An evaluation of atmospheric pressure ionization techniques for the analysis of N-Methyl carbamate pesticides by liquid chromatography mass spectrometry

Atmospheric pressure ionization (API) techniques are evaluated for the mass spectral analysis of N-methyl carbamate pesticides. Atmospheric pressure chemical ionization (APCI) using a heated nebulizer interface provided both protonated molecules and abundant, characteristic fragment ions. With ion spray (ISP; pneumatically assisted electrospray ionization), which utilizes a milder “ion evaporation” process, primarily protonated molecules were obtained, although fragment ions similar to those observed in APCI could be induced by variation of the API orifice voltage. Product ion spectra of ISP-derived protonated molecules, generated by tandem mass spectrometry using collision-induced dissociation, are also presented. The APCI and ISP spectra of the carbamates are compared to those obtained with a thermospray interface and also to their electron ionization and methane CI spectra obtained with a particle beam interface. For all four interfaces, combined liquid chromatography mass spectrometry methods using conventional (4.6 mm i.d.) columns are described for the separation and detection of pesticide mixtures. These methods are applied to the confirmatory analysis of three representative carbamate pesticides, spiked at the 0.1-ppm level in green peppers. For those carbamates amenable to gas chromatography mass spectrometry, comparative results are presented.     

Tune compounds for electrospray ionisation/in-source collision-induced dissociation with mass spectral library searching

Haloperidol, paracetamol, metronidazole and metamizole have been tested as tune compounds for electrospray ionisation in-source collision-induced dissociation MS (ESI-CID-MS) with two different mass spectrometers (Sciex API 365 and Agilent 1100 MSD SL). The different electrospray sources of API 365 and MSD 1100 SL consist of an orifice with nitrogen curtain gas and a capillary interface, respectively. In-source CID occurs in both interfaces in front of the skimmers, which separate a region with a vacuum of approximately 300 Pa and the high vacuum (<10(-3) Pa). Comparison of the breakdown curves of selected tune compounds, depending on collision energy (orifice or fragmentor voltage), showed, that very similar fragmentation can be obtained with both instruments, when adjusting the fragmentor voltage of the MSD 1100 SL to higher values than the orifice voltage of the API 365. For three energy levels--low, medium and high--the corresponding voltages were 20, 50 and 80 V for the API 365 and 110, 190, 230 V for the MSD 1100 SL. These voltages resulted in the most similar spectra for haloperidol and paracetamol with both instruments. The comparison of ESI-CID-MS of all tune compounds at three energy levels showed, that - despite variations in relative ion abundances - all significant ions were present in one of the three CID spectra. Therefore, mass spectral library searching of an ESI-CID-MS library set-up with one of the two instruments should be possible with the other instrument after adjusting the CID energies by means of at least two tune compounds such as haloperidol and paracetamol, metronidazole or metamizole.     

Tuning compounds for electrospray ionization/in-source collision-induced dissociation and mass spectra library searching

Tuning compounds for positive and negative electrospray ionization (ESI) were tested for the tuning of in-source collision-induced dissociation (ESI/CID) with three types of SCIEX API instruments (API 365, 2000 and 3000) in the single-quadrupole mode. The vacuum interfaces of these instruments differ slightly in geometry, but the principles of ionization and solvent evaporation by nebulizer and curtain gases, orifice and skimmer are identical. For comparison of in-source CID, breakdown curves of haloperidol, paracetamol, metronidazole and metamizole were acquired by increasing the orifice voltages. The API 2000 and 3000 required higher orifice voltages than did the API 365 to induce a similar degree of fragmentation of the protonated or deprotonated molecules to characteristic fragment ions. This increase of orifice voltage could be demonstrated with each of the four compounds tested by a shift of the maxima of the breakdown curves to higher orifice voltages. A procedure with three collision energy (CE) levels for drug identification with a mass spectra library set up with an API 365 therefore required an adjustment of the orifice voltages to higher values when being transferred to an API 2000 or API 3000. The corresponding orifice voltages for the three instruments were 20/50/80 V (API 365), 30/90/130 V (API 2000) and 40/80/120 V (API 3000). However, a change in orifice voltage of +/-10 V (with the API 2000 and 3000) hardly influenced the fit values of a library search for each single CE level. For adjusting orifice voltages with different instruments, a tuning procedure with haloperidol and paracetamol is presented. With this tuning procedure an ESI/CID mass spectra library set up for API 365 and API 150 could also be used for drug identification with an API 2000 and an API 3000 with good library search results.     

The effect of space charge in ion mobility spectrometry

We present the study of expansion of an ion cloud in the process of its drift in the ion mobility spectrometer taking into account the influence of diffusion and electric field of the space charge of the ion cloud. Nonlinear integro-differential equation describing these effects was obtained and solved numerically in the one-dimensional case. We found a threshold value of the ion density, above which the nonlinear effects associated with the space-charge field become significant, which agrees well with the experimental data and the criterion of Spangler. These nonlinear effects lead to the fact that the spatial ion distribution ceases to be Gaussian, and with good accuracy are approximated by the distribution of Kohlrausch–Williams–Watts.     

Electrospray mass spectrometry of methanol and water solutions suppression of electric discharge with SF6 gas

An equation by D. P. H. Smith predicts the capillary voltage required for the onset of electrospray (ES). For different solvents the voltage increases with the square root of the surface tension. Water requires a potential that is 1.8 times higher than that for methanol. This is verified experimentally. The higher potential required for water leads to ES in the presence of corona electric discharge. For low total ES plus corona currents, the electrosprayed analyte ion intensity is not adversely affected by the presence of discharge. At high total currents, there is a large decrease of analyte sensitivity. The sensitivity decrease is probably due to adverse space charge effect at high currents. The discharge can be suppressed by adding sulfur hexafluoride to the ambient gas. Both sensitivity and signal stability are improved. However, the sensitivity still remains lower by a factor of ≈ 4 relative to that observed with methanol. This is attributed to lower efficiency of gas-phase ion formation from charged water, relative to methanol, droplets.     

Chelate and intramolecular ion-pair formation: A guiding concept for structure/selectivity relationships in the liquid-liquid extraction of alkali and alkaline earth metal ions by anionic crown ether derivatives

Crown ether dyes with pendent anionic side-arms were synthesized for extractionspectrophotometry of alkali and alkaline earth metal ions. Dramatic changes in metal selectivity were obtained simply by changing the nature of the anionic side-arm on the same crown ether skeleton. A structure/metal selectivity relationship is discussed in detail in terms of “chelate” and “intramolecular ion-pair” formation. Small metal cations (high charge density) are preferred in the extraction by a crown ether reagent with a charge-localized anionic side-arm through the formation of a “chelate”. Large metal cations (low charge density) are preferred in the extraction by reagents with a charge-delocalized anionic side-arm through the formation of an “intramolecular ion-pair”. Steric restrictions imposed by the side-arm on the metal ion approaching the crown ether are also important factor in controlling the selectivity of these reagents.     

Ion transport by viscous gas flow through capillaries

The effects of a number of experimental parameters on the efficiency of ion transport by viscous gas flow through narrow capillaries have been studied. Both electrospray and corona ion sources were used. The experimental data are consistent with ions loss to the walls of the capillary, which initially is caused mainly by space-charge expansion, but later is caused by diffusion. These processes can result in severe discrimination against low mass ions. The extent of ion loss may be calculated by using a simple model for radial diffusional loss in long cylinders, with an exponential decay of the ion density along the transport capillary. However, such a simple model underestimates ion loss by ignoring the effects of space-charge, turbulent flow, and rapid decay of higher radial diffusion modes (enhanced loss of ions that enter the capillary close to the wall). In contrast, Monte Carlo simulations showed that the effect of the parabolic velocity profile, under laminar flow conditions, is to increase the transmitted ion current, sometimes by several orders of magnitude, relative to the predictions of the simple diffusion model. After considering all these factors, the transmitted current from a corona was well reproduced by using mobility values for ions formed in such discharges. However, the measured transmitted current from an electrospray source was much too high. To explain this, it was necessary to assume that about 2% of the electrospray current is carried by aerosol particles with radii in the 10-25-Å range. Finally, it is argued that in glass capillaries wall charging may explain why the transmitted ion current is observed to be very similar to that in metal capillaries.     

Polarized states of hybrid aligned nematic layers

Electric polarization arising in hybrid aligned nematic liquid crystal layers with rigid boundary conditions is studied numerically by solving the torques equation and Poisson equation. Three phenomena that give rise to the polarization are taken into account: flexoelectricity, surface polarization and adsorption of ions. The director orientation within the layer, as well as the distribution of electric potential and space charge density are calculated for layers deformed by an external magnetic field. The role of the ionic space charge is investigated. For a particular set of parameters of a model substance, the voltage arising between the layer surfaces varies from 10^-1 V (in an extremely pure nematic) to 10^-3 V (in material with a typical ion concentration). The surface polarization yields an additional voltage (of the order 10^-2 V) nearly independent of the ion concentration. The effect of simultaneous flexoelectric polarization and ion adsorption is evidently different from a linear superposition of their separate contributions. The flexoelectric polarization leads to partial separation of ions of opposite signs. In the case of positive flexoelectric coefficients, a thin sublayer of positive charge arises at the planar-orienting boundary plate. The negative charge is displaced towards the homeotropically aligning plate. The magnitude of this effect increases with the magnetic field. The surface phenomena introduce additional subsurface charges.     

From Chemical Kinetics to Streamer Corona Reactor and Voltage Pulse Generator

This paper discusses the global chemical kinetics of corona plasma-induced chemical reactions for pollution control. If there are no significant radical termination reactions, the pollution removal linearly depends on the corona energy density and/or the energy yield is a constant. If linear radical termination reactions play a dominant role, the removal rate shows experimental functions in terms of the corona energy density. If the radical concentration is significantly affected by nonlinear termination reactions, the removal rate depends on the square root of the corona energy density. These characteristics are also discussed with examples of VOC_s and NO_x removal and multiple processing. Moreover, this paper also discusses how to match a corona plasma reactor with a voltage pulse generator in order to increase the total energy efficiency. For a given corona reactor, a minimum peak voltage is found for matching a voltage pulse generator. Optimized relationship between the voltage rise time, the output impedance of a voltage pulse generator, and the stray capacitance of a corona reactor is presented. As an example, the paper discusses a 5.0-kW hybrid corona nonthermal plasma system for NO_x removal from exhaust gases.     

Modelling of pressure-driven membrane separation of electrolytes.: `High temperature' approximation

A model of pressure-driven membrane process of electrolyte separation is presented. The electric field potential assumed as being known, exact solution for permeate composition is readily obtained. All species are assumed to have the same convection velocity. Local electroneutrality condition is not used. The electric potential has been taken into account under high temperature approximation, thus reducing the problem to algebraic equation in exp(Ψ), where Ψ is dimensionless flow potential, and making it possible to calculate concentrations of ions in permeate. Negative retention is shown to be possible for one-component electrolyte solution. For electrolyte mixtures, concentration of ion with high charge is shown to “govern” the membrane selectivity in respect to low-charge ions. Results obtained are in qualitative accordance with the earlier experimental data on membrane separation of reaction mixtures in homogeneous catalysis.     

Coupling electrospray corona discharge, charge reduction and ion mobility mass spectrometry: From peptides to large macromolecular protein complexes

We present the design and implementation of a home-built point-to-plane corona discharge probe, which rapidly and efficiently charge reduces biological ions generated by electrospray ionization (ESI). The molecules analysed ranged from small peptides such as Glu-fibrinopeptide B (1.5 kDa), small proteins such as myoglobin (16.9 kDa), polymers such as polyethylene glycol (PEG 10 k) which all showed intense singly charged ions; to large native multiprotein complexes such as GroEL (802 kDa) which show a broad range of charge-reduced species. The corona discharge probe operates at atmospheric pressure and was directly interfaced with a standard-ESI or nanoflow-ESI source of quadrupole ion mobility time-of-flight mass spectrometer. The corona discharge probe is completely modular and could potentially be mounted to any commercial or research grade mass spectrometer with an ESI source. The level of charge reduction is precisely controlled by the applied voltage and/or probe gas flow rate and when in operation, results in approximately a 50 % reduction in total ion current. We also present the combination of corona discharge and travelling wave ion mobility and assign helium collision cross-section values (Ω_He) to the charge reduced species of the native protein complex pyruvate kinase. It would appear that the Ω_He of the +20 charge state for pyruvate kinase is approximately 20 % smaller than the +35 charge state. Finally, we discuss the potential benefits and concerns of utilising charge reduced protein species as a means of extending the travelling wave collision cross-section calibration range over that which is already published.     

Effect of the Solvent Flow Rate on the Ionization Efficiency in Atmospheric Pressure Photoionization-Mass Spectrometry

In the novel atmospheric pressure photoionization-mass spectrometry the ionization efficiency has been observed to decrease when the solvent flow rate is increased. The effect of the flow rate on the ionization efficiency was studied by comparing the behavior of two analytes, one of which is ionized through charge exchange, the other through proton transfer. Additional information about the ion loss mechanisms was obtained by comparing results obtained with two different APPI ion sources: a Sciex prototype and the Agilent/Syagen APPI source. In addition to the measurements done by using the mass analyzer, the total ion current in the ion source was obtained by measuring the currents of the ions arriving at curtain/end plate and orifice/capillary of the two mass spectrometers. The total ion current measurements showed a significant decrease at high solvent flow rates. Loss of dopant radical cations was thought to be the reason for the signal decrease of the analytes formed through charge exchange. Analytes formed through proton transfer were not as seriously ected by the high solvent flow rates, but some saturation of their signal was nevertheless observed. Loss of photons through absorption by solvent vapor is another mechanism that can be held responsible for a reduction of the total number of ions produced by the APPI source.     

Negative Corona Discharge Ion Source Under Ambient Conditions with Mini Line-cylinder Electrodes

A novel ambient negative corona discharge ion source with mini line-cylinder electrodes is designed. The diameters of inner and outer electrode are 0.16 and 4 mm respectively. With a special assembly method, a perfect coaxiality of the two electrodes is obtained. An injection system utilizing a temperature control technique, achieves a constant and stable concentration of the sample, which is critical to the experiment. The formulas of the corona onset voltage of line-cylinder electrodes are also introduced. The experiment results show that negative substances such as formic acid and acetic acid can be ionized under ambient conditions. When combined with micro electrical mechanical system fabricationprocess, the volume of the ion source can be reduced dramatically, but there is an undesirable surface discharge. To solve the surface discharge problem, an improved structure was designed and tested. The simplicity of the interface of the ion source makes it suitable for mass spectrometer, micro mass spectrometer, ion mobility spectrometer, and high-field asymmetric waveform ion mobility spectrometer applications.     

A review of: “Micelles: Theoretical and Applied Aspects.” Yoshikiyo Moroi. Plenum,New York, 1992. pp. ix + 252. 559.50. (ISBN 0-306-43996-4).

Abstract

We examine here the adsorption of weak, acidic, rigid macro‐ions onto oppositely charged surfaces using a mean field model. The analysis takes into account the effect of the nominal suspension pH on the charge distribution inside the macro‐ion layer, as well as the counter‐ion distribution in the adsorbed layer and in the solution surrounding the substrate. We find that, as expected, the adsorbed layer thickness decreases with the pH (namely, with the degree of charge dissociation) and with the solution ionic strength. The macro‐ion adsorption can, in some cases, over‐compensate for the substrate charge, thereby allowing layer‐by‐layer deposition. We find that charge inversion is obtained, for a given substrate, if the macro‐ion pK is lower than a critical value. For a given macro‐ion, charge inversion takes place if the substrate charge density exceeds a critical value that scales as the square root of the macro‐ion charge density. In both cases charge inversion is obtained only in the regime where the suspension pH is comparable to the pK.     

Space Charge Induced Nonlinear Effects in Quadrupole Ion Traps

A theoretical method was proposed in this work to study space charge effects in quadrupole ion traps, including ion trapping, ion motion frequency shift, and nonlinear effects on ion trajectories. The spatial distributions of ion clouds within quadrupole ion traps were first modeled for both 3D and linear ion traps. It is found that the electric field generated by space charge can be expressed as a summation of even-order fields, such as quadrupole field, octopole field, etc. Ion trajectories were then solved using the harmonic balance method. Similar to high-order field effects, space charge will result in an “ocean wave” shape nonlinear resonance curve for an ion under a dipolar excitation. However, the nonlinear resonance curve will be totally shifted to lower frequencies and bend towards ion secular frequency as ion motion amplitude increases, which is just the opposite effect of any even-order field. Based on theoretical derivations, methods to reduce space charge effects were proposed.

Electric mass transport of sodium chloride through cation-exchange membrane MK-40 in dilute sodium chloride solutions: A rotating membrane disk study

The polarization properties of an electromembrane system consisting of an MK-40 membrane and a dilute sodium chloride solution are investigated with an experimental apparatus, which includes a rotating membrane disk with a horizontally positioned membrane. For the electrochemical systems of MK-40/0.01 M NaCl and MK-40/0.001 M NaCl, effective ion transport numbers and partial current-voltage curves are determined for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. The space-charge distribution in the diffusion layer and in the membrane is calculated for various current densities and rotation rates of the membrane. It is shown that when electric-current densities are greater than the limiting value, ion fluxes of the salt increase as a result of a decrease in the effective thickness of the diffusion layer. This decrease is caused by the development of space charge, electroconvection, water dissociation, and the exaltation effect in the region near the membrane. It has been established that in dilute solutions the limiting current is not purely electrodiffusive in nature.     

Photoelectret state in poly(vinylcarbazole)

The photoelectret charge in the aluminum–poly(vinylcarbazole) (PVK) conducting-glass (NESA), sandwich cell configuration has been observed to decrease with voltage above some critical voltage, depending on the intensity of illumination. This critical voltage corresponds to the value at which a time-independent photocurrent is observed and also to the “knee-point” voltage in the current voltage plot. The results are explained on the basis of field-assisted detrapping setting in at high field.     

Fast,efficient generation of high‐quality atomic charges. AM1‐BCC model: I. Method

The AM1‐BCC method quickly and efficiently generates high‐quality atomic charges for use in condensed‐phase simulations. The underlying features of the electron distribution including formal charge and delocalization are first captured by AM1 atomic charges for the individual molecule. Bond charge corrections (BCCs), which have been parameterized against the HF/6‐31G* electrostatic potential (ESP) of a training set of compounds containing relevant functional groups, are then added using a formalism identical to the consensus BCI (bond charge increment) approach. As a proof of the concept, we fit BCCs simultaneously to 45 compounds including O‐, N‐, and S‐containing functionalities, aromatics, and heteroaromatics, using only 41 BCC parameters. AM1‐BCC yields charge sets of comparable quality to HF/6‐31G* ESP‐derived charges in a fraction of the time while reducing instabilities in the atomic charges compared to direct ESP‐fit methods. We then apply the BCC parameters to a small “test set” consisting of aspirin, d ‐glucose, and eryodictyol; the AM1‐BCC model again provides atomic charges of quality comparable with HF/6‐31G* RESP charges, as judged by an increase of only 0.01 to 0.02 atomic units in the root‐mean‐square (RMS) error in ESP. Based on these encouraging results, we intend to parameterize the AM1‐BCC model to provide a consistent charge model for any organic or biological molecule. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 132–146, 2000     

An NO+ reactant ion source for ion mobility spectrometry

The major reactant ion in conventional ion mobility spectrometry (IMS) is the hydronium ion, H₃O⁺ which is produced in the usual ionization sources such as corona discharge or radioactive sources. Using the hydronium reactant ion, mostly the analytes with proton affinity higher than that of water are ionized. A broader range of compounds can be detected by IMS if other alternative ionization channels, such as charge transfer from NO⁺, are employed. In this work we introduce a simple and novel method for producing NO⁺ as the major reactant ion in IMS. This was achieved by adding neutral NO to the corona discharge ionization source. The neutral NO was prepared via an additional discharge in an air stream, flowing into the corona discharge source. A curtain plate was mounted in front of the corona discharge to prevent the influence of the analyte on the production of NO⁺. Using this technique, the reactant ion could easily and quickly switch between the H₃O⁺ and NO⁺. The performance of the new source was evaluated by recording ion mobility spectra of test compounds with both H₃O⁺ and NO⁺ reactant ions.