Trap Secondary Ion Mass Spectrometer Research Articles

Structure, Energetics, Electronic, and Hydration Properties of Neutral and Anionic Al3O6, Al3O7, and Al3O8 Clusters

AbstractFor Abstract see ChemInform Abstract in Full Text.

Hydration of alumina cluster anions in the gas phase.

Hydration reactions of anionic aluminum oxide clusters were measured using a quadrupole ion trap secondary ion mass spectrometer, wherein the number of Lewis acid sites were determined. The extent of hydration varied irregularly as cluster size increased and indicated that the clusters possessed condensed structures where the majority of the Al atoms were fully coordinated, with a limited number of undercoordinated sites susceptible to hydrolysis. For maximally hydrated ions, the number of OH groups per Al decreased in an exponential fashion from 4.0 in Al(1) cluster to 1.4 in the Al(9) cluster, which was greater than that expected for a highly hydroxylated surface but less than that for solution phase alumina clusters.

Gas-Phase Hydration of U(IV), U(V), and U(VI) Dioxo Monocations

The formation of adduct ions consisting of uranium oxycations and water was studied using an ion trap-secondary ion mass spectrometer. The U(IV) and U(V) species [UO(OH)]+ and [UO2]+ were produced by bombarding the surface of UO3 using molecular primary ions, and the U(VI) species [UO2(OH)]+ was generated by O2 oxidation of [UO(OH)]+ in the gas phase. All three ions formed H2O adducts by termolecular association reactions: [UO(OH)]+ (a U(IV) species) added three water molecules, for a total of five ligands; [UO2]+ (U(V)) added three or four water molecules, for a total of five or six ligands; and [UO2(OH)]+ (U(VI)) added four water molecules for a total of six ligands. Addition of a seventh ligand was not observed in any of the systems. These analyses showed that the optimum extent of ligation increased with increasing oxidation state of the uranium metal. Hard kinetic models were fit to the time-dependent mass spectral data using adaptive simulated annealing (ASA) to estimate reaction rates and rate constants from kinetic data sets. The values determined were validated using stochastic kinetic modeling and resulted in rate data for all forward and reverse reactions for the ensemble of reactive ions present in the ion trap. A comparison of the forward rate constants of the hydration steps showed that in general, formation of the monohydrates was slow, but that hydration efficiency increased upon addition of the second H2O. Addition of the third H2O was less efficient (except in the case of [UO2]+), and addition of the fourth H2O was even more inefficient and did not occur at all in the [UO2(OH)]+ system. Reverse rate constants also decreased with increasing ligation by H2O, except in the case of [UO(OH)(H2O)4]+, which prefers to quickly revert to the trihydrate. These findings indicate that stability of the hydrate complexes [UOyHz(H2O)n]+ increases with increasing n, until the optimum number of ligands is achieved. The results enable correlation of uranium hydration behavior with oxidation state.

Ion-molecule reactions of gas-phase chromium oxyanions: Cr xO yH z− + O 2

Chromium oxyanions, Cr xO yH z −, were generated in the gas-phase using a quadrupole ion trap secondary ion mass spectrometer (IT-SIMS), where they were reacted with O 2. Only CrO 2 − of the Cr 1O yH z − envelope was observed to react with oxygen, producing primarily CrO 3 −. The rate constant for the reaction of CrO 2 − with O 2 was ∼38% of the Langevin collision constant at 310 K. CrO 3 −, CrO 4 −, and CrO 4H − were unreactive with O 2 in the ion trap. In contrast, Cr 2O 4 − was observed to react with O 2 producing CrO 3 − + CrO 3 via oxidative degradation at a rate that was ∼15% efficient. The presence of background water facilitated the reaction of Cr 2O 4 − + H 2O to form Cr 2O 5H 2 −; the hydrated product ion Cr 2O 5H 2 − reacted with O 2 to form Cr 2O 6 − (with concurrent elimination of H 2O) at a rate that was 6% efficient. Cr 2O 5 − also reacted with O 2 to form Cr 2O 7 − (4% efficient) and Cr 2O 6 − + O (2% efficient); these reactions proceeded in parallel. By comparison, Cr 2O 6 − was unreactive with O 2, and in fact, no further O 2 addition could be observed for any of the Cr 2O 6H z − anions. Generalizing, Cr xO yH z − species that have low coordinate, low oxidation state metal centers are susceptible to O 2 oxidation. However, when the metal coordination is >3, or when the formal oxidation state is ≥5, reactivity stops.

Ion−Molecule Reactions of Gas-Phase Chromium Oxyanions: CrxOyHz- + H2O

Chromium oxyanions having the general formula Cr x O v H z - play a key role in many industrial, environmental, and analytical processes, which motivated investigations of their intrinsic reactivity. Reactions with water areperhaps the most significant, and were studied by generating Cr x O y H z - in the gas phase using a quadrupole ion trap secondary ion mass spectrometer. Of the ions in the Cr 1 O y H z envelope (y = 2, 3, 4; z = 0, 1), only CrO 2 - was observed to react with H 2 O, producing the hydrated CrO 3 H 2 - at a slow rate (∼0.07% of the ion-molecule collision constant at 310 K). CrO 3 -, CrO 4 -, and CrO 4 H - were unreactive. In contrast, Cr 2 O 4 -, Cr 2 O 5 -, and Cr 2 O 5 H 2 - displayed a considerable tendency to react with H 2 O. Cr 2 O 4 - underwent sequential reactions with H 2 O, initially producing Cr 2 O 5 H 2 - at a rate that was ∼7% efficient. Cr 2 O 5 H 2 - then reacted with a second H 2 O by addition to form Cr 2 O 6 H 4 - (1.8% efficient) and by OH abstraction to form Cr 2 O 6 H 3 -(0.6% efficient). The reactions of Cr 2 O 5 - were similar to those of Cr 2 O 5 H 2 -: Cr 2 O 5 - underwent addition to form Cr 2 O 6 H 2 - (3% efficient) and OH abstraction to form Cr 2 O 6 H - (<1% efficient). By comparison, Cr 2 O 6 -was unreactive with H 2 O, and in fact, no further H 2 O addition could be observed for any of the Cr 2 O 6 H z -anions. Hartree-Fock ab initio calculations showed that reactive Cr x O y H z - species underwent nucleophilic attack by the incoming H 2 O molecules, which produced an initially formed adduct in which the water O was bound to a Cr center. The experimental and computational studies suggested that Cr 2 O y H z - species that have bi- or tricoordinated Cr centers are susceptible to attack by H 2 O; however, when the metal becomes tetracoordinate, reactivity stops. For the Cr 2 O y H z - anions the lowest energy structures all contained rhombic Cr 2 O 2 rings with pendant O atoms and/or OH groups. The initially formed [Cr 2 O y - + H 2 O] adducts underwent H rearrangement to a gem O atom to produce stable dihydroxy structures. The calculations indicated that rearrangement did not occur in the [Cr 2 O 5 H 2 - + H 2 O] adduct, possibly because the rearranged product could not accommodate the negative charge.

Hydrolysis of VX on concrete: rate of degradation by direct surface interrogation using an ion trap secondary ion mass spectrometer.

The nerve agent VX (O-ethyl S-2-diisopropylaminoethyl methylphosphonothiolate) is lethal at very low levels of exposure, which can occur by dermal contact with contaminated surfaces. Hence, behavior of VX in contact with common urban or industrial surfaces is a subject of acute interest. In the present study, VX was found to undergo complete degradation when in contact with concrete surfaces. The degradation was directly interrogated at submonolayer concentrations by periodically performing secondary ion mass spectrometry (SIMS) analyses after exposure of the concrete to VX. The abundance of the [VX + H]+ ion in the SIMS spectra was observed to decrease in an exponential fashion, consistent with first-order or pseudo-first-order behavior. This phenomenon enabled the rate constant to be determined at 0.005 min(-1) at 25 degrees C, which corresponds to a half-life of about 3 h on the concrete surface. The decrease in [VX + H]+ was accompanied by an increase in the abundance of the principal degradation product diisopropylaminoethanethiol (DESH), which arises by cleavage of the P-S bond. Degradation to form DESH is accompanied by the formation of ethyl methylphosphonic acid, which is observable only in the negative ion spectrum. A second degradation product was also implicated, which corresponded to a diisopropylvinylamine isomer (perhaps N,N-diisopropyl aziridinium) that arose via cleavage of the S-C bond. No evidence was observed for the formation of the toxic S-2-diisopropylaminoethyl methylphosphonothioic acid. The degradation rate constants were measured at four different temperatures (24-50 degrees C), which resulted in a linear Arrhenius relationship and an activation energy of 52 kJ mol(-1). This value agrees with previous values observed for VX hydrolysis in alkaline solutions, which suggests that the degradation of submonolayer VX is dominated by alkaline hydrolysis within the adventitious water film on the concrete surface.

Electron flood charge compensation device for ion trap secondary ion mass spectrometry

During secondary ion mass spectrometry (SIMS) analyses of organophosphorous compounds adsorbed onto soils, the measured anion signals were lower than expected and it was hypothesized that the low signals could be due to sample charging. An electron flood gun was designed, constructed and used to investigate sample charging of these and other sample types. The flood gun was integrated into one end cap of an ion trap secondary ion mass spectrometer and the design maintained the geometry of the self-stabilizing extraction optics used in this instrument. The SIMION ion optics program was used to design the flood gun, and experimental results agreed with the predicted performance. Results showed the low anion signals from the soils were not due to sample charging. Other insulating and conducting samples were tested using both a ReO 4 − and a Cs + primary ion beam. The proximity of the sample and electron source to the ion trap aperture resulted in generation of background ions in the ion trap via electron impact (EI) ionization during the period the electron gun was flooding the sample region. When using the electron gun with the ReO 4 − primary beam, the required electron current was low enough that the EI background was negligible; however, the high electron flood current required with the Cs + beam produced background EI ions that degraded the quality of the mass spectra. The consequences of the EI produced cations will have to be evaluated on a sample-by-sample basis when using electron flood. It was shown that the electron flood gun could be intentionally operated to produce EI spectra in this instrument. This offers the opportunity to measure, nearly simultaneously, species evaporating from a sample, via EI, and species bound to the surface, via SIMS.

Gas-Phase Condensation Reactions of SixOyHz- Oxyanions with H2O

Water was reacted with gas-phase oxyanions having the general composition SixOyHz- that were formed and isolated in an ion trap-secondary ion mass spectrometer (IT-SIMS). The radical SiO2•- reacted slowly with H2O to abstract HO•, forming SiO3H-, at a rate of 8 × 10-13 cm3 molecule-1 s-1, corresponding to an efficiency of about 0.03% compared with the theoretical collision rate constant (average dipole orientation). The product ion SiO3H- underwent a consecutive condensation reaction with H2O to form SiO4H3- at a rate that was approximately 0.4−0.7% efficient. SiO4H3- did not undergo further reaction with water. The multiple reaction pathways by which radical SiO3•- reacted with H2O were kinetically modeled using a stochastic approach. SiO3•- reacted with water by three parallel reaction pathways: (1) abstraction of a radical H• to form SiO3H-, which then reacted with a second H2O to form SiO4H3-; (2) abstraction of a radical OH• to form SiO4H-, which further reacted by consecutive H• abstractions to for...

Oxygen-for-Sulfur Exchange in the Gas Phase: Reactions of Al and Si Oxyanions with H2S

Gas-phase Si and Al oxyanions were formed by particle bombardment, isolated by mass, and then reacted with H2S in an ion trap secondary ion mass spectrometer (IT-SIMS). The reactions proceeded by different reaction pathways depending on whether the oxyanions were even- or odd-electron species. The radical anion SiO2•- reacted with H2S by abstracting a •SH radical to form the even-electron SiO2SH-. Once formed, the even electron SiO2SH- reacted with a second H2S molecule by O-for-S exchange to form SiOS2H-. The radical anion SiO3•- abstracted an •H radical from H2S to form even-electron SiO3H-, which then underwent two consecutive O-for-S exchange reactions with H2S to form SiO2SH- and SiOS2H-. For the reactions of the even-electron anion AlO2-, the products of two consecutive O-for-S exchange reactions with H2S were AlOS- and AlS2-. The radical abstraction reactions and the O-for-S exchange reactions of SiO3H-, AlO2-, and AlOS- were efficient in the 30−50% range. The efficiency of the O-for-S exchange reaction of SiO2SH- (producing SiOS2H-) was substantially less efficient at 8%.

Experimental and Computational Study of Hydration Reactions of Aluminum Oxide Anion Clusters

Hydration of aluminum oxide anion clusters was studied in the gas phase using an ion trap secondary ion mass spectrometer. Hydration of both AlO2- and Al2O4H- occurred by the consecutive addition of two H2O molecules. For hydration of AlO2-, the rate constants for addition of the first and second water molecules are 4 × 10-11 and 4 × 10-10 cm3 molecule-1 s-1, respectively. The first and second hydration rate constants for Al2O4H- are 2 × 10-9 and 8 × 10-10 cm3 molecule-1 s-1, respectively. A comparison of the experimental rate constants to the theoretical rate constants reveals that addition of the first H2O to AlO2- is only 2% efficient, whereas addition of the first H2O to Al2O4H- is 100% efficient. Ab initio calculations were performed to assist in the interpretation of the kinetic results. Reaction mechanisms and energetics for the hydration of the AlO2- system were calculated using the HF/6-311+G(d(Al),p), B3LYP/6-31+G(d), B3LYP/6-311+G(2d,p), B3LYP/6-311+G(3d2f,2p), and MP2/6-311+G(2d,p) levels of t...

Characterization of VX on concrete using ion trap secondary ionization mass spectrometry

The nerve agent VX (O-ethyl S-2-diisopropylaminoethyl methyl phosphonothiolate) was analyzed on the surface of concrete samples using an ion trap secondary ion mass spectrometer (IT-SIMS). It was found that VX could be detected down to an absolute quantity of 5 ng on a concrete chip, or to a surface coverage of 0.0004 monolayers on crushed concrete. To achieve these levels of detection, the m/z 268 → 128 ion fragmentation was measured using MS 2, where m/z 268 corresponds to [VX + H] +, and 128 corresponds to a diisopropylvinylammonium isomer, that is formed by the elimination of the phosphonothiolate moiety. Detection at these levels was accomplished by analyzing samples that had been recently exposed to VX, i.e., within an hour. When the VX-exposed concrete samples were aged, the SIMS signature for intact VX had disappeared, which signaled the degradation of the compound on the concrete surface. The VX signature was replaced by ions which are interpreted in terms of VX degradation products, which appear to be somewhat long lived on the concrete surface. These compounds include ethylmethylphosphonic acid (EMPA), diisopropyl taurine (DIPT), diisopropylaminoethanethiol (DESH), bis(diisopropylaminoethane)disulfide [(DES) 2], and a particularly tenacious compound that may correspond to diisopropylvinylamine (DIVA), or an isomer thereof. It was found that the thiolamine-derived degradation products DIPT, DESH, and (DES) 2 were removed with isopropyl alcohol extraction. However, the DIVA-related degradation product was observed to strongly adhere to the concrete surface for longer than one week. Although quantitation was not possible in this set of experiments, the results clearly show the rapid degradation of VX on concrete, as well as the surface sensitivity of the IT-SIMS for intact VX and its adsorptive degradation products.

Characterization of bis(alkylamine)mercury cations from mercury nitrate surfaces by using an ion trap secondary ion mass spectrometer

Mercury-cyclohexylamine (R-NH2, where R = c-C6H11) ions having the composition Hg(R-NH)2H+ can be formed by exposing the surface of Hg(NO3) n = 1,2 salts to gaseous primary amines and then sputtering the surface with a primary ion beam (ReO 4 − ). The resultant ions can be stabilized and detected using an ion trap secondary ion mass spectrometer (IT-SIMS) instrument, which provides collisional stabilization that facilitates their observation. Isolation of the Hg(R-NH)2H+ ions followed by collisional activation produces daughter ions corresponding to (C6H10NH2)+ and Hg(R-NH)+. These assignments were supported by deuterium labeling experiments, which resulted in the formation of Hg(R-NH)(R-NH-d 11)H+ and Hg(R-NH)2H+. The existence of the Hg(R-NH)2 compounds on the surface was verified using Raman spectroscopy, which showed a strong absorption at 590 cm−1 that corresponded to Hg-N stretching. Analogous ions could be formed with n-hexylamine, but no Hg-bearing ions were formed when starting with a secondary or a tertiary amine. This indicates that only primary amines will react with Hg-nitrate surfaces in this manner. Hg-amine ions could not be formed starting with other Hg salts: chloride, iodide, thiocyanate, sulfate, and oxide. These results suggest that surface derivatization using amines, coupled with an IT-SIMS instrument, may offer an approach for determining inorganic metal speciation on surfaces.