Glen P. Jackson

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Abstract Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s 5 ( 3 P 2 ) and 1s 3 ( 3 P 0 ) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the 3 P 2 (11.55 eV) and the 3 P 0 (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.

Spectral, spatial and temporal characterization of a millisecond pulsed glow discharge: copper analyte emission and ionization

Abstract Two-dimensional maps of the spatial distributions of excited and ionized sputtered copper atoms are presented for a millisecond pulsed argon glow discharge. These maps demonstrate the temporal as well as spatial dependence of different excitation and ionization processes over the pulse cycle. Transitions from the low energy electronic states for the atom, characterized by emission such as that at 324.75 nm (3.82→0.00 eV), dominate the plateau time regime at a distance of 2.5 mm from the cathode surface. These processes originate from the electron excitation of ground state copper atoms. Transitions from high-energy electronic states, such as that characterized by emission at 368.74 nm (7.16→3.82 eV), predominate during the afterpeak time regime at a distance of 5.0–6.0 mm from the cathode surface. This observation is consistent with the relaxation of highly excited copper atoms produced by electron recombination with copper ions during the afterpeak time regime. Analyses of afterpeak and plateau intensities for a series of copper emission lines indicate an electron excitation temperature equivalent to 5.78 eV at 0.8 torr and 1.5 W. Temporal profiles exhibit copper ion emission only during the plateau time regime.

Metastable atom‐activated dissociation mass spectrometry: leucine/isoleucine differentiation and ring cleavage of proline residues

Extensive backbone fragmentation resulting in a-, b-, c-, x-, y- and z-type ions is observed of singly and doubly charged peptide ions through their interaction with a high kinetic energy beam of argon or helium metastable atoms in a modified quadrupole ion trap mass spectrometer. The ability to determine phosphorylation-sites confirms the observation with previous reports and we report the new ability to distinguish between leucine and isoleucine residues and the ability to cleave two covalent bonds of the proline ring resulting in a-, b-, x-, y-, z- and w-type ions. The fragmentation spectra indicate that fragmentation occurs through nonergodic radical ion chemistry akin to electron capture dissociation (ECD), electron transfer dissociation (ETD) and electron ionization dissociation mechanisms. However, metastable atom-activated dissociation mass spectrometry demonstrates three apparent benefits to ECD and ETD: (1) the ability to fragment singly charged precursor ions, (2) the ability to fragment negatively charged ions and (3) the ability to cleave the proline ring that requires the cleavage of two covalent bonds. Helium metastable atoms generated more fragment ions than argon metastable atoms for both substance P and bradykinin regardless of the precursor ion charge state. Reaction times less than 250 ms and efficiencies approaching 5% are compatible with on-line fragmentation, as would be desirable for bottom-up proteomics applications.

Modeling of a millisecond pulsed glow discharge: Investigation of the afterpeak

We have developed a comprehensive modeling network for a millisecond pulsed glow discharge in argon (Ar) with copper (Cu) cathode, to describe the behavior of the various plasma species. Typical results of the model are shown, such as the potential distribution during and after the pulse, the densities of Ar+ ions, Ar atoms in the metastable and various other excited levels, Cu atoms and Cu+ ions, in ground state and excited levels, as well as optical emission intensities as a function of time during and after the pulse. Special attention is paid to the mechanisms giving rise to the so-called “afterpeak”, i.e., the peak in Ar and Cu excited level populations and optical emission intensities, which is experimentally observed in the afterglow of pulsed discharges. This afterpeak is attributed to electron-ion recombination to the highest excited levels, followed by radiative decay to lower levels. It is expected that the electron temperature decreases drastically upon pulse termination, resulting in a significant rise in electron density, making electron-ion recombination more plausible. Because we were not yet able to model these mechanisms, we worked in reversed order, to find out which recombination mechanisms account for the experimentally observed afterpeaks. Collisional-radiative recombination (i.e., three-body recombination with an electron as the third body) is the most plausible candidate, both for Ar and Cu, but it requires a rise in electron density in the afterglow, estimated to be about two orders of magnitude relative to the steady state, or voltage-on period. Therefore, as an alternative, we think that dissociative recombination of Ar2+ ions in high vibrational levels cannot yet completely be ruled out.

New fast screening method for organochlorine pesticides in water by using solid-phase microextraction with fast gas chromatography and a pulsed-discharge electron capture detector

A new fast screening method has been developed for the analysis of organochlorine pesticides in water that takes less than 10 min to perform and has detection limits of the order of 10 ng l–1. Pesticide extraction was achieved by solid-phase microextraction and the separation was performed using a micro-bore (0.1 mm) capillary column. A laboratory-built cryotrap system was used to focus the analytes prior to introduction to the column and a pulsed-discharge electron capture detector enabled sensitive, selective measurements of the pesticide peaks to be made. The 100 µm polydimethylsilicone (PDMS) coated fibre gave better recoveries than the 30 or 7 µm film PDMS fibres and an inter-fibre study comparing three 100 µm film fibres showed good reproducibility. With extraction times of 2 min, the procedure was found to be linear over the range 0.01–1.2 ng ml–1. The procedure was tested with a real river water sample.

Metastable Atom-Activated Dissociation Mass Spectrometry of Phosphorylated and Sulfonated Peptides in Negative Ion Mode

The dissociation behavior of phosphorylated and sulfonated peptide anions was explored using metastable atom-activated dissociation mass spectrometry (MAD-MS) and collision-induced dissociation (CID). A beam of high kinetic energy helium (He) metastable atoms was exposed to isolated phosphorylated and sulfonated peptides in the 3– and 2– charge states. Unlike CID, where phosphate losses are dominant, the major dissociation channels observed using MAD were Cα – C peptide backbone cleavages and neutral losses of CO2, H2O, and [CO2 + H2O] from the charge reduced (oxidized) product ion, consistent with an electron detachment dissociation (EDD) mechanism such as Penning ionization. Regardless of charge state or modification, MAD provides ample backbone cleavages with little modification loss, which allows for unambiguous PTM site determination. The relative abundance of certain fragment ions in MAD is also demonstrated to be somewhat sensitive to the number and location of deprotonation sites, with backbone cleavage somewhat favored adjacent to deprotonated sites like aspartic acid residues. MAD provides a complementary dissociation technique to CID, ECD, ETD, and EDD for peptide sequencing and modification identification. MAD offers the unique ability to analyze highly acidic peptides that contain few to no basic amino acids in either negative or positive ion mode.

Characterization of tyrosine nitration and cysteine nitrosylation modifications by metastable atom-activation dissociation mass spectrometry.

The fragmentation behavior of nitrated and S-nitrosylated peptides were studied using collision induced dissociation (CID) and metastable atom-activated dissociation mass spectrometry (MAD-MS). Various charge states, such as 1+, 2+, 3+, 2–, of modified and unmodified peptides were exposed to a beam of high kinetic energy helium (He) metastable atoms resulting in extensive backbone fragmentation with significant retention of the post-translation modifications (PTMs). Whereas the high electron affinity of the nitrotyrosine moiety quenches radical chemistry and fragmentation in electron capture dissociation (ECD) and electron transfer dissociation (ETD), MAD does produce numerous backbone cleavages in the vicinity of the modification. Fragment ions of nitrosylated cysteine modifications typically exhibit more abundant neutral losses than nitrated tyrosine modifications because of the extremely labile nature of the nitrosylated cysteine residues. However, compared with CID, MAD produced between 66% and 86% more fragment ions, which preserved the labile –NO modification. MAD was also able to differentiate I/L residues in the modified peptides. MAD is able to induce radical ion chemistry even in the presence of strong radical traps and therefore offers unique advantages to ECD, ETD, and CID for determination of PTMs such as nitrated and S-nitrosylated peptides.

Fast Gas Chromatography of Explosive Compounds Using a Pulsed-Discharge Electron Capture Detector*

ABSTRACT: The detection of a mixture of nine explosive compounds, including nitrate esters, nitroaromatics, and a nitramine in less than 140 sec is described. The new method employs a commercially available pulsed‐discharge electron capture detector (PDECD) coupled with a microbore capillary gas chromatography (GC) column in a standard GC oven to achieve on‐column detection limits between 5 and 72 fg for the nine explosives studied. The PDECD has the benefit that it uses a pulsed plasma to generate the standing electron current instead of a radioactive source. The fast separation time limits on‐column degradation of the thermally labile compounds and decreases the peak widths, which results in larger peak intensities and a concomitant improvement in detection limits. The combination of short analysis time and low detection limits make this method a potential candidate for screening large numbers of samples that have been prepared using techniques such as liquid–liquid extraction or solid‐phase microextraction.

Classification of Cultivation Locations of Panax quinquefolius L Samples using High Performance Liquid Chromatography Electrospray Ionization Mass Spectrometry and Chemometric Analysis

Panax quinquefolius L ( P. quinquefolius L) samples grown in the United States and China were analyzed with high performance liquid chromatography-mass spectrometry (HPLC-MS). Prior to classification, the two-way data sets were subjected to pretreatment including baseline correction and retention time (RT) alignment. Principal component analysis (PCA) and projected difference resolution (PDR) metrics were used to evaluate the data quality and the pretreatment effects. A fuzzy rule-building expert system (FuRES) classifier was used to classify the P. quinquefolius L samples grown in the United States and China with the optimized partial least-squares (o-PLS) classifier as the positively biased control method. A classification rate as high as 98 ± 3% with FuRES was obtained after baseline correction and RT alignment, which is equivalent to the result obtained by using the positively biased o-PLS control method (98 ± 3%). RT alignment improved the classification rates for both FuRES and o-PLS classifiers (18% improvement for the FuRES classification rate and 10% improvement for the o-PLS classification rate with baseline correction). From the rule obtained to classify the P. quinquefolius L samples grown in the United States and China, peaks were identified that can be prospective biomarkers for differentiating samples from different growth regions. HPLC-MS with chemometric analysis has the potential to be used as an authentication method for P. quinquefolius L grown in China and the United States.

Gas-phase reactions of bare and oxo-ligated actinide and lanthanide cations with pentamethylcyclopentadiene studied in a quadrupole ion trap mass spectrometer

Abstract Reactions of bare and oxo-ligated monopositive ions of uranium and thorium with 1,2,3,4,5-pentamethylcyclopentadiene, C 10 H 16 , (HCp ∗ ) were examined in a quadrupole ion trap (QIT) mass spectrometer. Representative lanthanide ions, Ln + and LnO + , and tantalum ions, Ta + and TaO + , were studied for comparison. The product branching ratios for both primary and secondary reactions of the actinide ions demonstrated gas-phase organoactinide chemistry that is quite disparate from organolanthanide chemistry under comparable conditions for this neutral reactant. Particularly revealing were product distributions for ThO + and UO + , which indicated chemical behavior similar to that of bare Sm + . We conclude that at least one valence electron at the metal center of the actinide oxide ions must remain chemically active. In the case of UO + , this provides evidence for the chemical engagement of the quasi-valence 5f electrons, which is in distinct contrast to the inert character of the 4f electrons of the lanthanides in both Ln + and LnO + . Mass-selective chemistry of two primary products, UC 10 H 10 + and UC 9 H 8 + , also showed behavior similar to that of Sm + and UO + , implying that there are two covalent organouranium bonds in these complex ions. In comparing the QIT results for the lanthanides with those from a low-pressure ion cyclotron resonance (ICR) mass spectrometry study [Organometallics 16 (1997) 3845], qualitative agreement was found, but significant quantitative differences were apparent. Based on results from collision-induced dissociation and effects of variations in bath gas pressure in the QIT, we conclude that the discrepancies arise from the very different pressure regimes in the ICR and QIT. Evidently, the QIT can be operated over a range of pressures that manifest effects of collisional cooling for some reactions. For the lowest pressure QIT experiments, the high degree of fragmentation is reminiscent of the ICR results. We propose that the QIT bath gas can essentially act as an inert “solvent,” which serves to mediate high-energy processes due to energy transfer from nascent hot intermediate products via energy-dissipating collisions.