Novel experimental arrangement developed for direct fullerene analysis by electrospray time-of-flight mass spectrometry

A novel electrospray setup was found effective for direct analysis of fullerene solutions by electrospray (ES) mass spectrometry. The electrospray capillary needle used for the analysis is equipped with a thin metal (copper, platinum or stainless steel) wire installed inside the capillary. The wire tip protrudes slightly from the capillary end. In this configuration the high electrical field formed by the wire tip stimulates a specific electrospray mode with a fine spray originating from the tip. The correlation of the acquired mass spectra with the magnified view of the spray at the capillary tip was investigated. The effective formation of fullerene ions in both negative and positive ion modes was observed in mass spectra only in the specific case of the electrospray originating from the wire tip. The fullerene di-anions observed in the negative ES mass spectra provide evidence for the electrochemical nature of this process occurring at the ES capillary tip. Observation of fullerene ions in mass spectra obtained using the suggested electrospray arrangement is assumed to be a consequence of the fine spray originating from the sharp metal wire tip. In this case the liquid/metal interface is near the Taylor cone apex.     

Negative ion electrospray mass spectrometry of nucleotides: ionization from water solution with SF6 discharge suppression

The total current and selected ion currents from the electrospray ionization (ES1) of 10^?5 M solutions of cocaine hydrochloride and deoxycytidine monophosphate (dCMP) monosodium salt in methanol and water solvents were compared in positive and negative ion modes, respectively, without and with SF₆, gas as a discharge suppressant. The ESI onset voltages (V_on), were the same for the positive and negative ion modes. The V_on, for methanol was much lower than that for water and in agreement with the equation of D. P. H. Smith, who attributes the difference to the higher surface tension of water. The onset of electric discharge (V_dis) without SF₆, occurred at lower capillary voltages for the negative relative to the positive ion mode for methanol; but V_dis is much higher than V_on for methanol, and discharges do not interfere with ESI operation. For water, V_on ≈ V_dis in the absence of SF₆, in the negative ion mode, and ESI operation is impossible without SF₆, discharge suppression. The discharge problem in the positive ion mode is less severe, but SF₆, is still very useful. A dynamic range of 10 ^?7–10^?5 M was obtained by selected ion monitoring of [dCMP - H]^? at 4.5 and 20 μL/min. flows. Subpicomole detection limits for the nucleotide salt were obtained under these conditions.     

Chromatographic and mass spectral studies of perfluorooctanesulfonate and three perfluorooctanesulfonamides

The chromatographic and mass spectral characteristics of perfluorooctanesulfonate (PFOS) and three nitrogen-substituted perfluorooctanesulfonamides have been obtained. A methyl/phenyl mixed-phase fused-silica capillary column was used for gas chromatographic (GC) analyses, while a C18 reversed-phase microbore column was used for liquid chromatographic (LC) analyses. Mass (MS) and tandem mass (MS/MS) spectra were generated using electron ionization (EI), argon CE, methane positive and negative ion CI, and ES ionization modes. EI spectra of the amides showed ions characteristic of both the fluorinated hydrocarbon and the sulfonamide portion of the molecules. The fragmentation pathway was studied using hydrogen/deuterium exchange, and was thought to involve a cyclic intermediate ion. Formation of molecular ions by CE and protonated molecule ions by CI to obtain molecular weight information was only partially successful. Negative ion ES-MS spectra provided intense [M-H]- anions for the amides, and an [M-K]- anion for PFOS from which molecular weight information could be obtained, while ES-MS/MS produced product ions that could be used to detect the presence of these compounds in biological or environmental samples.     

Comparison of the binding stoichiometries of positively charged DNA-binding drugs using positive and negative ion electrospray ionization mass spectrometry

Positive and negative ion electrospray ionization (ESI) mass spectra of complexes of positively charged small molecules (distamycin, Hoechst 33258, [Ru(phen)2dpq]Cl2 and [Ru(phen)2dpqC]Cl2) have been compared. [Ru(phen)2dpq]Cl2 and [Ru(phen)2dpqC]Cl2 bind to DNA by intercalation. Negative ion ESI mass spectra of mixtures of [Ru(phen)2dpq]Cl2 or [Ru(phen)2dpqC]Cl2 with DNA showed ions from DNA-ligand complexes consistent with solution studies. In contrast, only ions from free DNA were present in positive ion ESI mass spectra of mixtures of [Ru(phen)2dpq]Cl2 or [Ru(phen)2dpqC]Cl2 with DNA, highlighting the need for obtaining ESI mass spectra of non-covalent complexes under a range of experimental conditions. Negative ion spectra of mixtures of the minor groove binder Hoechst 33258 with DNA containing a known minor groove binding sequence were dominated by ions from a 1:1 complex. In contrast, in positive ion spectra there were also ions present from a 2:1 (Hoechst 33258: DNA) complex, suggesting an alternative binding mode was possible either in solution or in the gas phase. When Hoechst 33258 was mixed with a DNA sequence lacking a high affinity minor groove binding site, the negative ion ESI mass spectra showed that 1:1 and 2:1 complexes were formed, consistent with existence of binding modes other than minor groove binding. The data presented suggest that comparison of positive and negative ion ESI-MS spectra might provide an insight into various binding modes in both solution and the gas phase.     

单向气流负电晕放电离子迁移谱的研究及其在爆炸物检测中的应用

基于离子迁移谱的爆炸物探测仪多采用放射性电离源,发展非放射性电离源一直是该技术的研究热点。本研究基于电晕放电原理设计了一种新型负电晕放电电离源结构,结合自行研制的离子迁移谱仪,应用于痕量爆炸物的快速、高灵敏检测。单向气流模式下,对此电离源的气流、放电电压等运行参数进行了系统优化,得到最佳实验条件为：电晕放电电离源结构的电极环孔直径为3 mm,针-环距离为2 mm,放电电压为2400 V,漂气流速为1200 mL/min。在此条件下,避免了放电副产物氮氧化物和臭氧等引发的一系列复杂反应,得到了单一的反应试剂离子O-2(H2O)n。将其应用于爆炸物,如2,4,6-三硝基甲苯(TNT)、硝酸铵(AN)、硝化甘油( NG)、太安( PETN)、黑索金( RDX)等的高灵敏快速直接检测,对TNT的检测限达到200 pg/μL。结果表明,此负电晕放电电离源具有灵敏度高、结构简单、无辐射性、反应试剂离子单一等优点,在爆炸物快速高灵敏检测、公共安全保障等方面具有广阔的应用前景。     

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.     

Detection of metal ions by atomic emission spectroscopy from liquid-electrode discharge plasma

In this paper, the discharge ignited in a capillary connecting two beakers filled with electrolyte solution is investigated. During the experiment, an external electrical voltage is applied through two platinum electrodes dipped in the beakers. A gas bubble forms inside the capillary when the applied voltage is higher than 1000 V. Since the beakers are tilted slightly, after generation, the bubble moves slowly to the uphill outlet of the capillary due to buoyancy. When the bubble reaches the end of the capillary, it cracks and a bright discharge is ignited. The emission spectra of the discharge plasma are related to the metal ions dissolved in the solution and thus can be used for metal ion detection. An application of the system to measurement of water hardness is shown.     

Solvent effect on analyte charge state,signal intensity,and stability in negative ion electrospray mass spectrometry; implications for the mechanism of negative ion formation

Department of Chemistry, University of New Orleans, New Orleans, Louisiana, USA The effect of solvent composition on negative ion electrospray ionization (ESI) mass spectrometry was examined. The onset potentials for ES1 of a series of chlorinated solvents and methanol were found to be within the range predicted by D. P. H. Smith, based on differences in the surface tension of the solvents used. The tendency toward electric discharge decreased with increasing percent weight of chlorine in the solvent. This effect has been attributed to an increasing propensity for electron capture for more highly chlorinated solvents. Addition of the electron scavenger gas SF, was even more effective at suppressing corona discharge phenomena. In a comparison of ultimate signal intensity obtainable for a test analyte in 10% methanol, the highest signal, which was stable over the widest range of temperatures, was exhibited by chloroform compared to dichloromethane, 1,2-dichloroethane, carbon tetrachloride, and methanol (100%). Chloroform, thus, is a recommended solvent for negative ion electrospray mass spectrometry (ES/MS) when solubility is not a limiting issue. Solvent polarity was shown to exhibit a profound influence on the distribution of charge states in negative ion ES/MS. For both chlorinated and nonchlorinated organic solvents, the higher the solution dielectric constant, the more the charge-state distribution is shifted toward higher charge states. These observations build on the “electrophoretic” mechanism of droplet charging. Solvents with high solution dielectric constants are considered to be most effective at stabilizing multiply charged ions (where charge separation is greatest), and they are likely to increase the level of droplet charging. Solvents with high basicities (gas phase and solution phase) and high proton affinities, yet low dielectric constants, favor lower charge states in ES mass spectra of lipid A and cardiolipin from Escherichia coli. This indicates that gas-phase processes and solvent basicity contribute much less toward ion formation than solution-phase solvation via preferred orientation of the solvent dipole.     

Analysis of some metabolites of T-2 toxin, diacetoxyscirpenol and deoxynivalenol by thermospray high-performance liquid chromatography-mass spectrometry

Detection and identification of mycotoxin metabolites is a very challenging task. In order to achieve adequate sensitivity and specificity an analytical technique must overcome serious matrix interferences. Gas chromatography-mass spectrometry (GC-MS) which has the sensitivity and specificity to detect and identify mycotoxin metabolites requires hydrolysis of conjugated metabolites as well as derivatization. Thermospray high-performance liquid chromatography-mass spectrometry (HPLC-MS) offers the sensitivity, specificity, and structural information to detect and identify some mycotoxin metabolites in fecal and urine samples without derivatization. The mycotoxins evaluated in this study include deoxynivalenol (DON), T-2 toxin, and diacetoxyscirpenol. The de-epoxy and hydroxy metabolites of each toxin and the glucuronide conjugate of DON were isolated, extracted, and analyzed to detect their occurrence in animals. The thermospray mass spectra of the toxins showed an [M + H]+ ion and numerous structurally significant fragment ions in the positive ion detection mode. Negative ion detection exhibited primarily [M + acetate]- cluster ions with less fragmentation than observed by positive ion detection. The operation of the interface in the filament-on mode greatly increased the sensitivity in both positive and negative ion detection mode. Detection limits of 50-500 pg injected on column are obtained for these toxins and their metabolites using multiple ion detection. The urine and fecal extracts from rats, hens, and cows did not interfere with the HPLC-MS analysis for the specific metabolites or the glucuronide conjugate.     

Ion mobility spectrometric analysis of vaporous chemical warfare agents by the instrument with corona discharge ionization ammonia dopant ambient temperature operation

The ion mobility behavior of nineteen chemical warfare agents (7 nerve gases, 5 blister agents, 2 lachrymators, 2 blood agents, 3 choking agents) and related compounds including simulants (8 agents) and organic solvents (39) was comparably investigated by the ion mobility spectrometry instrument utilizing weak electric field linear drift tube with corona discharge ionization, ammonia doping, purified inner air drift flow circulation operated at ambient temperature and pressure. Three alkyl methylphosphonofluoridates, tabun, and four organophosphorus simulants gave the intense characteristic positive monomer-derived ion peaks and small dimer-derived ion peaks, and the later ion peaks were increased with the vapor concentrations. VX, RVX and tabun gave both characteristic positive monomer-derived ions and degradation product ions. Nitrogen mustards gave the intense characteristic positive ion peaks, and in addition distinctive negative ion peak appeared from HN3. Mustard gas, lewisite 1, o-chlorobenzylidenemalononitrile and 2-mercaptoethanol gave the characteristic negative ion peaks. Methylphosphonyl difluoride, 2-chloroacetophenone and 1,4-thioxane gave the characteristic ion peaks both in the positive and negative ion mode. 2-Chloroethylethylsulfide and allylisothiocyanate gave weak ion peaks. The marker ion peaks derived from two blood agents and three choking agents were very close to the reactant ion peak in negative ion mode and the respective reduced ion mobility was fluctuated. The reduced ion mobility of the CWA monomer-derived peaks were positively correlated with molecular masses among structurally similar agents such as G-type nerve gases and organophosphorus simulants; V-type nerve gases and nitrogen mustards. The slope values of the calibration plots of the peak heights of the characteristic marker ions versus the vapor concentrations are related to the detection sensitivity, and within chemical warfare agents examined the slope values for sarin, soman, tabun and nitrogen mustards were higher. Some CWA simulants and organic solvents gave the ion peaks eluting at the similar positions of the CWAs, resulting in false positive alarms.     

Investigation on the role of pneumatic aspects in electrospray, desorption electrospray surface ionization and surface activated chemical ionization

This review reports the results of some studies carried out by us on the role of pneumatic aspects in electrospray and desorption electrospray surface ionization, with the aim to propose some relevant aspects of the mechanisms involved in these ionization methods. Electrospray ion sources, with the exception of the nano- electrospray source, operate with the concurrent action of a strong electrical field and a supplementary coaxial gas flow. The electrical field is responsible for electrospraying of the analyte solution but the use of a coaxial gas flow leads to a significant increase of the analyte signal and allows the use of higher solution flows. However, by employing capillary voltages much lower than those necessary to activate the electrospray phenomenon, analyte ions are still observed and this indicates that different mechanisms must be operative for ion production. Under these conditions, ion generation could take place from the neutral pneumatically sprayed droplet by field-induced droplet ionization. Also in the case of desorption electrospray ionization (DESI), and without any voltage on the spraying capillary as well as on the surface of interest, ions of analytes present on the surface become detectable and this shows that desorption/ionization of analytes occurs by neutral droplets impinging the surface. Consequently, the pneumatic effect of the impinging droplets plays a relevant role, and for these reasons the method has been called pneumatic assisted desorption (PAD). Some analogies existing between PAD and surface activated chemical ionization (SACI), based on the insertion of a metallic surface inside an atmospheric pressure chemical ionization source operating without corona discharge, are discussed.     

Fast atom bombardment mass spectra of monosulphonated merocyanine dyestuffs

The FAB mass spectra of four sulphonated merocyanine dyes are investigated. Negative ion spectra are superior to those in the positive ion mode. The special structure of merocyanines with an acidic group at one end of the molecule and a basic group at the other leads to significantly different fragmentation pathways for positive and negative ions.     

The effect of the presence of foreign salts on the formation of gaseous ions for electrospray and laser spray

The effect of the presence of foreign salts (NaCl, aerosol OT, tetra-n-hexylammonium bromide, and CH3COONH4) on the formation of gaseous ions for electrospray (ES) and laser spray (LS) was studied in the positive and negative modes of operations. The ion signals for amino acids show sudden decrease with the concentration of foreign salts greater than 10(-5) M for both ES and LS. When the surface-active counter ions were added, the signal intensities showed a marked decrease for both ES and LS. This may be due to the enrichment of the surface-active counter ions on the surface of the charged droplets. When CH3COONH4 was added to an aqueous solution of 10(-6) M lysozyme chloride, an increase of the signal intensities for (lysozyme+nH)n+ and a decrease in the values of n were observed. The decrease in n may be due to the salt formation of (lysozyme+nH)n+ with the negative acetate ion leading to the reduction of positive charges.     

Ionization and transmission efficiency in an electrospray ionization—mass spectrometry interface

The ionization and transmission efficiencies of an electrospray ionization (ESI) interface were investigated to advance the understanding of how these factors affect mass spectrometry (MS) sensitivity. In addition, the effects of the ES emitter distance to the inlet, solution flow rate, and inlet temperature were characterized. Quantitative measurements of ES current loss throughout the ESI interface were accomplished by electrically isolating the front surface of the interface from the inner wall of the heated inlet capillary, enabling losses on the two surfaces to be distinguished. In addition, the ES current lost to the front surface of the ESI interface was spatially profiled with a linear array of 340-microm-diameter electrodes placed adjacent to the inlet capillary entrance. Current transmitted as gas-phase ions was differentiated from charged droplets and solvent clusters by measuring sensitivity with a single quadrupole mass spectrometer. The study revealed a large sampling efficiency into the inlet capillary (>90% at an emitter distance of 1 mm), a global rather than a local gas dynamic effect on the shape of the ES plume resulting from the gas flow conductance limit of the inlet capillary, a large (>80%) loss of analyte ions after transmission through the inlet arising from incomplete desolvation at a solution flow rate of 1.0 microL/min, and a decrease in analyte ions peak intensity at lower temperatures, despite a large increase in ES current transmission efficiency.     

Super-Atmospheric Pressure Electrospray Ion Source: Applied to Aqueous Solution

This is a follow-up paper of our previous report on an ion source, which was operated at an operating pressure higher than the atmospheric pressure. Besides having more working gas for desolvation, the reduction of mean free path of electrons in a higher pressure environment increases the threshold voltage for gaseous breakdown, thus enabling a stable electrospray for the sample solution with high surface tension without the occurrence of electric discharge. In our previous work, the ion source was not coupled directly to the mass spectrometer and significant amount of ions were lost before entering the vacuum of the mass spectrometer. In this paper, we report the new design of our second prototype in which, by using a modified ion transport capillary, the pressurized ESI ion source was coupled directly to the first pumping stage of the mass spectrometer without additional modification on the vacuum pumping system. Demonstrations of the new ion source on the sensitive detection of native proteins from aqueous solution in both positive and negative ion modes are presented.     

Formation of unexpected ions from a first‐generation polyamidoamine dendrimer by use of methanol: an artefact due to electrospray emitter corrosion?

We report the formation of unexpected ions during the analyses of a first‐generation polyamidoamine dendrimer in negative ion mode using an ion trap equipped with an electrospray ionisation source. These surprising ions corresponded to an increase of 12 m/z units over those expected. The formation of the unexpected ions was dependent on the tuning of the solution flow rate and the capillary high voltage. In addition, measurements of unusual value of the current suggested that a reaction was occurring in the corona plasma. The influence of methanol in this phenomenon was demonstrated by using CD₃OH in the sample preparation. We propose two structures to explain the observed adduct based on the results of MS² experiments and by referring to previous work dealing with 12 m/z units addition. We showed that a corona discharge caused by alterations taking place to the electrospray capillary emitter was the origin of these unexpected ions. Finally, we discuss the mechanism involved in the formation of the ions and we propose means to control such artefacts. Copyright © 2010 John Wiley & Sons, Ltd.     

A novel nanoflow interface for atmospheric pressure ionization mass spectrometry

A novel spray-ionization technique for nanoflow liquid chromatography/mass spectrometry (nLC/MS) has been developed by modifying the sonic spray ionization (SSI) technique. A solution from a tapered fused-silica capillary is sprayed by a gas flow coaxial to the capillary, and ions produced are analyzed with an ion-trap mass spectrometer. The ion intensity is shown to have a steep threshold at a low gas velocity and to be much less dependent on the gas velocity than that of conventional SSI, in which the ion intensity is strongly dependent on the gas velocity and reaches its maximum at sonic velocity. Thus, we conclude that the concentration of charge in the solution at the tapered capillary tip with an inner diameter of 15 microm is almost at saturation so that charged droplets are produced from the solution by electrical force, rather than by sheer stress due to the gas flow. The ions are readily produced from these charged droplets. Preliminary results are compared with results obtained with a miniaturized electrospray unit.     

Short-capillary electrophoresis with electrochemiluminescence detection using porous etched joint for fast analysis of lidocaine and ofloxacin

This paper describes the properties of a recently developed electrospray emitter coated with a fluorinated polymer of low surface energy. The emitters can produce stable electrospray from solvents of various surface tensions including distilled water with a high surface tension at a flow rate range of micro- to nanoliters without the aid of any nebulizing gas. The electrically non-conductive nature of the tips virtually eliminates electrical discharge and allows stable electrospray in the negative ion mode. The emitters are suitable for hyphenating HPLC to mass spectrometry in both positive and negative ion modes at low flow rates.     

Negative ion production from peptides and proteins by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

Negative ion production from peptides and proteins was investigated by matrix‐assisted laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectrometry. Although most research on peptide and protein identification with ionization by MALDI has involved the detection of positive ions, for some acidic peptides protonated molecules are not easily formed because the side chains of acidic residues are more likely to lose a proton and form a deprotonated species. After investigating more than 30 peptides and proteins in both positive and negative ion modes, [M–H]⁻ ions were detected in the negative ion mode for all peptides and proteins although the matrix used was 2,5‐dihydroxybenzoic acid (DHB), which is a good proton donor and favors the positive ion mode production of [M+H]⁺ ions. Even for highly basic peptides without an acidic site, such as myosin kinase inhibiting peptide and substance P, good negative ion signals were observed. Conversely, gastrin I (1‐14), a peptide without a highly basic site, will form positive ions. In addition, spectra obtained in the negative ion mode are usually cleaner due to absence of alkali metal adducts. This can be useful during precursor ion isolation for MS/MS studies. Copyright © 2008 John Wiley & Sons, Ltd.