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.     

Organic secondary ion mass spectrometry: sensitivity enhancement by gold deposition

Hydrocarbon oligomers, high-molecular-weight polymers, and polymer additives have been covered with 2-60 nmol of gold/cm2 in order to enhance the ionization efficiency for static secondary ion mass spectrometry (s-SIMS) measurements. Au-cationized molecules (up to -3,000 Da) and fragments (up to the trimer) are observed in the positive mass spectra of metallized polystyrene (PS) oligomer films. Beyond 3,000 Da, the entanglement of polymer chains prevents the ejection of intact molecules from a "thick" organic film. This mass limit can be overcome by embedding the polymer chains in a low-molecular-weight matix. The diffusion of organic molecules over the metal surfaces is also demonstrated for short PS oligomers. In the case of high-molecular-weight polymers (polyethylene, polypropylene, PS) and polymer additives (Irganox 1010, Irgafos 168), the metallization procedure induces a dramatic increase of the fingerprint fragment ion yields as well as the formation of new Aucationized species that can be used for chemical diagnostics. In comparison with the deposition of submonolayers of organic molecules on metallic surfaces, metal evaporation onto organic samples provides a comparable sensitivity enhancement. The distinct advantage of the metal evaporation procedure is that it can be used for any kind of organic sample, irrespective of thickness, opening new perspectives for "real world" sample analysis and chemical imaging by s-SIMS.     

Activity-based assay of matrix metalloproteinase on nonbiofouling surfaces using time-of-flight secondary ion mass spectrometry

A label-free, activity-based assay of matrix metalloproteinase (MMP) and its inhibition was demonstrated on peptide-conjugated gold nanoparticles (AuNPs) with nonbiofouling poly(oligo(ethylene glycol) methacrylate) (pOEGMA) films using time-of-flight secondary ion mass spectrometry (TOF-SIMS). Following surface-initiated atom-transfer radical polymerization of OEGMA on a Si/SiO2 substrate, the MMP activity was determined by analyzing the cleaved peptide fragments using TOF-SIMS on the peptide-conjugated AuNPs. The use of nonbiofouling pOEGMA films in conjunction with AuNPs synergistically enhanced the sensitivity of assays for MMP activity and its inhibition in human serum. The detection sensitivity of MMP-7 in serum was as low as 20 ng mL(-1) (1 pmol mL(-1)), and the half-maximal inhibitory concentration (IC50) of minocycline, which is a MMP-7 inhibitor, was estimated to be 450 nM. It is anticipated that the developed system will be broadly useful for conducting activity-based assays of serum proteases, as well as for screening of their inhibitors, with high sensitivity in a high-throughput manner.     

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.     

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.     

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.     

Molecular ion imaging and dynamic secondary ion mass spectrometry of organic compounds

An ion microscope equipped with a resistive anode encoder imaging system has been used to acquire molecular secondary ion images, with lateral resolution on the order of 1 microns, from several quaternary ammonium salts, an amino acid, and a polynuclear aromatic hydrocarbon which were deposited onto copper transmission electron microscope grids. All images were generated by using the secondary ion signal of the parent molecular species. The variation of parent and fragment molecular ion signals with primary ion dose indicates that, for many bulk organic compounds, bombardment-induced fragmentation of parent molecules saturates at primary ion doses of (1-8) X 10(14) ions/cm2. Subsequent ion impacts cause little further accumulation of damage in the sample, and intact parent molecular ions are sputtered even after prolonged ion bombardment (i.e. primary ion doses greater than 1 X 10(16) ions/cm2). This saturation process allows molecular images to be obtained at high primary ion doses and allows depth profiles to be obtained from simple molecular solid/metal test structures.     

Metallurgical applications of secondary ion mass spectrometry (SIMS)

This paper reviews the unique advantages provided by secondary ion mass spectrometry (SIMS) for the chemical characterization of heterogeneous materials, with particular attention paid to the field of materials science: detection and imaging of the lateral distribution of every element from hydrogen to uranium, even at very low concentration and excellent depth resolution in the nanometre range. The advantages brought by coupling SIMS with other conventional (SEM, electron probe microanalysis, TEM/SEM) and new (X-ray photoelectron spectroscopy, nuclear microprobe) microscopical and microanalytical techniques are mainly illustrated by examples taken from the author's laboratory (essentially in the field of aluminium metallurgy): quantitative analysis of the solid solution and within phases, surface, thin film and interface analysis by depth profiles. Special attention will be focused on the advantages of SIMS as an analytical microscope and the importance of high mass resolution to solve practical problems. The difficulties of quantification associated with the variations of sputtering rate of materials and ionization probability of the emitted ionic species in multiphase systems will also be discussed.     

Combed single DNA molecules imaged by secondary ion mass spectrometry

Studies of replication, recombination, and rearrangements at the level of individual molecules of DNA are often limited by problems of resolution or of perturbations caused by the modifications that are needed for imaging. The Combing-Imaging by Secondary Ion Mass Spectrometry (SIMS) (CIS) method helps solve these problems by combining DNA combing, cesium flooding, and quantitative imaging via the NanoSIMS 50. We show here that CIS can reveal, on the 50 nm scale, individual DNA fibers labeled with different, nonradioactive isotopes and, moreover, that it can quantify these isotopes so as to detect and measure the length of one or more short nucleic acid fragments associated with a longer fiber.     

Nanovolume analysis with secondary ion mass spectrometry using massive projectiles

A variant of secondary ion mass spectrometry is presented where the surface is bombarded with individual gold nanoparticles each resolved in time and space with a corresponding event-by-event detection of the secondary ions (SIs). The projectile used, Au400(4+), with impact energy of 136 keV, generates high SI yields. Typically, there is co-emission of multiple SIs from a single impact, i.e., emission of SIs from molecules co-located within a nanovolume with dimensions in the 10-nm range. The ability to detect co-located molecules was tested on samples consisting of alternating nanometric layers of oppositely charged polyions, poly(diallyldimethylammonium chloride), poly(styrenesulfonate) (PSS), and clay nanoplatelets. To achieve signal statistics, the chemical analysis was carried out with a sequence of stochastic impacts making this method suitable for characterization of similar nanoparticles or spots dispersed on a surface. Attomole detection sensitivity was achieved for PSS. The homogeneity of assembled layers could be assessed with approximately 10-nm resolution.     

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.