Anthony D. Appelhans

SIMION PC/PS2 electrostatic lens design program

SIMION PC/PS2 4.02 is a personal computer program for designing and analyzing charged particle (ions and electrons) lenses, ion transport systems, and various types of mass spectrometers and surface probes that utilize charged particles. The modification of an existing design or the generation of a completely new one is performed interactively with a graphics screen and mouse. Once the geometry has been defined, the operating conditions (electrode voltages and magnetic field configuration) can be quickly changed and the resultant fields viewed in several different 2D and 3D modes. The trajectories of charged particles moving in these fields are calculated utilizing sophisticated ‘‘look‐ahead’’ algorithms that dynamically control the time step to optimize speed and accuracy. A unique graphics display of the electrostatic fields and ion trajectories gives the user an intuitive, easily understood view of the performance characteristics. simion also includes a totally integrated capability for users to easily...

Secondary ion mass spectrometry of sodium nitrate: comparison of ReO4− and Cs+ primary ions

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 690A2 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.

Static SIMS investigation of tetraethylammonium bromide on soil particles using ReO4− and Ga+ projectiles

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 A2 using the quadrupole, and 560 A2 using the ion trap. The σ130 induced by Ga+ was measured at 450A2 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 A2), 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 A2, 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.

Detection of 2-chloroethyl ethyl sulfide and sulfonium ion degradation products on environmental surfaces using static SIMS

2-Chloroethyl ethyl sulfide (CEES) is a simultant for the chemical warfare agent bis(2-chloroethyl)sulfide (also known as HD or mustard), and both molecules undergo hydrolysis and subsequent condensation in aqueous solution to form stable sulfonium ions. The sulfonium ions derived from CEES are directly detected on quartzic surfaces using static SIMS instrumentation, which employs a molecular ReO 4 - (250 D) primary ion and pulsed secondary ion extraction. Pulsed extraction mitigates surface charging, and the ReO 4 - primary particle is efficient at sputtering molecular surface species into the gas phase. CEES eliminates Cl - to form an ethyl thiiranium intermediate, which is susceptible to nucleophilic attack by water and methanol to form 2-hydroxyethyl ethyl sulfide and 2-methoxyethyl ethyl sulfide. These two products and unhydrolyzed CEES also function as nucleophiles that condense with the ethyl thiiranium intermediate, resulting in the formation of sulfonium ion aggregates that are observable using SIMS. The previously unreported methoxy-substituted sulfonium ion suggests that a variety of derivatives are possible if different nucleophiles are present in the vicinity of the ethyl thiiranium intermediate. This work demonstrates that the sulfonium ion aggregates are stable on mineral surfaces and also demonstrates the potential value of SIMS for the detection of unanticipated ionic species in monitoring applications where mustard and its degradation products are suspected.

Ion-trap SIMS analysis of pinacolyl methylphosphonic acid on soil

Abstract The minimum detection limit and semi-quantitative determination of surface coverages of pinacolyl methylphosphonic acid (PMPA) on soil by static secondary-ion mass spectrometry (SIMS) are reported. The soil was exposed to aqueous solutions of PMPA and then analyzed on an ion-trap SIMS instrument. Mass spectrometry/mass spectrometry was utilized to discriminate against the chemical background inherent in environmental samples (such as soil). The quasimolecular ion [PMPA − H]− at m/z = 179 was trapped and then subsequently fragmented to form an ion at m/z = 95, which is interpreted as a loss of the pinacolyl olefin (C6H12) to form the methylphosphonic acid anion. The product ion at m/z = 95 was used to investigate the surface coverage of PMPA on the soil. The m/z = 95 product-ion abundance was observed to be linearly related to the PMPA surface coverage between 2 and 0.002 monolayers. The minimum detection limit is estimated at 0.008 monolayer (approximately 12 pg mm−2, three standard deviations of the blank). These data were compared with analyses performed by using a quadrupole SIMS instrument, which indicates an improvement in sensitivity by the ion-trap SIMS of a factor of 250. The results of this study demonstrate that ion-trap SIMS is a facile approach for determination of phosphonates on soil.

Characterization of copper chloride cluster ions formed in secondary ion mass spectrometry

Abstract The surfaces of copper chloride salts were investigated using three different secondary ion mass spectrometry (SIMS) instruments: a quadrupole instrument equipped with an ReO4− primary ion, a time-of-flight (ToF) instrument equipped with a Ga+ primary ion, and an ion trap instrument equipped with ReO4−. The research was conducted to identify copper chloride species sputtered from the surface of the copper chloride salts and to attempt to relate these species to the composition of the salt. Rich anion spectra were recorded using all three instruments for CuCl2 and CuCl, which were dominated by CuCl3− and CuCl2−. Other copper chloride adduct ions were also observed at higher masses. An examination of these ions revealed Cu primarily in the +1 and +2 oxidation state, irrespective of the oxidation state of the original salt. The presence of the Cu(+1)-bearing ions originating from the Cu(+2) salt has been attributed to reduction processes occurring during the bombardment event. However, oxidation processes must also be occurring, because Cu(+2)-bearing ions are observed in the spectra of the Cu(+1) salt. In contrast to the anion spectra, the only Cu-bearing cations contained in the CuCl(1,2) spectrum were acquired using the quadrupole- and ToF-SIMS corresponded to Cu+ and low abundance Cu2Cl+ and Cu3Cl2+ [(all Cu(+1)]. CuCl(1,2) were further investigated using the ion trap SIMS instrument: in these analyses, abundant cation clusters could be observed in addition to Cu+. The ions have been grouped into one of four categories: highly oxidized, one e− oxidized, redox neutral, and reduced. The most abundant species were redox neutral [e.g. (CuCl2)2CuCl+)] and one e− oxidized (e.g. (CuCl2)3+). Keywords: SIMS; Cluster ions; MS/MS; Ion trap; Copper chloride

Self imaging of surface ionization ion sources — where do the ions come from?

Abstract 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.

An autoneutralizing neutral molecular beam gun

A high‐energy (up to 28 keV) neutral molecular beam gun has been developed and put into routine use that takes advantage of the autoneutralization properties of the sulfur hexafluoride anion for the production of high‐energy sulfur hexafluoride neutral molecules. The anions are produced in an electron‐capture source, accelerated, and focused in a lens assembly designed to minimize residence time, allowed to drift at their terminal velocity for a suitable distance during which up to 30% auto‐eject an electron, and all remaining charged particles are electrostatically skimmed, resulting in a focused neutral beam. Rasterable neutral beams focused to a 5‐mm spot size up to 3 m from the source have been produced with beam currents up to 40 pA equivalent. Spot sizes of 1 mm can be produced with intensity levels of a few picoamperes equivalent.

Fast neutral beam ion source coupled to a Fourier transform ion cyclotron resonance mass spectrometer

The coupling of an autoneutralizing SF−6 fast ion‐beam gun to a Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometer is described. The fast neutral beam provides for secondary‐ion‐type FT/ICR mass analysis [e.g., production of abundant pseudomolecular (M+H)+ ions] of involatile samples without the need for external ion injection, since ions are formed at the entrance to the ICR ion trap. The design, construction, and testing of the hybrid instrument are described. The feasibility of the experiment (for both broadband and high‐resolution FT/ICR positive‐ion mass spectra) is demonstrated with tetra‐butylammonium bromide and a Tylenol■ sample. The ability to analyze high molecular weight polymers with high mass resolution is demonstrated for Teflon■. All of the advantages of the fast neutral beam ion source previously demonstrated with quadrupole mass analysis are preserved, and the additional advantages of FT/ICR mass analysis (e.g., high mass resolving power, ion trapping) are retained.

ION/NEUTRAL MASS SPECTROMETRY OF THERMAL IONIZERS : APPLICATION TO IONIZATION MECHANISMS FROM ZEOLITES

Abstract 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.