Sampling Glow Discharge Ionization Source Research Articles

Continuous Real-Time Analysis of Products from the Reaction of Some Monoterpenes with Ozone Using Atmospheric Sampling Glow Discharge Ionization Coupled to a Quadrupole Ion Trap Mass Spectrometer

An on-line technique has been demonstrated for the analysis of photochemical oxidation reaction products. The technique is based on the direct introduction of gas and particulate oxidation products into a custom-built atmospheric sampling glow discharge ionization source (ASGDI) coupled to a quadrupole ion trap mass spectrometer (QITMS). Operational parameters of the ASGDI system were investigated to determine their influence on the ion signal for the analysis of oxidation products in real time. These parameters include the discharge current, ion accumulation time, and type of reagent gas. Reference mass spectra from standards were generated for a variety of biogenic compounds and terpene reaction products containing keto, hydroxy, aldehyde, carboxylic acid, or epoxy groups to better understand the fragmentation that occurs in the glow discharge ion source. Results are presented for ozonolysis reactions of four biogenic monoterpenes (alpha-pinene, beta-pinene, D-limonene, Delta(3)-carene) monitored with the ASGDI quadrupole ion trap to demonstrate the ability to obtain real-time measurements. The reaction products identified with ASGDI-QITMS correspond to those products identified with other techniques, including on-line atmospheric pressure chemical ionization techniques. Efficient differentiation of multifunctional products including mono-/di-/hydroxy-/keto-carboxylic acid and keto-/hydroxy-aldehyde was possible by use of the MS/MS capability of the instrument.

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

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.

A quadrupole ion trap mass spectrometer with three independent ion sources for the study of gas-phase ion/ion reactions.

An instrument for the study of gas-phase ion/ion reactions in which three independent sources of ions, namely, two electrospray ionization sources and one atmospheric sampling glow discharge ionization source, are interfaced to a quadrupole ion trap mass analyzer is described. This instrument expands the scope of gas-phase ion/ion reaction studies by allowing for manipulation of the charge states of multiply charged reactant and product ions. Examples are provided involving the formation of protein-protein complexes in the gas phase. Complexes with charge states that cannot be formed from reactant ion charge states present in the normal electrospray charge state distributions can be formed in the new apparatus. Strategies that rely on both reactant ion charge state manipulation and product ion charge state manipulation are demonstrated. In addition, simplification of product ion spectra generated from dissociation of complexes formed via ion/ion reactions can be effected by using the discharge source to reduce the charge state of the product ions to primarily 1+.

Characterization of switched direct current gas sampling glow discharge ionization source for the time-of-flight mass spectrometer

A variable frequency, switched, direct-current gas sampling glow discharge ionization source was interfaced to an orthogonally extracted time-of-flight mass spectrometer for the purpose of dynamically generating atomic and molecular mass spectra of halogenated hydrocarbons. The plasma duty cycle is 50% in each mode of operation. The operating mode is selected simply by switching the polarity of voltage applied to the sample-introduction plate. The self-igniting helium discharge is contained within the first vacuum stage of the differentially pumped spectrometric interface. Both atomic and molecular mass spectra could be obtained with similar ion-optical lens settings, with the exception of the steering plate potential, which had to be alternated between two values (at the plasma switching frequency). A range of discharge switching rates can be used, although a non-linear drop in the modulated signal occurs with frequency. Switching rates above 100 Hz appears to be impractical with current experimental apparatus. Long-term temporal stability of the atomic (79Br+) and molecular (172CHBr2+) signals was measured over a period greater than 1 h, with an overall precision of 13.3 and 6.1% RSD, respectively. The 79Br+/81Br+ isotope ratio precision was 1.1% RSD over this period, while the 172CHBr2+/173CHBr2+ molecular ratio was determined to be 1.6% RSD. The precision of the isotope and ionic ratios are counting statistics limited. The atomic detection limit is in the range 3–110 pg s−1 (as the halogen) and 15–250 pg s−1 (molecular) for analytes manually introduced into an exponential-dilution sample-introduction device, with boxcar averagers being employed for data collection. A variety of chlorinated hydrocarbons were introduced into the discharge via the flow cell, and it was possible to differentiate (or speciate) the compounds based on their 35Cl+/12C+ elemental ratios with a correlation coefficient (R) of 0.995.

Development of a direct current gas sampling glow discharge ionization source for the time-of-flight mass spectrometer

A direct current, reduced-pressure, gas sampling glow discharge (GSGD) ionization source has been developed and interfaced to an orthogonally extracted time-of-flight mass spectrometer for the purpose of generating both atomic and molecular fragmentation mass spectra. The discharge is contained within the first vacuum stage of the differentially pumped interface of the mass spectrometer. The source is mechanically and logistically simple to construct, operate, and maintain. Switching between the atomic and molecular modes of operation is achieved by altering the discharge gas composition, the operating pressure, and the current. Helium was used to generate atomic mass spectra, whereas molecular spectra were produced by use of argon. Gas flow rates were less than 1 l min –1 for each mode of ionization. This report focuses primarily upon the atomic (elemental) analytical capabilities of the GSGD interface. Atomic detection limits are in the range of 20-90 pg s –1 (as the halogen) for analytes introduced into the system with an exponential dilution device, and with boxcar averagers employed for data collection. Precision is better than 0.4% relative standard deviation (RSD) for measurement of the 79 Br + / 81 Br + isotope ratio (presented to the source as bromoform vapor) over a period of 2.5 h. A variety of chlorinated hydrocarbons were introduced into the discharge via a flow cell, and it was possible to differentiate (i.e., speciate) the compounds based upon their 35 Cl + / 12 C + elemental ratios with a correlation coefficient (R) of 0.996.

Structures of NO −3 formed via glow discharge in atmospheric gases

Tandem mass spectrometry experiments using both ion-molecule reactions and collision-induced dissociation (CID) in an ion trap mass spectrometer have been performed to characterize the structures of the NO − 3 ions that issue from an atmospheric sampling glow discharge ionization source. The experimental evidence suggests that significant abundances of at least two stable forms of NO − 3 can be obtained from an air-sustained glow discharge. Ab initio calculations have been performed to identify the structures of the likely isomers. Rate constants have been measured for the reaction of the less thermodynamically stable isomer(s) with carbon dioxide, and evidence is given for a new reaction, (OONO) − + NO ∓ 2 → NO − 2 + O 2 + NO . The combination of CID with a kinetic study is shown to be a useful approach for obtaining rate constant information when a mixture of reactive and unreactive isomers is present in the reactant population.

Atmospheric sampling glow discharge ionization source for the determination of trace organic compounds in ambient air

A new atmospheric sampling ion source, based on the establishment of a glow discharge in ambient air drawn into a region of reduced pressure, is described. The source is simple, rugged, and relatively maintenance-free, exhibits a very short memory, and is extremely sensitive for compounds with high proton affinities, high electron affinities, high gas-phase acidities, and/or low ionization potentials. The effects of discharge voltage and ion source pressure on the nature of the mass spectra observed are described. These operating parameters affect the absolute number of ions observed and, particularly for positive ions, affect the distribution of the reagent ions and the degree of fragmentation. For illustrative purposes, the limit of detection of dynamic range of the ion source coupled with a mass spectrometer are discussed for 2,4,6-trinitrotoluene. For the present system, a detection limit of 1-2 parts per trillion and a linear dynamic range of at least 6 orders of magnitude are observed.