David A. Dahl

simion for the personal computer in reflection

Abstract This article is a reflective overview of the origins, history, and capabilities of the ion optics simulation program simion for the PC (versions 2.0–7.0) from the author’s perspective. It provides insight into the rationale and events that contributed to the direction of the evolution and current capabilities of the program. The capabilities of version 7.0 are presented along with tests of its computational accuracy. Future developmental areas are discussed.

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.

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 extraction from insulating fibers in ToF-SIMS

Extraction of ions in a triple focusing time-of-flight secondary ion mass spectroscopy system is studied as a function of substrate potential with samples consisting of insulating dielectric fibers mounted on a metallic substrate. It is determined that ions extracted from the surfaces of the dielectric require, in general, a higher substrate potential than those extracted from the surface of the metallic substrate. Results are explained in terms of the electrostatic potential distribution in the extraction region and the dielectric properties of the fibers.

Sample charge compensation via self-charge-stabilizing ion optics

Abstract A self-charge-stabilizing ion optics method to control electrostatic sample charging of insulators bombarded with energetic particles has been developed and evaluated. In this method, ion optical elements form a simple self-controlling electrostatic field servo loop that automatically limits sample charging. The method has been applied successfully with insulating samples for secondary ion mass spectrometry in both quadrupole and ion trap based instruments. Self-charge-stabilizing ion optics appears to be a direct way to prevent or control sample charging in those circumstances where the approach is applicable. Keywords: Ion optics; Sample charging; Secondary ion mass spectrometry; Ion trap

A MODULAR ION BEAM DEFLECTOR

Abstract A modular electrostatic ion beam deflector has been designed and tested that provides well-isolated xy deflection with minimal beam distortion using a single supply voltage per deflector stage. The convenient stacked electrode design of these deflectors facilitates precise assembly while providing good field termination and isolation characteristics.

Rapid detection of tri-n-butyl phosphate on environmental surfaces using static SIMS

Abstract Static secondary ion mass spectrometry (SIMS) is an analytical method that can be used to detect the presence of tri-n-butyl phosphate (TBP) on environmental surfaces including minerals (e.g., basalts, quartz) and vegetation. Static SIMS instrumentation equipped with pulsed secondary ion extraction and a ReO4− primary ion gun permits the rapid acquisition of cation and anion mass spectra of samples surfaces with virtually no sample preparation: samples are merely attached to a sample holder using double-stick tape. SIM spectra were demonstrated to be sensitive to the mode of TBP adsorption to mineral surfaces: TBP adsorbed to Fe(II)-bearing phases, Fe(III)-bearing phases, silicate, and vegetation surfaces could be distinguished from one another. These results indicate that SIMS has broad applicability for the rapid characterization of environmental surfaces, and in some cases, is capable of identifying the mode of contaminant-surface interactions. The technique is also attractive because it can analyze milligram-size samples, and no waste is generated during analysis.

A univoltage ion gun design

Abstract A broad beam ion gun has been designed and tested that requires only a single high voltage supply. It creates 1–2-mm-diameter 2–10 kV beams that are visually uniform in spatial intensity with a sharp edge cutoff using a Perrhenate solid state ion source. The gun’s electrode geometry is designed to make beam diameter independent of acceleration voltage. Moreover, the source is imaged at the focal length of the acceleration optics to obtain a beam with visually uniform spatial intensity. With this approach, all points on the source’s emission surface contribute equally to the brightness of each point in the beam. The univoltage ion gun design has demonstrated its utility in broad beam static secondary ionization mass spectrometry (SIMS) instruments and is being adopted as the ion gun of choice for many of our group’s instruments.

Investigation of surface charging of insulating materials due to secondary particle emission using a new dual beam instrument

Abstract A new instrument has been developed to study the characteristics of charge build-up on insulating materials caused by the sputtering of secondary particles by ion, electron, neutral or photon beams. The instrument utilizes two beam lines: the charging ion beam irradiates the sample while a measurement ion beam is passed parallel to the sample surface and is then deflected by the charge on the sample. The deflection of the measurement beam, which is proportional to the charge on the sample, is measured with a microchannel plate and photodiode array combination. For these experiments the instrument was configured to simulate a secondary ion mass spectrometer using a negative ion primary beam and a secondary ion extraction lens element. The charge build-up on Mylar and Teflon bulk films was measured as a function of extraction voltage and the results were analyzed using the SIMION ion optics code. The samples could be charged negative or positive, depending upon extraction voltage, and the dominant parameter controlling the charging was the shape of the potential field near the sample. The results indicate that the extraction voltage and the design of the sample mount and near-field regions are the important factors controlling sample charging.

Mass separator for radioactive isotopes

A new isotope separator has been designed, constructed, and put into routine operation for separation of 133Xe providing a major advancement and significant cost reduction in preparation of this radioactive isotope. The design features and advantages are discussed that expedite high purity separation of relatively small quantities of this isotope. These advantages could be easily used to expedite separation of other shorter-lived radioactive isotopes.