Three-dimensionally (3D) resolved ion trajectory calculations within the complex viscous flow field of an atmospheric pressure ion source are presented. The model calculations are validated with spatially resolved measurements of the relative sensitivity distribution within the source enclosure, referred to as the distribution of ion acceptance (DIA) of the mass analyzer. In previous work, we have shown that the DIA shapes as well as the maximum signal strengths strongly depend on ion source operational parameters such as gas flows and temperatures, as well as electrical field gradients established by various source electrode potentials (e.g., capillary inlet port potential and spray shield potential). In all cases studied, distinct, reproducible, and, to some extent, surprising DIA patterns were observed. We have thus attempted to model selected experimental operational source modes (called operational points) using a validated computational flow dynamics derived 3D-velocity field as an input parameter set for SIMION/SDS, along with a suite of custom software for data analysis and parameter set processing. Despite the complexity of the system, the modeling results reproduce the experimentally derived DIA unexpectedly well. It is concluded that SIMION/SDS in combination with accurate computational fluid dynamics (CFD) input data and adequate analysis software is capable of successfully modeling operational points of an atmospheric pressure ion (API) source. This approach should be very useful in the computer-aided design of future API sources.
Understanding ion transport properties from the ion source to the mass spectrometer (MS) is essential for optimizing device performance. Numerical simulation helps in understanding of ion transport properties and, furthermore, facilitates instrument design. In contrast to previously reported numerical studies, ion transport simulations in a continuous injection mode whilst considering realistic space-charge effects have been carried out. The flow field was solved using Reynolds-averaged Navier-Stokes (RANS) equations, and a particle-in-cell (PIC) method was applied to solve a time-dependent electric field with local charge density. A series of ion transport simulations were carried out at different cone gas flow rates, ion source currents, and capillary voltages. A force evaluation analysis reveals that the electric force, the drag force, and the Brownian force are the three dominant forces acting on the ions. Both the experimental and simulation results indicate that cone gas flow rates of ≤250 slph (standard liter per hour) are important for high ion transmission efficiency, as higher cone gas flow rates reduce the ion signal significantly. The simulation results also show that the ion transmission efficiency reduces exponentially with an increased ion source current. Additionally, the ion loss due to space-charge effects has been found to be predominant at a higher ion source current, a lower capillary voltage, and a stronger cone gas counterflow. The interaction of the ion driving force, ion opposing force, and ion dispersion is discussed to illustrate ion transport mechanism in the ion source at atmospheric pressure.
In this study, the validation and analysis of steady state numerical simulations of the gas flows within a multi-purpose ion
source (MPIS) are presented. The experimental results were obtained with particle image velocimetry (PIV) measurements in
a non-scaled MPIS. Two-dimensional time-averaged velocity and turbulent kinetic energy distributions are presented for two
dry gas volume flow rates. The numerical results of the validation simulations are in very good agreement with the experimental
data. All significant flow features have been correctly predicted within the accuracy of the experiments. For technical reasons,
the experiments were conducted at room temperature. Thus, numerical simulations of ionization conditions at two operating
points of the MPIS are also presented. It is clearly shown that the dry gas volume flow rate has the most significant impact
on the overall flow pattern within the APLI source; far less critical is the (larger) nebulization gas flow. In addition to
the approximate solution of Reynolds-Averaged Navier-Stokes equations, a transport equation for the relative analyte concentration
has been solved. The results yield information on the three-dimensional analyte distribution within the source. It becomes
evident that for ion transport into the MS ion transfer capillary, electromagnetic forces are at least as important as fluid
dynamic forces. However, only the fluid dynamics determines the three-dimensional distribution of analyte gas. Thus, local
flow phenomena in close proximity to the spray shield are strongly impacting on the ionization efficiency. 相似文献
Results obtained with two computational approaches for the simulation of ion motion at elevated pressure are compared with
experimentally derived ion current data. The computational approaches used are charged particle tracings with the software
package SIMION ver. 8 and finite element based calculations using the software package Comsol Multiphysics ver. 4.0/4.0a.
The experimental setup consisted of a tubular corona discharge ion source coupled to a cylindrical measurement chamber held
at atmospheric pressure. Generated ions are flown into the chamber at essentially subsonic laminar isothermal conditions.
In the simulations, strictly stationary conditions were assumed. The results show very good agreement between the SIMION/SDS
model and experimental data. For the Comsol model, only qualitative agreement is observed. 相似文献
This study examines the reagent gas pressure and ion source temperature dependence for dimethyl ether chemical ionization (DME CI) mass spectra recorded with an external source ion trap mass spectrometer (ITMS). Information for better controls of the reagent gas pressure in order to obtain fair CI spectra is provided. The origin of M+? ions observed in DME CIMS is discussed in detail. Furthermore, the ion source temperature effect on the DME CI is also investigated. 相似文献
Efforts to analyze trace levels of cyclic peroxides by liquid chromatography/mass spectrometry gave evidence that acetonitrile suppressed ion formation. Further investigations extended this discovery to ketones, linear peroxides, esters, and possibly many other types of compounds, including triazole and menadione. Direct ionization suppression caused by acetonitrile was observed for multiple adduct types in both electrospray ionization and atmospheric pressure chemical ionization. The addition of only 2% acetonitrile significantly decreased the sensitivity of analyte response. Efforts to identify the mechanism were made using various nitriles. The ion suppression was reduced by substitution of an acetonitrile hydrogen with an electron-withdrawing group, but was exacerbated by electron-donating or steric groups adjacent to the nitrile. Although current theory does not explain this phenomenon, we propose that polar interactions between the various functionalities and the nitrile may be forming neutral aggregates that manifest as ionization suppression.
A new ion generation method, named plasma-spray ionization (PLASI) for direct analysis of liquid streams, such as in continuous infusion experiments or liquid chromatography (LC), is reported. PLASI addresses many of the analytical limitations of electrospray ionization (ESI) and has potential for real time process stream analysis and reaction monitoring under atmospheric conditions in non-ESI friendly scenarios. In PLASI-mass spectrometry (MS), the liquid stream is pneumatically nebulized and partially charged at low voltages; the resultant aerosol is thus entrained with a gaseous plasma plume from a distal glow discharge prior to MS detection. PLASI-MS not only overcomes ESI-MS limitations but also generates simpler mass spectra with minimal adduct and cluster formation. PLASI utilizes the atomization capabilities of an ESI sprayer operated below the ESI threshold to generate gas-phase aerosols that are then ionized by the plasma stream. When operated at or above the ESI threshold, ionization by traditional ESI mechanisms is achieved. The multimodal nature of the technique enables readily switching between plasma and ESI operation. It is expected that PLASI will enable analyzing a wide range of analytes in complex matrices and less-restricted solvent systems, providing more flexibility than that achievable by ESI alone. Figure
The use of the change in the oscillation frequency of the current of a new atmospheric helium glow discharge for sensitive
signal detection for gas chromatography is studied. The effluent of a capillary column is directed into the glow discharge
cell perpendicular to the axis of the glow discharge that existed between a platinum anode and cathode. A stable discharge
is obtained when several hundred volts are applied between the 0.2-mm gap between the anode and cathode. The effects of the
electrode gap, discharge voltage and gas flow rate on the baseline frequency and discharge current were investigated. The
chromatogram shows that the discharge current and discharge gap have a strong influence on the detector response. The discharge
current shows positive peaks; however, frequency peaks are positive or negative depending on the discharge conditions. The
response of the detector is in the femto-mole and pico-mole range for nonane and decane.
Received August 5, 1997. Revision September 2, 1999 相似文献
Allowing water/hydrogen or water/hydrogen/He gas mixture to flow through micro- hollow type of electrodes and applying 60 Hz AC power between the electrodes made it possible to sustain large area and atmospheric pressure discharge. The electrode assembly was constructed by sandwiching a dielectric spacer with two thin metal sheets and boring an array of micro holes through them. Another variation of the assembly was prepared by stacking thin metallic sheets so that the stack functions as an electrode through which the gas mixture flows for generating dielectric barrier discharge. A large volume of the gas mixture, while producing plasma, underwent instantaneous hydrogen isotope exchange reactions between H2O and D2O or between D2O and H2 gas molecules. The efficiency of the atmospheric pressure discharge was assessed by measuring the extent of the exchange reactions at a given flow rate of the gas mixture. 相似文献
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. 相似文献
During the analysis of neonicotinoid pesticide standards (thiamethoxam, clothianidin, imidacloprid, acetamiprid, and thiacloprid) by mass spectrometry, the degradation of these pesticides (M-C=N-R is degraded into M-C=O, M is the skeleton moiety, and R is NO2 or CN) was observed in the atmospheric pressure ionization interfaces (ESI and APCI). In APCI, the degradation of all the five neonicotinoid pesticides studied took place, and the primary mechanism was in-source ion/molecule reaction, in which a molecule of water (confirmed by use of H218O) attacked the carbon of the imine group accompanying with loss of NH2R (R=NO2, CN). For the nitroguanidine neonicotinoid pesticides (R=NO2, including thiamethoxam, clothianidin, and imidacloprid), higher auxiliary gas heater temperature also contributed to their degradation in APCI due to in-source pyrolysis. The degradation of the five neonicotinoid pesticides studied in ESI was not significant. In ESI, only the nitroguanidine neonicotinoid pesticides could generate the degradation products through in-source fragmentation mechanism. The degradation of cyanoamidine neonicotinoid pesticides (R=CN, including acetamiprid and thiacloprid) in ESI was not observed. The degradation of neonicotinoid pesticides in the ion source of mass spectrometer renders some adverse consequences, such as difficulty interpreting the full-scan mass spectrum, reducing the sensitivity and accuracy of quantitative analysis, and misleading whether these pesticides have degraded in the real samples. Therefore, a clear understanding of these unusual degradation reactions should facilitate the analysis of neonicotinoid pesticides by atmospheric pressure ionization mass spectrometry.
Collision-induced dissociation (CID) experiments were performed on atmospheric ion adducts [M + R]– formed between various types of organic compounds M and atmospheric negative ions R- [such as O2–, HCO3–, COO–(COOH), NO2–, NO3–, and NO3–(HNO3)] in negative-ion mode atmospheric pressure corona discharge ionization (APCDI) mass spectrometry. All of the [M + R]– adducts were fragmented to form deprotonated analytes [M – H]– and/or atmospheric ions R–, whose intensities in the CID spectra were dependent on the proton affinities of the [M – H]– and R– fragments. Precursor ions [M + R]– for which R- have higher proton affinities than [M – H]– formed [M – H]– as the dominant product. Furthermore, the CID of the adducts with HCO3– and NO3-(HNO3) led to other product ions such as [M + HO]– and NO3–, respectively. The fragmentation behavior of [M + R]– for each R– observed was independent of analyte type (e.g., whether the analyte was aliphatic or aromatic, or possessed certain functional groups). 相似文献
Plasma Chemistry and Plasma Processing - Analysis of the plasma parameters of a tungsten-inert gas microarc with a length of 0.4 mm is carried out by means of a unified one-dimensional... 相似文献
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. 相似文献
We introduce a new atmospheric pressure-electron capture dissociation (AP-ECD) source in which conventional nanospray emitters
are coupled with the source block and photoionization lamp of a PhotoSpray APPI source. We also introduce procedures for data
collection and processing, aimed at maximizing the signal-to-background ratio of ECD products. Representative data from Substance
P are presented to demonstrate the performance of the technique. Further, we demonstrate the effects of two important experimental
variables, source temperature and vacuum-interface declustering potential (DP), on the method. Last, we show that even when
a high source temperature is used to maximize efficiency, AP-ECD fragments of a model phosphorylated peptide retain the modification. 相似文献
The effect of bleeding oxygen or a mixture of inert gases (Ne, Ar, and Xe) in the ion source of an EMAL-2 mass spectrometer on the results of elemental analysis (EI 696 steel, L 68 brass, and Br 663 bronze standard reference samples) was examined. Filling the ion source with gases to a pressure of 10–2 Pa increased the number of singly charged ions and decreased the number of multiply charged ions in a laser-induced plasma. This fact allowed us to enhance analytical signals by a factor of 1.5. The effect was most pronounced for ions of elements whose second ionization potentials lie in a range of 15–20 eV. The effects were almost identical for bleeding oxygen and inert gases. 相似文献
In this paper, we demonstrate the first use of an atmospheric pressure microplasma-based vacuum ultraviolet (VUV) photoionization source in atmospheric pressure mass spectrometry applications. The device is a robust, easy-to-operate microhollow cathode discharge (MHCD) that enables generation of VUV photons from Ne and Ne/H2 gas mixtures. Photons were detected by excitation of a microchannel plate detector and by analysis of diagnostic sample ions using a mass spectrometer. Reactive ions, charged particles, and metastables produced in the discharge were blocked from entering the ionization region by means of a lithium fluoride window, and photoionization was performed in a nitrogen-purged environment. By reducing the output pressure of the MHCD, we observed heightened production of higher-energy photons, making the photoionization source more effective. The initial performance of the MHCD VUV source has been evaluated by ionizing model analytes such as acetone, azulene, benzene, dimethylaniline, and glycine, which were introduced in solid or liquid phase. These molecules represent species with both high and low proton affinities, and ionization energies ranging from 7.12 to 9.7 eV. Figure
