Secondary Yield Research Articles

C60 Secondary Ion Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Secondary ion mass spectrometry (SIMS) has seen increased application for high spatial resolution chemical imaging of complex biological surfaces. The advent and commercial availability of cluster and polyatomic primary ion sources (e.g., Au and Bi cluster and buckminsterfullerene (C(60))) provide improved secondary ion yield and decreased fragmentation of surface species, thus improving accessibility of intact molecular ions for SIMS analysis. However, full exploitation of the advantages of these new primary ion sources has been limited, due to the use of low mass resolution mass spectrometers without tandem MS to enable enhanced structural identification capabilities. Similarly, high mass resolution and high mass measurement accuracy would greatly improve the chemical specificity of SIMS. Here we combine, for the first time, the advantages of a C(60) primary ion source with the ultrahigh mass resolving power and high mass measurement accuracy of Fourier transform ion cyclotron resonance mass spectrometry. Mass resolving power in excess of 100 000 (m/Δm(50%)) is demonstrated, with a root-mean-square mass measurement accuracy below 1 part-per-million. Imaging of mouse brain tissue at 40 μm pixel size is shown. Tandem mass spectrometry of ions from biological tissue is demonstrated and molecular formulas were assigned for fragment ion identification.

Analysis of native biological surfaces using a 100 kV massive gold cluster source.

In the present work, the advantages of a new, 100 kV platform equipped with a massive gold cluster source for the analysis of native biological surfaces are shown. Inspection of the molecular ion emission as a function of projectile size demonstrates a secondary ion yield increase of ~100× for 520 keV Au(400)(4+) as compared to 130 keV Au(3)(1+) and 43 keV C(60). In particular, yields of tens of percent of molecular ions per projectile impact for the most abundant components can be observed with the 520 keV Au(400)(4+) probe. A comparison between 520 keV Au(400)(4+) time-of-flight-secondary ion mass spectrometry (TOF-SIMS) and matrix assisted laser desorption ionization-mass spectrometry (MALDI-MS) data showed a similar pattern and similar relative intensities of lipid components across a rat brain sagittal section. The abundant secondary ion yield of analyte-specific ions makes 520 keV Au(400)(4+) projectiles an attractive probe for submicrometer molecular mapping of native surfaces.

High-Reactivity Matrices Increase the Sensitivity of Matrix Enhanced Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry (SIMS) is a desorption/ionization method in which ions are generated by the impact of a primary ion beam on a sample. Classic matrix assisted laser desorption and ionization (MALDI) matrices can be used to increase secondary ion yields and decrease fragmentation in a SIMS experiment, which is referred to as matrix enhanced SIMS (ME-SIMS). Contrary to MALDI, the choice of matrices for ME-SIMS is not constrained by their photon absorption characteristics. This implies that matrix compounds that exhibit an insufficient photon absorption coefficient have the potential of working well with ME-SIMS. Here, we evaluate a set of novel derivatives of the classical MALDI matrices α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) for usability in ME-SIMS. This evaluation was carried out using peptide mixtures of different complexity and demonstrates significant improvements in signal intensity for several compounds with insufficient UV absorption at the standard MALDI laser wavelengths. Our study confirms that the gas-phase proton affinity of a matrix compound is a key physicochemical characteristic that determines its performance in a ME-SIMS experiment. As a result, these novel matrices improve the performance of matrix enhanced secondary ion mass spectrometry experiments on complex peptide mixtures.

Molecular ion emission from alkanethiol-SAMs by HCI bombardment

In this work, we employ highly charged ions to study the sputtering of positive molecular fragments from two different alkanethiol self-assembled monolayers (SAMs) on gold surfaces: undecanethiol and dodecanethiol. The SAMs are bombarded with a pulsed Arq+ beam (3<q<12) with kinetic energies ranging from 1 to 15 keV. The desorbed positive molecular ions were detected and analyzed from time-of-flight spectra, and thereby the masses and yields of secondary ions were obtained. The proton yields are dependent on the charge state of the incident ion. On the other hand, the positive molecular ion yields, such as CnHm+, are charge state independent. The positive molecular ion yields decay with the molecular size n.

TOF-SIMS with Argon Gas Cluster Ion Beams: A Comparison with C60+

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an established technique for the characterization of solid sample surfaces. The introduction of polyatomic ion beams, such as C(60), has provided the associated ability to perform molecular depth-profiling and 3D molecular imaging. However, not all samples perform equally under C(60) bombardment, and it is probably naïve to think that there will be an ion beam that will be applicable in all situations. It is therefore important to explore the potential of other candidates. A systematic study of the suitability of argon gas cluster ion beams (Ar-GCIBs) of general composition Ar(n)(+), where n = 60-3000, as primary particles in TOF-SIMS analysis has been performed. We have assessed the potential of the Ar-GCIBs for molecular depth-profiling in terms of damage accumulation and sputter rate and also as analysis beams where spectral quality and secondary ion yields are considered. We present results with direct comparison with C(60) ions on the same sample in the same instrument on polymer, polymer additive, and biomolecular samples, including lipids and small peptides. Large argon clusters show reduced damage accumulation compared with C(60) with an approximately constant sputter rate as a function of Ar cluster size. Further, on some samples, large argon clusters produce changes in the mass spectra indicative of a more gentle ejection mechanism. However, there also appears to be a reduction in the ionization of secondary species as the size of the Ar cluster increases.

Soft‐sputtering of insulin films in argon‐cluster secondary ion mass spectrometry

In secondary ion mass spectrometry (SIMS) of organic substances, the dissociation of the sample molecules is crucial. We have developed SIMS equipment capable of bombardment, where the primary ions are argon cluster ions with kinetic energy per atom controllable down to 1 eV. We previously reported the detection of intact ions of insulin and cytochrome C using this equipment. In this paper, we present a detailed characterization of the emission of secondary ions from insulin, focusing on the difference in secondary ion yield between intact ions and fragment ions by varying the incident angle of the cluster ions. The emission intensity of the intact ions was changed drastically due to the exposed dosage and incident angle of the cluster ions in contrast to the fragment ions. We discuss these results based on the manner in which the argon-cluster ions collide with the organic solid.

Highly sensitive molecular detection with swift heavy ions

Various imaging techniques using microbeam have been applied in biology. Secondary ion mass spectrometry (SIMS) is one of the prominent tools for biological imaging; SIMS can provide data on molecular distribution in biological samples smaller than 1μm. However, conventional SIMS has only low sensitivity for molecular ions; therefore there is a need for beams of more sensitive primary ions. Plasma desorption mass spectrometry (PDMS) is a method using high energy fission fragments from excitation of a 252Cf source, and it allows ionization of large molecules (typically up to 20kDa) due to the dense electronic excitation. Although PDMS is not in use today because of the development of soft ionization methods, ionization induced by high energy ion collision still remains the only method which combines high spatial resolution and sensitive detection of large molecules. In this work, the secondary ion yield of amino acid and phospholipid was measured for 6MeV Cu4+. The yields were compared to bismuth cluster ions, which achieve relatively high yield. It was confirmed that the swift heavy ion has a couple of hundred times higher yield for large molecules than bismuth cluster ions.

Study of the desorption/ionization mechanism in electrospray droplet impact secondary ion mass spectrometry

Electrospray droplet impact (EDI) secondary ion mass spectrometry (SIMS) is a desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from an atmospheric-pressure electrospray are accelerated in vacuum by several kV and impact on the sample deposited on the metal substrate. The abundances of the secondary ions for C(60) and amino acids are measured as a function of the acceleration voltage of the primary charged water droplets. Two desorption/ionization mechanisms are suggested in the EDI ionization processes: low-energy and high-energy regimes. In the low-energy regime, the excess charges in the primary droplets play a role in the formation of secondary ions. In the high-energy regime, samples are ionized by the supersonic collision of the primary droplets with the sample. The yield of secondary ions increases by about three orders of magnitude with increase in the acceleration voltage of the primary droplets from 1.75 kV to 10 kV.

A new approach to measuring D/H ratios with the Cameca IMS‐7F

AbstractSIMS offers spatial resolution on the scale of micrometers and consumes an exceedingly small amount of sample. In addition, easy sample preparation, rapid analysis and high sensitivity allow SIMS to be applied to hydrous minerals. Hydrogen isotopes normally can be measured as negative or positive secondary ions using either a Cs+ or an O− primary ion beam, respectively. The hydrogen negative secondary ion yields are only marginally higher than hydrogen positive secondary ion yields. There are advantages and disadvantages to using either method to analyze hydrogen isotopes by SIMS. For insulating samples, O− is commonly used to avoid using the electron gun. The main disadvantage in using an O− beam is hydride interference (H2+) associated with deuterium (D) that must be resolved using a mass resolving power of ∼1000–2000. We have developed a method where we combine a lower mass resolution and a small voltage offset to obtain rapid (12 min), high‐precision (∼2‰, 2σ) D/H ratios in turquoise using a Cameca IMS‐7F ion microprobe. This technique can be applied to a variety of hydrous minerals in geological materials. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Investigations of secondary ion yield‐enhancing methods in combination

AbstractToday, several methods are used for secondary ion yield enhancement in time‐of‐flight SIMS (ToF‐SIMS). Some deal with the modification of sample properties, e.g. with monolayer preparation of analytes on metal substrates instead of thick layers, or the metalization of thick‐layer samples with noble metals like gold or silver (MetA‐SIMS). Other methods deal with primary ion beam parameters, e.g. the use of heavier ions or the use of polyatomic instead of monoatomic primary ions.To investigate the yield‐enhancing effects of the combination of these techniques, (sub‐) monolayers of polyethylene imine, both pristine and metalized with different amounts of gold, were prepared on substrate materials with different stopping powers. For primary ion (PI) bombardment, monoatomic (Ne+, Ar+, Kr+, Xe+, Bi+) and polyatomic (Bin+, Bin++) primary ions were used.It was found that all techniques used in this contribution increase the secondary ion yield mainly by increasing the stopping power, thereby increasing the amount of PI energy that is deposited near the sample surface (upper 5 nm). Only a small additional yield enhancement was observed when to a yield‐enhancing technique such as polyatomic PI bombardment, which deposits nearly all the PI energy near the surface, another technique was added that is based on the same mechanism for yield enhancement. In other words, the yield enhancement achieved by two enhancement techniques used in combination was much smaller than the product of the yield enhancements obtainable with each technique taken by itself. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Development of a TOF‐SIMS technology as a predictive tool for the needs of the mineral processing industry

AbstractRecently, upgrades towards a semiquantitative approach to mineral processing applications using the time of flight (TOF) SIMS (TOF‐SIMS) technique have been developed and implemented. Secondary ion yield at specific instrument parameters for matrix elements in the predominant ore minerals and their comparative normalization factors have been determined. Surface loading quantification for Cu on a variety of ore minerals has shown that signal intensity variability is related to the substrate matrix. Relative sensitivity factors for component loading have been determined and calibration curves for Cu loading on mineral surfaces have established with lower limits of detection in the range of 10 ppm.Given the new semiquantitative approach for surface characterization of minerals, a new test was developed to be used as a predictive tool in mineral flotation separation. The test protocol involved a two‐chamber ball mill where Cu transfer between the pulp and specimen surface was measured by the semiquantitative TOF‐SIMS approach. The test was applied to 13 ores. The reported experimental data on these ores demonstrated the ability of this technique to differentiate Cu transfer over a large dynamic range. The data also demonstrated that the surface loading of Cu on pyrite can be correlated, in some cases, with mineralogy. In others, however, the surface Cu loading observed is not congruent with the mineralogical assessment of the ore sample, but still linked with flotation behavior. This shows that the test could be used with mineralogy to better benchmark a sample before embarking on a flotation flowsheet development programme. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Ultra‐low‐angle microtomy to back up S‐SIMS molecular depth profiling with C60+ and Bin+ for the nanoscale analysis of high‐tech industrial materials

AbstractPolyatomic projectiles such as C60+ have been used for molecular sputter depth profiling in time‐of‐flight static secondary ion mass spectrometry (ToF‐S‐SIMS). However, the practical application for nanoscale analysis problems still depends on the likely change of secondary ion yield with depth. Therefore, ultra‐low‐angle microtomy (ULAM) has been applied in conjunction with dual beam depth profiling (Bi3+C60+ + +) on polymer systems relevant to printing industry.Sputter depth profiles have been recorded from two 200 nm poly(methyl methacrylate) (PMMA) layers with additives. Furthermore, also a poly(vinyl pyrrolidone) (PVP)‐PMMA bilayer on Si (thickness of each layer 200 nm) was analyzed using sputter depth profile measurements. Identical samples have been prepared using UV‐cured acrylate as substrate allowing ULAM to be performed. The exposed surfaces have been analyzed using Binq+ liquid metal ion gun (LMIG). Use of a cutting angle of 0.3° and 0.1° makes that, in principle, surface analysis off the section with a spot of 150 nm characterises a depth range of 0.3–0.8 nm in the initial sample.The results show that ULAM is a valuable tool to complement the exploratory use of direct sputter depth profiling. To the best of our knowledge, this feasibility study is the first to show the experimental verification of molecular C60+ sputter depth profiles. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Energy‐dependent fragmentation of polystyrene molecule using size‐selected Ar gas cluster ion beam projectile

AbstractA size‐selected Ar gas cluster ion beam (GCIB) was applied to the SIMS of a polystyrene thin film. The size of Ar cluster ions are selected between 5000 to 500 atoms at an acceleration voltage of 5 and 10 kV using time‐of‐flight technique. Under this condition, the kinetic energy per constituent atom (Eatom) of the GCIB projectile was estimated to be 1 ∼ 20 eV. The Eatom dependence of secondary ion yield of fragment species of polystyrene can be essentially classified into three types which are attributable to the relationship between the Eatom and the dissociation energy of the specified bonding site of the molecule. These results indicate that the site‐specific bond breaking of molecules could be possible by adjusting the Eatom of size‐selected GCIB. Copyright © 2010 John Wiley &amp; Sons, Ltd.

SIMS quantification of SiGe composition with low‐energy ion beams

AbstractSystematic SIMS analyses with low‐energy (250 eV ∼1 keV) oxygen, cesium and krypton primary beams have been carried out on a set of fully strained uniform epitaxial Si1−xGex films (x = 5 ∼ 60%), as well as a germanium ion‐implanted silicon standard to investigate the matrix effect under various conditions. It is shown that preferential ion yield enhancement of one matrix component over the other can occur as the result of primary ion incorporation. Through defining a matrix yield factor, this work demonstrated that constant secondary ion yield ratios between Si ion and Ge ion over a large concentration range are only valid under some very specific analysis conditions. Emphases were placed on oxygen beam analyses with regard to steady‐state ion yields and surface transients. Both show some unique features only accessible under low‐energy conditions. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Alkaline earth metal salts of CaCO3, BaCO3, and SrCO3 as matrix for tryptophan SIMS analysis

AbstractOur experiment is focused on an increase of the molecular secondary ion yield, observed using primary ion beam analysis of biomolecular solids. D‐tryptophan, C11H12N2O2, is determined by SIMS under impact of 25 keV Bi+ beam. Matrix‐embedded samples with the alkaline earth metal salts of CaCO3, BaCO3, SrCO3, are prepared in forms of pellets and films. The experiment with a CaCO3 pellet shows that D‐tryptophan parent‐like ions are cationized with alkali metal ions and small functional groups in monomer mass region. Signals of Ca ions are observed with very low intensity. D‐tryptophan dimers and trimers are also observed. In contrast, in CaCO3, BaCO3, SrCO3 pellet, D‐tryptophan parent‐like ion emission is suppressed by strong fragmentation of cellulose, added in pellets. Results from CaCO3 film show ionization of D‐tryptophan through proton donation. These comparative experiments can shed some light on details of the cationization phenomena in secondary ion mass spectrometry of biomolecules. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Quantitative analysis of the surface composition of mixed self‐assembled monolayers using ToF‐SIMS and FT‐IR analyses

AbstractMixed self‐assembled monolayers (SAMs) consisting of different molecules have been used to modify surface wettability, functionalize end groups for chemical binding, and control adsorption of proteins, DNA, or cells. In these applications, control over the quantification of the surface composition of a mixed SAM is especially important when designing biochips and biosensors. Previously, we presented a method by which ToF‐SIMS can be used to quantify the composition of two adsorbates with acetylene and propene functional groups from their secondary ion yield ratio in mixed SAMs on gold substrates. We concluded that although the ion yield ratio of the two adsorbates in mixed SAM was not equal to the mole fraction of Diyne (‐acetylene) and Diene (‐propene) in a solution, this was due to a different secondary ion formation, and not to the difference in the amount of adsorbates on the surface. In this study, we revisit the same sample system, this time applying a FT‐IR technique to test the accuracy of our previous findings. We found that the absorbances of the acetylene and propene functional groups neatly and linearly correlated with the mole fraction in the solution, which were consistent with our previous conclusions. We also used this methodology to quantify mixed silane SAMs on a silica surface. Our study shows that the ToF‐SIMS and FT‐IR techniques can complement one another and be useful to quantify mixed SAMs on various substrates. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Secondary ion yield enhancement in organic samples due to Au/Pt nanoparticle condensation and their substrate effects

AbstractUpon atomic ion bombardment, metal nanoparticles at the surface of organic materials (polymers, additives, biomolecules) induce a strong increase of the analytically significant sample ion yields in secondary ion mass spectrometry. In order to unravel the physics behind the Metal Assisted SIMS (MetA SIMS) effect, this study focuses, first, on the comparison between thin layers of polystyrene (PS) on gold‐covered substrates and thick layers of PS covered with different thicknesses of Au. The evolutions of the characteristic fragment ion yields are almost identical up to 8 nm equivalent Au thickness but the yields of Au‐cationized oligomers are consistently one order of magnitude larger when gold is used as substrate. This difference might be due to the limited diffusion of PS on the Au condensates. Second, the effect of Au and Pt on the ion yield enhancement has been studied by casting thin layers of PS on Au and Pt‐covered Si substrates. The Au and Pt‐induced fragment ion yields evolve similarly with the metal thickness up to 6 nm. The yields of metal cationized oligomers, however, are 7–8 times lower in the case of Pt compared to that of Au. Tentative interpretation of this effect involves the nature of the Me‐PS bond and the abundance of the respective species. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Investigating the relation between the secondary yield enhancement and the structure of the metallic overlayer in metal‐assisted SIMS

AbstractPristine and Au‐covered molecular films have been analyzed by ToF‐SIMS for different amounts of evaporated gold. In contrast to other MetA‐SIMS studies, the metallization of the high‐mass polymers (polycarbonate and polystyrene) was carried out in situ. The evaporation flux was monitored with a quartz‐crystal microbalance, and the exact quantity of deposited gold was determined by XPS. It was found that the sticking coefficient of gold on these polymers is less than one. The evolution of the secondary ion yield using Argon ion bombardment is studied as a function of the total amount of deposited metal. Furthermore, the SIMS results are accompanied by a detailed characterization of the sample surfaces morphology, in order to check any possible relation between the structure of the deposited metallic film and the yield enhancement of S‐SIMS signals. Copyright © 2010 John Wiley &amp; Sons, Ltd.

NanoSIMS50 imaging of thin samples coupled with neutral cesium deposition

AbstractThanks to the 50 nm of lateral resolution of the NanoSIMS50, new fields of application have been opened to Secondary Ion Mass Spectrometry. The NanoSIMS50 imaging capabilities are however often hampered by the fact that, due to a high impact energy (16 keV) and normal incidence of the primary ion beam, the steady state is only reached after sputtering several nanometers (10–20 nm) of the sample. As a consequence, the secondary ion emission is very low during the sputtering of these first nanometers. This phenomenon, which is most pronounced for organic and biologic samples, induces artifacts of measurement. In a previous study, we have shown that negative secondary ion yields can be enhanced in Secondary Ion Mass Spectrometry (SIMS) by coupling the primary ion bombardment with a simultaneous neutral cesium deposition. As this method cannot be used directly on the NanoSIMS50, we propose here to perform SIMS analysis after a prior deposition of Cs0 in a standalone UHV system. With this new method of analysis, organic thin films and proteins have been 3D monitored. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Photon, Electron and Secondary Ion Emission from Single C(60) keV Impacts.

This paper presents the first observation of coincidental emission of photons, electrons and secondary ions from individual C(60) keV impacts. An increase in photon, electron and secondary ion yields is observed as a function of C(60) projectile energy. The effect of target structure/composition on photon and electron emissions at the nanometer level is shown for a CsI target. The time-resolved photon emission may be characterized by a fast component emission in the UV-Vis range with a short decay time, while the electron and secondary ion emission follow a Poisson distribution.