Instrumental aspects of secondary ion mass spectrometry and secondary ion imaging mass spectrometry

A short survey of the varieties of the Secondary Ion Mass Spectrometry (SIMS) known at present is given. The principle of quantitative analysis with respect to thin film analysis is discussed. The properties of SIMS and SIIMS (Secondary Ion Imaging Mass Spectrometry) are compared with those of Electron Microprobe Analysis. Results of an analysis of a thin film of titanium oxide and of an FeMn ferrite by means of SIMS and SIIMS are given.     

Subsurface biomolecular imaging of Streptomyces coelicolor using secondary ion mass spectrometry

Imaging using time-of-flight secondary ion mass spectrometry (TOF-SIMS) with buckministerfullerene (C(60)) primary ions offers the possibility of mapping the chemical distribution of molecular species from biological surfaces. Here we demonstrate the capability of the technique to provide biomolecular information from the cell surface as well as from within the surface, as illustrated with the distribution of two antibiotics in Streptomyces coelicolor (a mycelial bacterium). Differential production of the two pigmented antibiotics under salt-stressed and normal conditions in submerged cultivations could be detected from the TOF-SIMS spectra of the bacteria, demonstrating the potential of the technique in studying microbial physiology. Although both the antibiotics were detected on the cell surface, sputter etching with C(60)(+) revealed the spectral features of only one of the antibiotics within the cells. Exploratory analysis of the images using principal component analysis assisted in analyzing the spectral information with respect to peak contributions and their spatial distributions. The technique allows the study of not only lateral but also the depthwise distribution of biomolecules, uniquely enabling exploration of the processes within biological systems with minimal system intervention and with little a priori biochemical knowledge of relevance.     

Gold nanoparticle-enhanced secondary ion mass spectrometry imaging of peptides on self-assembled monolayers

We demonstrate the use of gold nanoparticles (AuNPs) to enhance the secondary ion emission of peptides in time-of-flight secondary ion mass spectrometry (TOF-SIMS). The signal intensity of peptides adsorbed onto AuNPs was significantly increased when compared to that of self-assembled monolayers (SAMs). This gold nanoparticle-enhanced SIMS, termed NE-SIMS, enabled the sensitive detection of subtle modifications of peptides, such as phosphorylation. From a quantitative analysis of the amounts of adsorbed peptides and AuNPs on SAMs using quartz crystal microbalance and surface plasmon resonance spectroscopy, the ratio of peptide molecule to AuNP on amine-SAMs was revealed to be 18-19:1. When considering the ratio of peptide to matrix (1:10(3)-10(6)) employed in a matrix-enhanced SIMS, the use of AuNPs gave rise to a significantly increased secondary ion emission of peptides. Peptides were adsorbed onto patterned AuNPs on SAMs using a microfluidic system, and well-contrasted molecular ion images were obtained. NE-SIMS is expected to be applied to a chip-based analysis of modification of biomolecules in a label-free manner.     

Molecular depth profiling of multilayer polymer films using time-of-flight secondary ion mass spectrometry

The low penetration depth and high sputter rates obtained using polyatomic primary ions have facilitated their use for the molecular depth profiling of some spin-cast polymer films by secondary ion mass spectrometry (SIMS). In this study, dual-beam time-of-flight (TOF) SIMS (sputter ion, 5 keV SF(5)(+); analysis ion, 10 keV Ar(+)) was used to depth profile spin-cast multilayers of poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (PHEMA), and trifluoroacetic anhydride-derivatized poly(2-hydroxyethyl methacrylate) (TFAA-PHEMA) on silicon substrates. Characteristic positive and negative secondary ions were monitored as a function of depth using SF(5)(+) primary ion doses necessary to sputter through the polymer layer and uncover the silicon substrate (>5 x10(14) ions/cm(2)). The sputter rates of the polymers in the multilayers were typically less than for corresponding single-layer films, and the order of the polymers in the multilayer affected the sputter rates of the polymers. Multilayer samples with PHEMA as the outermost layer resulted in lowered sputter rates for the underlying polymer layer due to increased ion-induced damage accumulation rates in PHEMA. Additionally, the presence of a PMMA or PHEMA overlayer significantly decreased the sputter rate of TFAA-PHEMA underlayers due to ion-induced damage accumulation in the overlayer. Typical interface widths between adjacent polymer layers were 10-15 nm for bilayer films and increased with depth to approximately 35 nm for the trilayer films. The increase in interface width and observations using optical microscopy showed the formation of sputter-induced surface roughness during the depth profiles of the trilayer polymer films. This study shows that polyatomic primary ions can be used for the molecular depth profiling of some multilayer polymer films and presents new opportunities for the analysis of thin organic films using TOF-SIMS.     

Copper diffusivity in boron-doped silicon wafer measured by dynamic secondary ion mass spectrometry

The effective copper diffusivity (D_eff) in boron-doped silicon wafer was measured using a Dynamic Secondary Ion Mass Spectrometry (D-SIMS) that was incorporated with an out-drift technique. By this technique, positive interstitial copper ions (Cu_I⁺) migrated to the surface region when a continuous charge of electrons showered on the oxidized silicon wafer, which was also bombarded by primary O₂⁺ ions. The Cu_I⁺ ions at the surface region diffused back to the bulk when the electron showering stopped. The D-SIMS recorded the real-time distribution of Cu_I⁺ ions, generating depth profiles for in-diffusion of copper for silicon-wafer samples with different boron concentrations. These were curve-fitted using the standard diffusion expressions to obtain different D_eff values, and compared with other measurement techniques.     

Three dimensional analysis of self-structuring organic thin films using time-of-flight secondary ion mass spectrometry

Selective sub-micrometer structuring of phase-separating organic semiconductor materials has recently got into focus for providing the opportunity of further improvements in optoelectronic device applications. Here we present a 3D-time-of-flight secondary ion mass spectrometry (3D-TOF-SIMS) depth profiling investigation on spin-coated blends consisting of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and a cationic cyanine dye (1,1′-diethyl-3,3,3′,3′-tetramethylcarbocyanine iodide). TOF-SIMS provides the required lateral and depth resolution to resolve material and molecular inhomogeneities and phase separation in the blend. The data are illustrating the three-dimensional arrangement of the substances involved and confirm results of earlier studies using atomic force microscopy, UV-vis spectroscopy and x-ray photoelectron spectroscopy, and which have shown well distinguishable morphological features. The formation of this domain structure has been found to be dependent on the absolute as well as the individual film thickness, in accordance with models based on thin liquid two-layer films. Honey-comb like primary structures with micrometer dimension were found in samples containing small amounts of dye molecules in the deposition solution. In this case a thin dye deposit on PCBM was detected, which is well separated from the dye layer at the substrate. For this type of sample, we discuss an extended model of film formation based on partial depletion of dye molecules during film solidification, resulting in two individual dye layers.     

Theoretical and experimental aspects of secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) can be used to investigate the composition of solids in the following way: by means of an ion bombardment (energy 3–15 keV) the sample is sputtered away. The sputtered particles which are ejected immediately as ions (secondary ions) are characteristic for the composition of the sample. Separated in a mass spectrometer they can be used in principle for a quantitative analysis.A short survey on the different models, proposed to explain the emission of secondary ions, will be given.The experimental techniques, which have to be carefully chosen for each experiment, will be discussed in more detail, viz. (1) choice of the primary particle: state of charge, element, current-density; (2) selection of the secondary particles: positively or negatively charged, atomic or cluster ions, post-ionization of sputtered neutrals, high or low mass resolution.The parameters which are important for solving practical problems, viz. element-dependent ionic yield, quantitative analysis, limit of detection, material composition, resolution in depth, will be discussed.Some examples from practice of SIMS will be used to illustrate the wide range of applicability of the method.     

Metal salts for molecular ion yield enhancement in organic secondary ion mass spectrometry: a critical assessment

In a search for molecular ion signal enhancement in organic SIMS, the efficiency of a series of organic and inorganic salts for molecular cationization has been tested using a panel of nonvolatile molecules with very different chemical characteristics (leucine enkephalin, Irganox 1010, tetraphenylnaphthalene, polystyrene). The compounds used for cationization include alkali bromide and group Ib metal salts (XBr with X = Li, Na, K; CF3CO2Ag; AgNO3; [CH3COCH=C(O-)CH3]2Cu; AuCl3). Alkali ions, very good for polar molecule cationization, prove to be of limited interest for nonpolar molecules such as polystyrene. Silver trifluoroacetate displays excellent results for all the considered molecules, except for leucine enkephalin (which might be due to the use of different solvents for the analyte and the salt). Instead, silver nitrate mixed with leucine enkephalin in an ethanol solution provides intense molecular signals. The influence of the respective concentrations of analyte and salt in solution, of the silver trifluoroacetate solution stability, and of the sample microstructure on the secondary ion intensities are also investigated. The results of other combinations of analyte and salts are reported. Finally, the use of salts is critically compared to other sample preparation procedures previously proposed for SIMS analysis of large organic molecules.     

Postacquisition mass resolution improvement in time-of-flight secondary ion mass spectrometry

Good mass resolution can be difficult to achieve in time-of-flight secondary ion mass spectrometry (TOF-SIMS) when the analysis area is large or when the surface being analyzed is rough. In most cases, a significant improvement in mass resolution can be achieved by postacquisition processing of raw data. Methods are presented in which spectra are extracted from smaller regions within the original analysis area, recalibrated, and selectively summed to produce spectra with higher mass resolution than the original. No hardware modifications or specialized instrument tuning are required. The methods can be extended to convert the original raw file into a new raw file containing high mass resolution data. To our knowledge, this is the first report of conversion of a low mass resolution raw file into a high mass resolution raw file using only the data contained within the low mass resolution raw file. These methods are applicable to any material but are expected to be particularly useful in analysis of difficult samples such as fibers, powders, and freeze-dried biological specimens.     

Analysis of dark spots growing in organic EL devices by time-of-flight secondary ion mass spectrometry

Chemical structural analysis of tape-stripped surfaces at dark spots growing in organic electroluminescent (EL) devices during exposure to the atmosphere was done by time-of-flight secondary ion mass spectrometry (OF-SIMS). The EL devices consist of indium-tin-oxide, triphenylamine-tetramer, tris(8-hydroxyquinoline)aluminum (Alq3), and a Mg-Ag cathode deposited in order under vacuum on a glass substrate. It was found that the interface between the Alq3 layer and the Mg-Ag cathode was exposed as a result of tape-stripping, where a large number of dark spots were observed on both sides. Secondary ion images of O-, Mg+, and Alq2+ were observed from the dark spots on the cathode side. On the other hand, Mg+ and O- images with a nucleus in the center were observed from the Alq3 side. It is concluded from the results that the constituent element Mg of the cathode was oxidized at the interface adjacent to the Alq3 layer during exposure to the atmosphere, forming a dark spot with a nucleus in the center. Finally, it was confirmed that the TOF-SIMS analysis of the tape-stripped surface is useful for the analysis of the mechanism of dark spot formation.     

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.     

Corona discharge ion mobility spectrometry with orthogonal acceleration time of flight mass spectrometry for monitoring of volatile organic compounds

We demonstrate the application of corona discharge ion mobility spectrometry with orthogonal acceleration time of flight mass spectrometry (CD IMS-oaTOF) for volatile organic compounds (VOCs) monitoring. Two-dimensional (2D) IMS-oaTOF spectra of VOCs were recorded in nearly real time. The corona discharge atmospheric pressure chemical ionization (APCI) source was operated in positive mode in nitrogen and air. The CD ion source generates in air H(3)O(+)(H(2)O)(n) and NO(+). The NO(+) offers additional possibility for selective ionization and for an increase of the sensitivity of monoaromatic compounds. In addition to H(3)O(+)(H(2)O)(n) and NO(+), we have carried out ionization of VOCs using acetone as dopant gas ((CH(3))(2)COH(+)). Sixteen model VOCs (tetrahydrofuran, butanol, n-propanol, iso-propano, acetone, methanol, ethanol, toluene, benzene, amomnia, dioxan, triethylamine, acetonitrile, formaldehyde, m-xylene, 2,2,2-trifluoroethylamine) were tested using these ionization techniques.     

Organic ion imaging using tandem mass spectrometry.

A triple-quadrupole mass spectrometer has been interfaced with a wide-angle secondary ion microprobe. The combination permits acquisition of data necessary to determine the distribution of targeted organic analytes even in the presence of overwhelming isobaric interference. Micrographs generated from secondary ion intensity alone are compared to those generated using secondary ionization with tandem mass spectrometry (MS/MS), both for image reference and to show the improvement in image quality that can be attained when MS/MS is employed. Inhomogeneous mixtures of glycerol, KCl, and asparagine on 1-cm-diameter aluminum targets were used to demonstrate the instrument's selectivity. Secondary ions generated from samples of this system include isobaric 133Cs+ implanted from the primary ion beam, the 41K(+)-glycerol adduct, and protonated asparagine.     

Characterization of polar compounds and oligomers in secondary organic aerosol using liquid chromatography coupled to mass spectrometry

A generic method has been developed for the analysis of polar compounds and oligomers in secondary organic aerosol (SOA) formed during atmospheric simulation chamber experiments. The technique has been successfully applied to SOA formed in a variety of systems, ranging from ozonolysis of biogenic volatile organic compounds to aromatic photooxidation. An example application of the method is described for the SOA produced from the reaction of ozone with cis-3-hexenyl acetate, an important biogenic precursor. A range of solvents were tested as extraction media, and water was found to yield the highest recovery. Extracts were analyzed using reversed-phase liquid chromatography coupled to ion trap mass spectrometry. In order to determine correct molecular weight assignments and increase sensitivity for less polar species, a series of low-concentration mobile-phase additives were used (NaCl, LiBr, NH4OH). Lithium bromide produced better fragmentation patterns, with more structural information than in the other cases with no reduction in sensitivity. The main reaction products identified in the particle-phase were 3-acetoxypropanal, 3-acetoxypropanoic acid, and 3-acetoxypropane peroxoic acid and a series of dimers and trimers up to 500 Da. Structural identification of oligomers indicates the presence of linear polyesters possibly formed via esterfication reactions or decomposition of peroxyhemiacetals.