An electron impact ion source has been designed for generation of noble gas ions in a compact isotope separator. The source utilizes a circular filament that surrounds an ionization chamber, enabling multiple passes of electrons through the ionization chamber. This report presents ion optical design and the results of efficiency and sensitivity measurements performed in an ion source test chamber and in the compact isotope separator. The cylindrical design produced xenon ions at an efficiency of 0.37% with a sensitivity of ~24 µA /Pa at 300 µA of electron current.

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 O-2 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.

Carbonate is a somewhat enigmatic anion in static secondary ion mass spectrometry (SIMS) because abundant ions containing intact CO32- are not detected when analyzing alkaline-earth carbonate minerals common to the geochemical environment. In contrast, carbonate can be observed as an adduct ion when it is bound with alkali cations. In this study, carbonate was detected as the adduct Na2CO3.Na+ in the spectra of sodium carbonate, bicarbonate, hydroxide, oxalate, formate and nitrite and to a lesser extent nitrate. The appearance of the adduct Na2CO3.Na+ on hydroxide, oxalate, formate and nitrite surfaces was interpreted in terms of these basic surfaces fixing CO2 from the ambient atmosphere. The low abundance of Na2CO3.Na+ in the static SIMS spectrum of sodium nitrate, compared with a significantly higher abundance in salts having stronger conjugate bases, suggested that the basicity of the conjugate anions correlated with aggressive CO2 fixation; however, the appearance of Na2CO3.Na+ could not be explained simply in terms of solution basicity constants. The oxide molecular ion Na2O+ and adducts NaOH.Na+ and Na2O.Na+ also constituted part of the carbonate spectral signature, and were observed in spectra from all the salts studied. In addition to the carbonate and oxide ions, a low-abundance oxalate ion series was observed that had the general formula Na2-xHxC2O4.Na+, where 0 < x < 2. Oxalate adsorption from the laboratory atmosphere was demonstrated but the oxalate ion series also was likely to be formed from reductive coupling occurring during the static SIMS bombardment event. The remarkable spectral similarity observed when comparing the sodium salts indicated that their surfaces shared common chemical speciation and that the chemistry of the surfaces was very different from the bulk of the particle. Copyright (C) 2003 John Wiley Sons, Ltd.

Chromium oxyanions, CrxOyHz−, were generated in the gas-phase using a quadrupole ion trap secondary ion mass spectrometer (IT-SIMS), where they were reacted with O2. Only CrO2− of the Cr1OyHz− envelope was observed to react with oxygen, producing primarily CrO3−. The rate constant for the reaction of CrO2− with O2 was ∼38% of the Langevin collision constant at 310 K. CrO3−, CrO4−, and CrO4H− were unreactive with O2 in the ion trap. In contrast, Cr2O4− was observed to react with O2 producing CrO3− + CrO3 via oxidative degradation at a rate that was ∼15% efficient. The presence of background water facilitated the reaction of Cr2O4− + H2O to form Cr2O5H2−; the hydrated product ion Cr2O5H2− reacted with O2 to form Cr2O6− (with concurrent elimination of H2O) at a rate that was 6% efficient. Cr2O5− also reacted with O2 to form Cr2O7− (4% efficient) and Cr2O6− + O (2% efficient); these reactions proceeded in parallel. By comparison, Cr2O6− was unreactive with O2, and in fact, no further O2 addition could be observed for any of the Cr2O6Hz− anions. Generalizing, CrxOyHz− species that have low coordinate, low oxidation state metal centers are susceptible to O2 oxidation. However, when the metal coordination is >3, or when the formal oxidation state is ≥5, reactivity stops.

Tetraethylammonium (TEN+) adsorbed to soil particles (primarily silicate) was investigated using static secondary ion mass spectrometry (SIMS) in order to assess the behavior of the adsorbate under atomic and polyatomic projectile bombardment. Three different instruments were used for the investigation; a quadrupole-SIMS instrument equipped with a ReO4− primary ion gun; an ion trap SIMS instrument equipped with ReO4−; and an imaging time-of-flight (ToF) SIMS equipped with Ga+. In all experiments, TEN+ was observed to decrease in abundance with increasing primary ion dose. The disappearance cross-section (σ130) for intact TEN+ (mz 130), induced by ReO4−, was measured at 670 Å2 using the quadrupole, and 560 Å2 using the ion trap. The σ130 induced by Ga+ was measured at 450Å2 using the ToF-SIMS, indicating that the polyatomic projectile was perturbing an area 20–50% larger than the monoatomic. These values are significantly larger than Ga+-induced cross-sections in the literature (100–200 Å2), for similar compounds in a more fluid matrix (gelatin). The comparison was extended by measuring the cross-section using ReO4− projectiles and a gelatin matrix: σ130 in this case was 480 Å2, which is of the order of 150% greater than the same experiment using Ga+. It is concluded that ReO4− produces a larger σ than does Ga+. In addition, the results suggest that disappearance cross-sections are larger on a refractory solid surface (silicate), than they are on a fluid surface (gelatin). The minimum detection limit was estimated for TEN+ on soil using ReO4− with the quadrupole SIMS instrument, at approximately 5 × 10−4 monolayers (ML), which corresponds to about 500 ppb. Consideration of this result suggests that a lower detection limit may be achievable using a brighter primary ion beam together with a trapped ion mass spectrometer.

The use of perrhenate (rhenium tetroxide, ReO4−) as a bombarding particle was compared with Cs+ for its ability to generate molecular species from sodium nitrate. The purpose of the study was to quantitatively evaluate the enhancement in sputtering to be gained using a heavy, polyatomic primary particle. It was found that ReO4− is three to five times more efficient at generating ions such as Na2NO3+ and Na(NO3)2−. The nitrate-bearing molecular ions were observed to decrease in intensity as primary ion dose increases; at the same time, nitrite-bearing ions were observed to increase. This observation is interpreted in terms of beam damage to the surface of the target. Disappearance cross sections (σ) using ReO4− bombardment were measured as 960 and 690Å2 for Na2NO3+ and Na(NO3)2−, respectively. σ values measured using Cs+ bombardment were slightly larger. These measurements show that for an equivalent area of the sample disrupted, ReO4− is more effective for the production of nitrate-bearing secondary ions, which increases the probability of completing a measurement before extensive beam damage occurs. Secondary ion energies were evaluated and shown to be comparable for the ReO4− and Cs+ bombardment experiments; for this reason, sample charging is not deemed to be a significant factor in these experiments.

Phosphazene polymers are a class of materials that are finding applications in membrane separations. A series of substituted bis(phenoxy)phosphazene polymers were characterized using static secondary ion mass spectrometry. The anion spectra contained ions which originate from the phosphazene backbone, as well as from the pendant aromatic moieties. The cation spectra also contained ions derived from the pendant moieties, but consisted primarily of ions which arise from adventitious surface contaminants, such as siloxane compounds and hydrocarbons. The backbone-derived ions could be distinguished from the pendant-derived ions on the basis of their response to prolonged primary ion bombardment: the pendant ions decay at rates that are generally two to four times the rates observed for tbe backbone ions. This observation is interpreted in terms of the pendant moieties being more easily removed during ablation than the backbone ions.

Ion emission mechanisms from high temperature inorganic matrices are poorly understood, and pathways for sample loss are largely unknown. In an effort to gain better understanding of these processes, an instrument was designed, built and tested which measured in sequence the ions, and then the neutrals coming from the surface of high temperature matrices. Switching between modes was rapid enough to follow changes. This instrument was applied to the study of ion formation from potassium and silver zeolites. It was shown that the only form in which potassium sublimes from potassium zeolite is as monatomic cations. This helps to explain the high alkali metal ion formation efficiency observed from zeolite matrices and supports the concept that alkali metal ions are subliming directly from the solid state into the gas phase. In contrast, silver sublimes from silver zeolite as a combination of monatomic cations and neutrals, with the ion emission intensity about 3 orders of magnitude less than the emission of ions from alkali metal impurities.

Images of the ion emitting regions of solid state ion sources have been produced using the ions emitted from the sources during operation to help elucidate the chemistry and physics of surface ionization. Examples are presented of a ceramic negative ion source producing perrhenate anions and of a zeolite ion source producing cesium cations, both of which are used as ion emitters in SIMS guns. In both cases it is shown that the ions orginate from the surface of the ceramic or zeolite matrix, and not from interfacial regions between the matrix and the metal support structure. It is argued that for these two systems the gas phase ions are formed predominantly by direct sublimation of preformed ions from the hot surface of the matrix, due to the established fact that the ions observed in the gas phase are also known to exist in the solid, while it is improbable that the neutral species exist in the solid in appreciable concentrations. It is further shown that conventional surface ionization filament designs introduce asymmetric voltage gradients in the ion lens, leading to a loss in focusing. This can be corrected by using an indirectly heated ion source that has no voltage gradient across the face of the ion emitting region.