique applicable in different areas (e.g. analysis of pharmaceuticals, identification of biologically active compounds in tissues, imaging mass spectrometry). Its modification termed desorption nano-electrospray (nano-DESI) was tested for analysis of anthocyanins. Acidifying of samples and acidic spray liquid (methanol:water = 75:25 with 0.2% HCOOH) were essential for obtaining good quality spectra. Profiles of main anthocyanins in wine samples, two vintages (2005 and 2007) of three cultivars (Alibernet, Neronet and Rubinet), were successfully acquired. They were in agreement with results of LC/MS experiments (anthocyanins isolated by solid phase extraction were separated by μ-HPLC with gradient elution and detected by ESI-MS). Nano-DESI-MS data also allowed to determine ratio of two cultivars (Neronet and Rubinet) in their mixture and to detect coloring of wine by tenturier or elderberry extract. Detection of main anthocyanins in slices of wine grape, chokeberries and elderberries or in a wine stain on cotton fabric is also presented.     

Chelation reactions of acetylacetone on a field ion emitter

Reactions of metal chlorides adsorbed on a field ion emitter with acetylacetone admitted from the gas phase lead to the formation of a number of different acetylacetonate ions. It is shown that these ions originate mainly from chelation reactions in which acetylacetonate molecules are formed prior to field ionization.     

Towards a miniaturized non-radioactive electron emitter with proximity focusing

Most state of the art gas sensor systems based on atmospheric pressure ionization, such ion mobility spectrometers use radioactive beta-sources (e.g. ³H or ⁶³Ni) to provide free electrons with high kinetic energy to initiate a chemical gas phase ionization of the analytes to be detected. Here, we introduce a non-radioactive electron emitter which generates free electrons at atmospheric pressure. Therefore, electrons are generated in a vacuum by field emission and accelerated towards a 300 nm thin 1 mm² silicon nitride membrane separating the vacuum from atmospheric pressure. Electron currents of about a few hundred microamps can be reached. High energetic electrons of about 10 keV can easily penetrate the membrane without significant loss of kinetic energy. The concept of proximity focusing avoids complex electron lenses to focus the electron beam onto the membrane. The used field emitter tips are made of multi-walled carbon nanotubes. Another benefit of our system is that no insulated power supply operating at high voltage is needed, as necessary for thermo emitters. Here, we show a first prototype of a proximity focused electron gun with field emitting carbon nanotubes. The system is coupled to our drift tube ion mobility spectrometer for validation. Ion mobility spectra similar to those of a ³H ionization source were achieved.     

On the correlation between emitter heating current and emitter temperature in field desorption mass spectrometry

Calibration curves have been established that give an estimate of the temperature in the emission region of high temperature activated field desorption emitters. Two characteristic types of field anodes have been investigated: (1) emitter wires carrying microneedles of 25 to 30 μm length, preferentially used for field desorption of organic compounds; (2) emitters with microneedles of 50 to 60 μm length, which are used for the investigations of polymers by pyrolysis field desorption mass spectrometry.     

Electrochemical methods in understanding the redox processes of drugs and biomolecules and their sensing

Progression of the biochemical information of drugs and its interaction with biological substances require efficient methodologies suitable for generating information without incurring any significant damage to the analytes. Electrochemical methods provide useful information and compatible to the biological environments, without any significant alterations in the analyte, the methods are capable in onsite applications. Present review discusses about the application of the electrochemical techniques in evaluating the mechanism of interaction of drugs with the biomaterials like proteins, lipids, and DNA. The redox process leads to the determination of the drugs and biomolecules through the development of modified electrodes. The modified ultra-microelectrodes (UMEs) or nano electrodes are useful in sensing at the extracellular level of single cells. The electrochemical collision using UMEs provides information about the single molecular level. Present article discusses a brief review of some of the drug and biomolecule interactions using macro and also UMEs.     

Electrochemical nanogravimetric studies of sulfur/sulfide redox processes on gold surface

Electrochemical quartz crystal microbalance, combined with cyclic voltammetric, chronoamperometric, and potentiostatic measurements, was used to study electrodeposition/dissolution phenomena at a gold electrode in solutions containing Na₂S. Spontaneous, open-circuit deposition processes as well as dissolution of the deposits in sulfide-free solutions have also been investigated. The potential range, scan rate, sulfide concentration, and pH have been varied. The results of the piezoelectric nanogravimetric studies are elucidated by a rather complex scheme involving underpotential deposition of sulfur at approximately −0.85 V vs. sodium calomel electrode, reductive dissolution of the deposited sulfur-containing layer at potentials more negative than approximately −0.9 V, and formation of a sulfur-containing multilayer at potentials more positive than −0.2 V. During the reduction of sulfur deposited on Au, a mass increase due to the formation of polysulfide species in the surface layer, accompanied by incorporation of Na⁺ counterions, can be observed that starts at approximately −0.4 V. This is a reversible process, i.e., during the reoxidation, counterions leave the surface layers. Frequency excursions during the electroreduction and reoxidation processes reveal existence of several competitive dissolution–deposition steps. Spontaneous interaction between Au and HS⁻ species results in a surface mass increase at the open-circuit potential, and it also manifests itself in the substantial decrease of the open-circuit potential after addition of Na₂S to the supporting electrolyte.