Situ Secondary Ion Mass Spectrometry Research Articles

Oxygen isotope fractionation between calcite and fluid as a function of growth rate and temperature: An in situ study

The 18O content of carbonates (expressed as δ18O) is widely used in paleothermometry, yet it is clear from comparison of experimental results and natural carbonate samples that chemical (isotopic) equilibrium is not always achieved in nature. This observation underscores the importance of exploring possible effects of growth rate on isotopic fractionation, which is the focus of this study. In situ Secondary Ion Mass Spectrometry (SIMS) analyses of δ18O were performed on single crystals of experimentally grown calcite. The change in growth rate (V) over time within each crystal was monitored by addition of multiple rare earth element (REE) spikes into the fluid from which the calcite grew. The values of δ18O in bulk calcites and experimental fluids were measured by stable isotope ratio mass spectrometers. The average SIMS δ18O overlaps with those determined by conventional mass spectrometry within analytical uncertainty. The fractionation factor expressed as Δ18O approaches its equilibrium value in slowly grown calcites (V<0.02nm/s) and decreases by 1.5‰ with increasing growth rate from 10−3 to 0.34nm/s. Here Δ18O=103⋅ln(α18O), and α18O=(18O/16O)calcite/(18O/16O)fluid. Our results provide the first in situ evidence that under equilibrium conditions 18O may be depleted in the near-surface region of calcite relative to the bulk crystal lattice, and that this 18O-depleted zone can be “captured” during rapid crystal growth. Crystal growth rate is therefore a potentially important consideration when using δ18O in natural carbonates as a proxy for ocean and terrestrial temperature.

Vibrational Spectroscopy and Mass Spectrometry for Characterization of Soft Landed Polyatomic Molecules

We report implementation of two powerful characterization tools, in situ secondary ion mass spectrometry (SIMS) and ex situ surface enhanced Raman spectroscopy (SERS), in analyzing surfaces modified by ion soft landing (SL). Cations derived from Rhodamine 6G are soft landed onto Raman-active silver colloidal substrates and detected using SERS. Alternatively and more conveniently, high-quality SERS data are obtained by spin coating a silver colloidal solution over the modified surface once SL is complete. Well-defined SERS features are observed for Rhodamine 6G in as little as 15 min of ion deposition. Deposition of ~3 pmo1 gave high-quality SERS spectra with characteristic spectroscopic responses being derived from just ~0.5 fmol of material. Confocal SERS imaging allowed the enhancement to be followed in different parts of deposited dried droplets on surfaces. Characteristic changes in Raman spectral features occur when Rhodamine 6G is deposited under conditions that favor gas-phase ion fragmentation. Simultaneous deposition of both the intact dye and its fragment ion occurs and is confirmed by SIMS analysis. The study was extended to other Raman active surfaces, including Au nanostar and Au coated Ni nanocarpet surfaces and to SL of other molecules including fluorescein and methyl red. Overall, the results suggest that combination of SERS and SIMS measurements are effective in the characterization of surfaces produced by ion SL with significantly enhanced molecular specificity.

In situ δ7Li, Li/Ca, and Mg/Ca analyses of synthetic aragonites

In situ secondary ion mass spectrometry (SIMS) analyses of δ7Li, Li/Ca, and Mg/Ca were performed on five synthetic aragonite samples precipitated from seawater at 25°C at different rates. The compositions of δ7Li in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasma–mass spectrometry (MC‐ICP‐MS). Both techniques yielded similar δ7Li in aragonite when SIMS analyses were corrected to calcium carbonate reference materials. Fractionation factors α7Li/6Li range from 0.9895 to 0.9923, which translates to a fractionation between aragonite and fluid from −10.5‰ to −7.7‰. The within‐sample δ7Li range determined by SIMS is up to 27‰, exceeding the difference between bulk δ7Li analyses of different aragonite precipitates. Moreover, the centers of aragonite hemispherical bundles (spherulites) are enriched in Li/Ca and Mg/Ca relative to spherulite fibers by up to factors of 2 and 8, respectively. The Li/Ca and Mg/Ca ratios of spherulite fibers increase with aragonite precipitation rate. These results suggest that precipitation rate is a potentially important consideration when using Li isotopes and elemental ratios in natural carbonates as a proxy for seawater composition and temperature.

Soft-Landing of CoIII(salen)+ and MnIII(salen)+ on Self-Assembled Monolayer Surfaces

Soft-landing of mass-selected CoIII(salen)+ and MnIII(salen)+ complexes [salen = N,N′-ethylenebis(salicylideneaminato)] was performed using self-assembled monolayer surfaces of alkanethiol (HSAM) and fluorinated alkanethiol (FSAM) on gold as targets. Physical processes associated with ion deposition were studied using time-resolved in situ secondary ion mass spectrometry (SIMS) in a specially designed Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). We demonstrate charge retention by a significant fraction of precursor ions on the FSAM and complete neutralization on the HSAM surface. Our results indicate efficient electron transfer from gold to the deposited species for the HSAM surface and inefficient, if any, electron transfer on the FSAM surface. Formation of abundant cluster ions observed in SIMS spectra is attributed to gas-phase reactions in the SIMS plume. Detailed analysis allowed us to extract the kinetics of both ionic and neutral complexes trapped on the FSAM surface. The results indicate that neutralization on this surface most likely takes place on the defect sites.

Effect of the Surface on Charge Reduction and Desorption Kinetics of Soft Landed Peptide Ions

Charge reduction and desorption kinetics of ions and neutral molecules produced by soft-landing of mass-selected singly and doubly protonated Gramicidin S (GS) on different surfaces was studied using time dependant in situ secondary ion mass spectrometry (SIMS) integrated in a specially designed Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) research instrument. Soft-landing targets utilized in this study included inert self-assembled monolayers (SAMs) of 1-dodecane thiol (HSAM) and its fluorinated analog (FSAM) on gold and hydrophilic carboxyl-terminated (COOH-SAM) and amine-terminated (NH(2)-SAM) surfaces. We observed efficient neutralization of soft-landed ions on the COOH-SAM surface, partial retention of only one proton on the HSAM surface, and efficient retention of two protons on the FSAM surface. Slow desorption rates measured experimentally indicate fairly strong binding between peptide molecules and SAM surfaces with the binding energy of 20-25 kcal/mol.

First Observation of Charge Reduction and Desorption Kinetics of Multiply Protonated Peptides Soft Landed onto Self-Assembled Monolayer Surfaces

The kinetics of charge reduction and desorption of different species produced by soft-landing of mass-selected ions was studied using in situ secondary ion mass spectrometry (SIMS) in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). The improved SIMS capability described in this work utilizes an in-line 8 keV Cs+ ion gun and allows us to interrogate the surface both during the ion deposition and after the deposition is terminated. As a model system doubly protonated ions of Gramicidin S were deposited onto a fluorinated self-assembled monolayer (FSAM) surface. Our results demonstrate for the first time that various peptide-related peaks in FT-ICR SIMS spectra follow very different kinetics. We obtained unique kinetics signatures for doubly protonated, singly protonated and neutral peptides retained on the surface and followed their evolution as a function of time. The experimental results are in excellent agreement with a kinetic model that takes into account charge reduction and thermal desorption of different species from the surface.

Identification of LEIS contributions observed in the ion kinetic energy distributions collected on a cameca IMS-3f SIMS instrument

Kinetic energy distributions of the secondary ions and scattered ions of O + , Cs + and Ar + (bombardment with primary ions of O-, O 2+, Cs + and Ar + was utilized) were obtained from the elemental surfaces of Mg, Al, Si, Ag and Pb using a modified Cameca IMS-3f magnetic sector mass spectrometer. In addition, the distributions of O + were acquired for O 2+ ion surface bombardment of several standard reference materials (Ti-base alloys of SRM648 and SRM649) and geological samples [pyrrhotite (Fe (1-x) S) and arsenopyrite (FeAsS)]. Distinct features attributable to elastic binary projectile/target backscattering collisions were found to be superimposed in the data. Hence, a full characterization of these kinetic ion energy distributions could be made. As a result it is possible to identify both the secondary ion and scattered ion contributions present in the collected kinetic energy distributions and to perform low-energy primary ion surface scattering (LEIS) experiments combined with in situ secondary ion mass spectrometry (SIMS) analysis, albeit to a reduced sensitivity and resolution compared to those possible on dedicated LEIS instruments.

Quantitative secondary ion mass spectrometry analysis of SiO 2 desorption during in situ heat cleaning

Analysis of silicon oxide (SiO 2) desorption was performed by a combination of thermal annealing in an ultra high vacuum (UHV), consecutive growth of a thin silicon (Si) layer by molecular beam epitaxy (MBE) and ex situ secondary ion mass spectrometry (SIMS). Together with the Si substrate the Si-MBE layer creates a uniform Si matrix which allows quantitative analysis by SIMS. The protective oxide on as-received wafers stored for a few months turned out to be 0.8–1.6 nm thick. Desorption in a vacuum takes place in a narrow temperature window below 875°C. The main residual contaminations were carbon (C) with a surface concentration of about 10 12 cm −2 and boron (B) with a concentration of 6×10 10–6×10 11 cm −2. Metallic impurities were below the SIMS detection limit.

The mechanism and kinetics of the oxidation of Cr(100) single-crystal surfaces studied by reflection high-energy electron diffraction, x-ray emission spectroscopy, and secondary ion mass spectrometry/Auger sputter depth profiling

Reflection high-energy electron diffraction, x-ray emission spectroscopy, and ex situ secondary ion mass spectrometry (SIMS)/Auger analysis by sputter depth profiling have been used to study the oxidation of Cr(100) single-crystal surfaces in the oxygen partial pressure range 10−9–10−7 Torr from room temperature to 700 °C. It was found that at all temperatures and for oxygen partial pressures as high as 10−7 Torr, that the initial sticking probability for oxygen was unity. The rate of oxidation was then found to decrease exponentially to a limiting thickness determined by the sample temperature. Various oxide phases have been observed depending on the temperature and the oxide thickness: two face centered cubic phases with a0’s of 4.2 and 11.8 Å, a tetragonal phase, and finally hexagonal α-Cr2O3. Initial oxidation of the Cr(100) surface results in conversion of bcc Cr to fcc oxide. The fcc and tetragonal structures are thought to arise from the formation of different Cr vacancy superlattices due to oxygen incorporation, which leads eventually to the precipitation of α-Cr2O3. Oxygen 18/16 isotope experiments coupled with ex situ SIMS/Auger analysis have clearly demonstrated that oxygen uptake is accompanied by oxygen movement into the fcc Cr–O lattice for temperatures up to about 500 °C. After the appearance of α-Cr2O3, growth is primarily by cation transport.