Victor H. Vartanian

Identification of tetracycline antibiotics by electrospray ionization in a quadrupole ion trap

Tetracycline antibiotics, tetracycline, chlortetracycline, demeclocycline, doxycycline, minocycline, methacycline, oxytetracycline, and anhydrotetracycline, are examined by electrospray ionization in a quadrupole ion trap. Studies were undertaken to evaluate the use of metal complexation as an alternative to conventional proton attachment. A variety of metal cationization processes, including attachment of Na+, Mg2+, Ca2+, Co2+, Ni2+, and Cu2+ were probed. Infrared multiphoton photodissociation and collisionally activated dissociation (CAD) were compared for generation of diagnostic fragmentation patterns of protonated and metal cationized tetracyclines. The photodissociation spectra provide a more informative signature, including more low mass ions that are not observed upon CAD. The metal complexes dissociate by pathways that are similar to those observed for the protonated molecules.

Dissociation of polyether-transition metal ion dimer complexes in a quadrupole ion trap

The formation and dissociation of dimer complexes consisting of a transition metal ion and two polyether ligands is examined in a quadrupole ion trap mass spectrometer. Reactions of three transition metals (Ni, Cu, Co) with three crown ethers and four acyclic ethers (glymes) are studied. Singly charged species are created from ion-molecule reactions between laser-desorbed monopositive metal ions and the neutral polyethers. Doubly charged complexes are generated from electrospray ionization of solutions containing metal salts and polyethers. For the singly charged complexes, the capability for dimer formation by the ethers is dependent on the number of available coordination sites on the ligand and its ability to fully coordinate the metal ion. For example, 18-crown-6 never forms dimer complexes, but 12-crown-4 readily forms dimers. For the more flexible acyclic ethers, the ligands that have four or more oxygen atoms do not form dimer complexes because the acyclic ligands have sufficient flexibility to wrap around the metal ion and prevent attachment of a second ligand. For the doubly charged complexes, dimers are observed for all of the crown ethers and glymes, thus showing no dependence on the flexibility or number of coordination sites of the polyether. The nonselectivity of dimer formation is attributed to the higher charge density of the doubly charged metal center, resulting in stronger coordination abilities. Collisionally activated dissociation is used to evaluate the structures of the metal-polyether dimer complexes. Radical fragmentation processes are observed for some of the singly charged dimer complexes because these pathways allow the monopositive metal ion to attain a more favorable 2 + oxidation state. These radical losses are observed for the dimer complexes but not for the monomer complexes because the dimer structures have two independent ligands, a feature that enhances the coordination geometry of the complex and allows more flexibility for the rearrangements necessary for loss of radical species. Dissociation of the doubly charged complexes generated by electrospray ionization does not result in losses of radical neutrals because the metal ions already exist in favorable 2+ oxidation states.

Open cell analog of the screened trapped-ion cell using compensation electrodes for Fourier transform ion cyclotron resonance mass spectrometry

Abstract The open cell collinear electrode geometry permits the inclusion of an auxiliary set of electrodes adjacent to the trap electrodes to perform several functions. Here they are used as compensation electrodes that virtually eliminate the radial electric field at the z = 0 midplane of the cell. The function of the compensation electrodes in reducing the radial electric field is analogous to the grounded transmissive screen inserted between the trap electrodes and the trapping volume in the closed screened cell whereby the interior of the cell is shielded from the trapping field. Segmenting the trap electrodes and applying oppositely biased potentials to each set of segments reduces the radial electric field at the z = 0 midplane of the cell by superposition of the opposing electric fields. In addition, dynamic adjustment of the potential well contour is performed by adjusting the relative potentials on each set of electrodes. The supplementary voltage applied to the inner set of compensation electrodes reduces the radial electric field by nearly two orders of magnitude and increases the potential well depth for greater ion capacity. In addition, the potential well assumes increased particle-in-a-box character without increasing the physical size of the cell, thereby reducing the effect of space charge. A FORTRAN program is developed that models the cell geometry and predicts the relationship between trap and compensation electrode voltage required to minimize the radial electric field throughout a specified cell volume. A theoretically optimum ratio of -0.33 V applied to the compensation electrodes for 1.0 V applied to the trap electrodes is predicted and is in close agreement with an experimentally determined optimum ratio of -0.36 V applied to the compensation electrodes for 1.0 V applied to the trap electrodes. This ratio reduces the cyclotron frequency shift from -70.8 Hz V-1 in an uncompensated open cell to -0.50 Hz V-1, a reduction of more than 99%. The radial electric field at the z = 0 midplane of the cell and 90% of the cell radius is reduced 97%, from 0.0139 V mm-1 to 0.0004 V mm-1. The reduction in frequency shift is accomplished without compromising mass accuracy. By collisionally damping ions to the center of the cell, mass accuracy over a one-decade range (60–600 u) approaches the mass accuracy of the hyperbolic cell geometry.

Optimization of a fixed-volume open geometry trapped ion cell for Fourier transform ion cyclotron mass spectrometry

Abstract Several open cylindrical trapped ion cells of fixed volume but varying electrode dimension are evaluated for the Fourier transform ion cyclotron resonance mass spectrometry experiment. In closed geometry cells the z-axis dimension is increased to form elongated cells with reduced radial electric field. In contrast, the aspect ratio for open cells can be increased without increasing the overall cell volume, an important consideration when magnetic field homogeneity is a concern. The increase in aspect ratio is achieved by reducing the relative dimensions of the trap electrodes with respect to the excitation and detection electrodes. The radial electric field magnitude for a fixed-length open cell with aspect ratios ranging from 0.84 to 1.60 is evaluated both theoretically and experimentally. Experimental frequency shifts for fixed-volume cells are reduced from −74.3 to −18.1 Hz V−1, a 411% reduction, by increasing the aspect ratio of an open cell from 1.20 to 1.60 respectively. As an alternative to the present standard open cell with electrodes of equal length, an elongated open cell geometry is optimized for maximized well depth (ion capacity) and minimized radial electric field.

The Use of Unsaturated Fluorocarbons for Dielectric Etch Applications

Six unsaturated fluorocarbon (UFC) gases as well as a fluorinated ether were examined for dielectric etch and global warming emissions performance and compared to three perfluorocompound (PFC) gases. All of the gases were capable of etch performance comparable to that of a typical C 3 F 8 process, while exhibiting superior global warming emissions performance compared to the PFCs. A low-flow hexafluoro-2-butyne process was found to have a significant emissions benefit, showing a normalized emissions reduction of 88.2% compared to the C 3 F 8 process. Two other C 4 F 6 isomers (hexafluoro-1,3-butadiene and hexafluorocyclobutene) also exhibited reductions greater than 80%, while hexafluoropropene and octafluorocyclopentene exhibited emissions reductions greater than 70% compared to the typical C 3 F 8 process. For the C 4 F 6 isomers, a large portion of the emissions were a result of CHF 3 formation with photoresist as the sole source of the hydrogen. An extended 4 min etch with hexafluoro-1,3-butadiene resulted in a deep via with an aspect ratio of 5:1, very high selectivity to photoresist, and no evidence of etch stopping.

Debye-shielding mechanism for trapping ions formed by laser desorption Fourier transform ion cyclotron resonance mass spectrometry

Abstract The trapping of metal ions in laser desorption/ionization (LDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is attributed to an electrostatic shielding mechanism promoted by the sustained quasi-neutral plasma behavior of the desorbed particle plume in strong magnetic fields. This shielding process allows low energy ions to penetrate the applied trapping potentials at the trapped-ion cell, thus resulting in the introduction of ions with energies less than the effective depth of the trapping potential well. Subsequent deshielding of these ions while in the cell exposes them to the trapping field and results in their retention. Data from time-of-flight (TOF) studies indicate that large spatially and temporally overlapped populations of high energy ions and low energy electrons are generated by LDI of a variety of metal targets when laser power density exceeds 107–108W cm−2. The charge density in the desorbed plasma is shown to increase during flight along converging magnetic field lines but to dissipate rapidly on exiting the field. Retarding potential studies in the magnetic field performed with both TOF and FT-ICR detection indicate that the Debye shielding exhibited by these quasi-neutral populations is sufficient in some cases to allow ions with energies on the order of 1 eV to penetrate retarding potentials as high as 500 V. Further indication that such effects are present in the LDI FT-ICR experiment is given by TOF kinetic energy analysis of ions acquired in the trapped-ion cell from LDI and then dumped to an external detector. This analysis indicates that the average kinetic energy of such ions is typically only 60% of the applied trapping potential.

Evolution of trapped ion cells in Fourier transform ion cyclotron resonance mass spectrometry

Abstract Factors shaping the development of trapped ion cells in Fourier transform ion cyclotron resonance (FTICR) mass spectrometry have evolved along several lines. Areas of improvement include maximizing the quadrupolar character of the cell, reducing the radial electric trapping field, and improving the linearity of the excitation event. This review focuses on cell development for optimum FTICR performance.

Evaluation of Oxalyl Fluoride for a Dielectric Etch Application in an Inductively Coupled Plasma Etch Tool

The goal of the work presented in this article was to provide a preliminary screening for a novel fluorinated compound, oxalyl fluoride, C 2 O 2 F 2 (F-(C=O)-(C=O)-F), as a potential replacement for perfluorocompounds in dielectric etch applications. Both process and emissions data were collected and the results were compared to those provided by a process utilizing a standard perfluorinated etch chemistry (C 2 F 6 ). In this evaluation, oxalyl fluoride produced very low quantities of global warming compounds under the conditions in which it was tested, as compared to the C 2 F 6 process A preliminary evaluation of the compounds process performance was also carried out. Patterned tetraethoxysilane-deposited silicon oxide masked with deep UV photoresist having 0.6, 0.45, and 0.35 μm via hole features was used as the test vehicle. Although C 2 O 2 F 2 was capable of etching silicon dioxide, low oxide etch rate and poor selectivity to the mask layer were observed. Finally, in addition to the experimental work performed, a set of ab initio quantum chemical calculations was undertaken to obtain enthalpies of dissociation for each of the bonds in the oxalyl fluoride molecule in order to better understand its dissociation pathways in plasma environments.

High performance fourier transform ion cyclotron resonance mass spectrometry via a single trap electrode

An open-ended cylindrical cell with a single annular trap electrode located at the center of the excitation and detection region is demonstrated for Fourier transform ion cyclotron resonance mass spectrometry. A trapping well is created by applying a static potential to the trap electrode of polarity opposite the charge of the ion to be trapped, after which conventional dipolar excitation and detection are performed. The annular trap electrode is axially narrow to allow the creation of a potential well without excessively shielding excitation and detection. Trapping is limited to the region of homogeneous excitation at the cell centerline without the use of capacitive coupling. Perfluorotributylamine excitation profiles demonstrate negligible axial ejection throughout the entire excitation voltage range even at an effective centerline potential of only −0.009 V. High mass resolving power in the single-trap electrode cell is demonstrated by achievement of mass resolving power of 1.45 × 106 for benzene during an experiment in which ions created in a high pressure source cubic cell are transferred to the low pressure analyzer single-trap electrode cell for detection. Such high performance is attributed to the negligible radius dependent radial electric field for ions cooled to the center of the potential well and accelerated to less than 60% of the cell radius. An important distinction of the single-trap electrode geometry from all previous open and closed cell arrangements is exhibition of combined gated and accumulated trapping. Because there is no potential barrier, all ions penetrate into the trapping region regardless of their translational energy as in gated trapping, but additional ions may accumulate over time, as in accumulated trapping. Ions of low translational kinetic energy are demonstrated to be preferentially trapped in the single-trap electrode cell. In a further demonstration of the minimal radial electric field of the single-trap electrode cell, positive voltages can be applied to the annular trap electrode as well as the source cell trap electrode to achieve highly efficient transfer of ions between cells.

The Evaluation of Hexafluorobenzene as an Environmentally Benign Dielectric Etch Chemistry

Hexafluorobenzene was evaluated as an alternative chemistry for dielectric etch applications in a high density plasma etch chamber with reduced global warming emissions. Processes based on hexafluorobenzene exhibited global warming emissions reductions as high as 97% compared to a C 3 F 8 -based process, which is the greatest reductions level of any alternative chemistry examined to date on this tool. Using hexafluorobenzene, it is possible to operate in a regime of high etch rate and high polymerization. There are several issues, however, that need to be addressed if this chemistry is to be used for high performance dielectric etching. This material is a liquid at room temperature, which makes it difficult to deliver process gas to the chamber. In addition, this chemistry is highly polymerizing, resulting in excess polymer deposition on chamber walls leading to significant process variability for standard chamber clean times Significantly longer chamber clean times were required between each etch to remove the excess polymer.