Comparison of positive ions formed in nickel-63 and corona discharge ion sources using nitrogen, argon, isobutane, ammonia and nitric oxide as reagents in atmospheric pressure ionization mass spectrometry

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133

Two relatively new ambient ionization sources, direct analysis in real time (DART) and the flowing atmospheric-pressure afterglow (FAPA), use direct current, atmospheric-pressure discharges to produce reagent ions for the direct ionization of a sample. Although at a first glance these two sources appear similar, a fundamental study reveals otherwise. Specifically, DART was found to operate with a corona-to-glow transition (C-G) discharge whereas the FAPA was found to operate with a glow-to-arc transition (G-A) discharge. The characteristics of both discharges were evaluated on the basis of four factors: reagent-ion production, response to a model analyte (ferrocene), infrared (IR) thermography of the gas used for desorption and ionization, and spatial emission characteristics. The G-A discharge produced a greater abundance and a wider variety of reagent ions than the C-G discharge. In addition, the discharges yielded different adducts and signal strengths for ferrocene. It was also found that the gas exiting the discharge chamber reached a maximum of 235°C and 55°C for the G-A and C-G discharges, respectively. Finally, spatially resolved emission maps of both discharges showed clear differences for N 2 ϩ and O(I). These findings demonstrate that the discharges used by FAPA and DART are fundamentally different and should have different optimal applications for ambient desorption/ionization mass spectrometry (ADI-MS). irect-current (DC) discharges have been widely used for elemental analyses since they were first introduced for alloy characterization [1]. When DC discharges were coupled with mass spectrometry, the result was a very sensitive and powerful tool for elemental [1] and molecular analyses [2,3]. Of the many electrical regimes of DC discharges, three forms have been found to have particular analytical merit: the arc, the glow, and the corona. Among these three types of discharges, the fundamental distinction is the operating current and voltage. The arc occurs at very high currents (hundreds of amperes) with a low voltage drop between electrodes (tens of volts). It also exhibits negative resistance; that is, the sustaining voltage drops as the current rises. The glow discharge (GD), which has conventionally been operated between 0.1 to 10 Torr, exists at much lower currents (tens of milliamperes) and a higher voltage drop (hundreds of volts). Lastly, the corona discharge operates with very low currents (a few microamperes) and a much higher voltage drop (several kilovolts).Corona discharges find their most common analytical application in atmospheric pressure chemical ionization (APCI) [4,5]. In conventional APCI, a corona discharge is formed by applying ϳ4 kV to a needle electrode in a selected atmosphere, to yield currents of ϳ5 A. After a series of reactions [5], reagent ions are produced that can then ionize a sample. Protonated water clusters are typically observed because of the presence of water vapor in the air. Such protonated clusters promote proton transfer ionization, resulting in mass spect...

Section: Instrumentationmentioning

confidence: 99%

Characterization of direct-current atmospheric-pressure discharges useful for ambient desorption/ionization mass spectrometry

Shelley

Wiley

Chan

et al. 2009

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133

“…A corona discharge ion source later replaced the 63 Ni as the source of ionization [6]. Horning et al were the first to interface a GC instrument to an APCI ion source [10,11]. Since these initial publications, a series of papers originating at the National Center for Toxicological Research in Jefferson, Arkansas, have been published in which the effluent from a gas chromatograph is ionized at atmospheric pressure [12][13][14][15][16][17][18][19][20][21].…”

mentioning

confidence: 99%

“…Supercritical fluid chromatography has also been interfaced to APCI-MS [30 -32]. However, the primary use of this ionization method has been as an ionization interface between liquid chromatography and mass spectrometry [11].…”

mentioning

confidence: 99%

A combination atmospheric pressure LC/MS:GC/MS ion source: Advantages of dual AP-LC/MS:GC/MS instrumentation

McEwen

McKay

2005

105

Modification of commercial LC/MS instrumentation to allow both atmospheric pressure (AP) LC/MS and GC/MS is described. Advantages of this additional capability versus LC/MS alone include higher chromatographic resolution in the GC versus LC mode, greater peak capacity for complex mixture analysis, higher sensitivity for a variety of volatile compounds, and the ability to observe compounds of low polarity that are not readily observed in LC/MS. Advantages over conventional GC/MS include the ability to use higher carrier gas flow and shorter columns for passing less volatile materials through the gas chromatograph, selective ionization, and rapid switching between positive and negative ion modes. Other advantages include application of the enhanced capabilities of LC/MS instrumentation to GC/MS analyses such as cone voltage fragmentation, MS n , high mass resolution, and accurate mass measurement. Limitations of APGC/MS include the inability to observe saturated hydrocarbon and certain other highly nonpolar compounds and less odd-electron fragmentation for computer aided library searching. For some analyses, the limitation related to ionization of highly nonpolar compounds is advantageous, as is the simplified mass spectrum and easy molecular weight identification that results from less fragmentation observed in the AP ionization mode. (J Am Soc Mass Spectrom 2005, 16, 1730 -1738

“…More widely used approaches for direct sampling are based on atmospheric pressure chemical ionization (APCI) [11] which is capable of making positive or negative ions. APCI is effected by a sequence of electron-and ion-molecule reactions initiated by electrons that are generally produced via either a corona discharge [12] or a ␤-emitter such as 63 Ni [1,13]. Because the equilibrium distribution in such reactions under normal APCI operating conditions often favors terminal ions that are characteristic of trace impurities rather than major components of the support gas [14], the ionization method has proven to be highly sensitive in analytical applications.…”

mentioning

confidence: 99%

“…Ions are lost primarily by drift due to electric fields or by diffusion when they are present at relatively low number densities such as exist in standard EI and chemical ionization (CI) sources. At the other extreme in ion number density are unipolar devices such as the corona APCI source [12,17], for which ion loss due to drift in the electric field created by the space charge becomes dominant. In contrast to corona APCI, charged particles of both polarities are present in bipolar ion sources.…”

mentioning

confidence: 99%

Factors influencing the analytical performance of an atmospheric sampling glow discharge ionization source as revealed via ionization dynamics modeling

Goeringer

2003

A kinetic model is developed for the dynamic events occurring within an atmospheric sampling glow discharge that affect its performance as an ion source for analytical mass spectrometry. The differential equations incorporate secondary electron generation and thermalization, reagent and analyte ion formation via electron capture and ion-molecule reactions, ion loss via recombination processes, diffusion, and ion-molecule reactions with matrix components, and the sampling and pumping parameters of the source. Because the ion source has a flow-through configuration, the number densities of selected species can be estimated by applying the steady-state assumption. However, understanding of its operation is aided by knowledge of the dynamic behavior, so numerical methods are applied to examine the time dependence of those species as well. As in other plasma ionization sources, the ionization efficiency is essentially determined by the ratio of the relevant ion formation and recombination rates. Although thermal electron and positive reagent ion number densities are comparable, the electron capture/ion-molecule reaction rate coefficient ratio is normally quite large and the ion-electron recombination rate coefficient is about an order of magnitude greater than that for ion-ion recombination. Consequently, the efficiency for negative analyte ion formation via electron capture is generally superior to that for positive analyte ion generation via ion-molecule reaction. However, the efficiency for positive analyte ion formation should be equal to or better than that for negative analyte ions when both ionization processes occur via ion-molecule reaction processes (with comparable rate coefficients), since the negative reagent ion density is considerably less than that for positive reagent ions. Furthermore, the particularly high number densities of thermal electrons and reagent ions leads to a large dynamic range of linear response for the source. Simulation results also suggest that analyte ion number densities might be enhanced by modification of the standard physical and operating parameters of the source. . In these and other applications, a number of methods have been utilized for generation of ions from compounds of interest. Electron ionization (EI) has been employed for formation of positive ions in situations for which air is leaked directly into the mass spectrometer [5,6]. To enhance ionization selectivity over that exhibited by EI, single-and multi-photon ionization techniques have been used for determination of targeted aromatic compounds in automobile exhaust [7][8][9][10]. More widely used approaches for direct sampling are based on atmospheric pressure chemical ionization (APCI) [11] which is capable of making positive or negative ions. APCI is effected by a sequence of electron-and ion-molecule reactions initiated by electrons that are generally produced via either a corona discharge [12] or a ␤-emitter such as 63 Ni [1,13]. Because the equilibrium distribution in such reactions under normal APCI operati...