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

Abstract

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...

Parametric Sensitivity in a Generalized Model for Atmospheric Pressure Chemical Ionization Reactions

“…53,56 Predicted and measured ion abundances were comparable until reaction times >100 μs after which divergence occurred and was attributed to recombination reactions. Other models for glow discharge sources in mass spectrometry 57 and for proton transfer reaction mass spectrometry 44 are noteworthy yet limited for use in models here by differences in ionization chemistry.…”

“…Recently, response of an ion mobility spectrometer to DMMP was compared to models derived from the work of Siegel and included the formation of proton bound dimer with reaction times of 10–100 μs. , Predicted and measured ion abundances were comparable until reaction times >100 μs after which divergence occurred and was attributed to recombination reactions. Other models for glow discharge sources in mass spectrometry and for proton transfer reaction mass spectrometry are noteworthy yet limited for use in models here by differences in ionization chemistry.…”

Atomic spectrometry update. Atomic mass spectrometry

“…Modeling of the ionization dynamics in atmospheric sampling GDMS identified the key experimental factors for optimization of the analytical performance. 35 The simulations accounted for the generation and thermalization of secondary electrons, the formation of reagent and analyte ions by electron capture (EC) and ion-molecule reactions, the removal of ions by recombination, diffusion and ion-molecule reactions and the sampling and pumping parameters of the source. Although thermal electrons and positive reagent ions reached similar number densities, the high rate of EC relative to ion-molecule reactions caused the formation of analyte anions by EC to dominate the generation of positive analyte ions by ion-molecule reaction.…”

“…The detectors were cross-calibrated daily by measuring the 96 Zr 1 isotope. The RSF of 35 Cl was determined on a zirconium standard disc with a Cl concentration of 155 ¡ 5 mg g 21 and checked by analyzing samples by both GDMS and pyrohydrolysis-ion chromatography (IC). Surface Cl contamination was believed to be responsible for the long stabilization time of y50 min.…”

“…Surface Cl contamination was believed to be responsible for the long stabilization time of y50 min. At concentrations v3 mg g 21 , the 35 Cl ion was measured because ArH 1 interfered with 37 Cl. Analytical figures of merit included a LOD of 22 ng g 21 and an overall precision of 1% and 10% for concentrations in the mg g 21 and ng g 21 ranges, respectively.…”