Organic secondary ion mass spectrometry: Signal enhancement by water vapor injection

The enhancement of the static secondary ion mass spectrometry (SIMS) signals resulting from the injection, closely to the sample surface, of H₂O vapor at relatively high-pressure, was investigated for a set of organic materials. While the ion signals are generally improved with increasing H₂O pressure upon 12 keV Ga⁺ bombardment, a specific enhancement of the protonated ion intensity is clearly demonstrated in each case. For instance, the presence of H₂O vapor induces an enhancement by one order of magnitude of the [M+ H]⁺ static SIMS intensity for the antioxidant Irgafos 168 and a ∼1.5-fold increase for polymers such as poly(vinyl pyrrolidone).     

Organic ion imaging beyond the limit of static secondary ion mass spectrometry

Secondary ion mass spectra and images were obtained from spikes of choline chloride, acetylcholine chloride, and methylphenylpyridinium iodide deposited onto specimens of porcine brain tissue. Samples were subsequently subjected to a dose of 10-keV Cs⁺ sufficient to suppress secondary ion emission characteristic of the targeted analytes. Following ablation of the samples by massive glycerol clusters generated by electrohydrodynamic emission, secondary ion mass spectra and images could be obtained that reflected the identity and location of the spiked analytes. The absolute intensity of secondary ion emission that followed ablation was found to be between 30 and 100% of the intensity obtained prior to exposure to the high dose of Cs’. Not all chemical noise is removed by ablation, however, so that the signal-to-noise ratios after ablation correspond to between 10 and 85% of their values observed under conditions of low primary ion dose.     

Molecular weight determination of bulk polymer surfaces by static secondary ion mass spectrometry

The determination of molecular weights at surfaces of bulk polymer materials can be accomplished by static secondary ion mass spectrometry (SIMS) via fragments originating from repeat units and end groups. The intensity ratio of these fragments depends on the polymer chain length as seen for bisphenol-A-polycarbonate and perfluorinated polyethers (Krytox). A kinetic model of fragment ion formation explains the molecular weight dependent fragment intensities and links them to properties of the molecular weight distribution. In the most simple case one obtains the number average molecular weight <M_n> at the surface. This technique can be used for the determination of the molecular weight at bulk polymer surfaces such as a CD-ROM made from polycarbonate by injection molding.     

Characterization of beam-induced reactions occurring in liquid secondary ion mass spectrometry/fast-atom bombardment by tandem mass spectrometry

Department of General and Physical Chemistry, University of Liege, Liege, Belgium A specific beam-induced secondary reaction involving the condensation of hydroxylic matrices with some organic groups (aldehydes, ketones, etc.) accompanied by the loss of a water molecule was investigated by liquid secondary ion mass spectrometry/fast-atom bombardment (LSIMS/FAB). A mechanistic scheme and a structure of the induced product are proposed. The features of this secondary reaction were studied and the influence of the types of solutes, acidic additives, and matrices analyzed. Rather than a drawback, LSIMS/FAB mass spectrometry can take advantage of this matrix effect to infer analytical information through tandem mass spectrometry experiments. Specific neutral loss scans can be conducted to highlight beam-induced reactive molecules, even when the detection of these species is prevented in normal scan spectra by other surface-active components.     

Substitution reactions in the secondary ion mass spectrometry of N-methylpyridinium halides

Secondary ion mass spectra of N-methylpyridinium halides (C⁺X^?, where C⁺ is a pyridinium cation and X^? is a halogen anion) exhibit the C⁺ ions, a series of cluster ions ((C⁺)_n(X^?)_n–1) and, furthermore, remarkable [CX – R]⁺ ions (R = H or Me). The mechanism of the formation of [CX – R]⁺ ions was investigated by the use of deuterated compounds and B/E and B²/E constant linked-scan measurements. A possible explanation is proposed in which the ions are produced through substitution reactions between species constituting the C₂X⁺ cluster ions in the gas phase.     

Characterization of polymeric surfaces and interfaces using time-of-flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used for chemical analysis of surfaces. ToF-SIMS is a powerful tool for polymer science because it detects a broad mass range with good mass resolution, thereby distinguishing between polymers that have similar elemental compositions and/or the same types of functional groups. Chemical labeling techniques that enhance contrast, such as deuterating or staining one constituent, are generally unnecessary. ToF-SIMS can generate both two-dimensional images and three-dimensional depth profiles, where each pixel in an image is associated with a complete mass spectrum. This Review begins by introducing the principles of ToF-SIMS measurements, including instrumentation, modes of operation, strategies for data analysis, and strengths/limitations when characterizing polymer surfaces. The sections that follow describe applications in polymer science that benefit from characterization by ToF-SIMS, including thin films and coatings, polymer blends, composites, and electronic materials. The examples selected for discussion showcase the three standard modes of operation (spectral analysis, imaging, and depth profiling) and highlight practical considerations that relate to experimental design and data processing. We conclude with brief comments about broader opportunities for ToF-SIMS in polymer science.     

Depth profile measurement by secondary ion mass spectrometry

The possibilities of measuring depth profiles by secondary ion mass spectrometry are evaluated. The influence of different instrumental and experimental parameters on depth resolution in the profiles are studied: the effects of primary ion beam characteristics, reactive gas adsorption and mechanical aperturing in secondary ion extraction are discussed. Beam effects are studied from the point of view of surface damage. The effects of secondary processes, such as crater edge effects, element mixing, preferential sputtering, background signals, (residual) gas contamination and ion-induced topographical and compositional changes are studied for thin metal and binary materials.     

Liquid secondary ion mass spectrometry of porphyrin dimers: reduction reactions and structural characterisation

New synthesised porphyrin dimers, with an amide or ester linkage between the two porphyrin units, were studied using liquid secondary ion mass spectrometry (LSIMS). The formation of reduced species was observed for all the compounds and it was found that the extent of reduction is dependent on the matrix used and on the structure of the porphyrin dimer. The main fragmentation pathways lead to monomer fragments resulting from cleavage of the amide or ester linkage between the two porphyrin units. The consistency of the fragmentations for all the dimers studied leads to the proposal of a common designation for the fragment ions. LSIMS, in addition to molecular weight determination, can provide important structural information for this type of compound.     

A discussion on mass resolution in secondary ion mass spectrometry

Mass resolution is a very important parameter for mass spectrometry. It is necessary to compare the mass resolution between the newly developed TOF-SIMS and the conventionally high-performance magnetic SIMS. However, the definitions of mass resolution for these two types of instruments are quite different. Whether it is possible to compare mass resolution and how to do such comparison is a challenge. This problem was raised officially during the 2012 ISO/TC 201 meeting at Tampa, Florida, the United States and the long-term cooperation with ISO started afterwards. The definition of mass resolution is one of the most important and fundamental problems for mass spectrometry and should attract significant attention. Here, some detail discussions on mass resolution as well as the related experimental studies in the past few years, including the collaborations with ISO/TC 201/SC6 and SC1 are summarized. This summary covers the common problem for almost all the current existing and still used definitions of mass resolution. A reasonable new definition for mass resolution considering the peak shape or resolution function has been proposed, which has also been confirmed by using experimental studies of the mass resolution comparison between TOF and magnetic SIMS. This study lays a foundation for the future mass resolution comparisons between different mass spectrometry.     

Protein denaturation detected by time-of-flight secondary ion mass spectrometry

In the present work we investigate the denaturation of a functional protein, horseradish peroxidase (HRP), under various experimental conditions using time-of-flight secondary ion mass spectrometry. HRP was immobilized on TiO(2), and the samples were stored under different conditions. The activity of the enzyme was assessed colorimetrically and compared to ToF-SIMS spectra. We show that denaturation of the protein can be monitored using the ToF-SIMS signal of the disulfide bonds, which is related to the tertiary structure of the protein. As disulfide bonds appear in a vast range of proteins, the present findings may be of wide significance; i.e., a tool is provided that can allow the investigation of the presence of an active protein structure by a comparably simple surface analytical method.     

A pulsed time-of-flight mass spectrometer for liquid secondary ion mass spectrometry

The design and performance of a new time-of-flight mass spectrometer is reported. The instrument combines the advantages of a pulsed drawout TOF analyzer with a liquid secondary ion source. Differences from commercially available pulsed TOF analyzers (Wiley/McLaren type) are discussed with regard to operation with ion desorption from a liquid matrix.     

Quantitative analysis of microstructures by secondary ion mass spectrometry.

The focus of this review is on trace-element quantitation of microstructures in solids. This review is aimed at the nonspecialist who wants to know how secondary ion mass spectrometry (SIMS) quantitation is achieved. Despite 35 years of SIMS research and applications, SIMS quantitation remains a fundamentally empirical enterprise and is based on standards. The most used standards are "bulk standards"-solids with a homogeneous distribution of a trace element-and ion-implanted solids. The SIMS systematics of bulk standards and ion-implanted solids are reviewed.     

Image processing in secondary ion mass spectrometry

The application of image processing in secondary ion mass spectrometry is discussed. The Cameca 4f SIMS uses a single microchannel plate and a highly sensitive camera in combination with an image processor with real time capabilities (Kontron IBAS). An automation procedure with image integration, extended dynamic range image acquisition and retro depth profiling is presented and illustrated with practical applications.     

Surface analysis of polymer materials by secondary ion mass spectrometry

Atomic as well as molecular secondary ions are emitted from the uppermost monolayer of a solid during ion bombardment. Mass analysis of these positive and negative secondary ions supplies detailed information on the chemical composition of the bombarded surface. High mass range (> 10,000 u), high mass resolution (m/Δm > 10,000), accurate mass determination (ppm range) and high sensitivity (ppm of a monolayer) are achieved by applying time-of-flight (TOF) mass analyzers. TOF-SIMS has been successfully applied to a wide variety of polymer materials, including polymer blends, chemically or plasma modified surfaces, and plasma polymerization layers. Detailed information on the composition of repeat units, endgroups, oligomer distributions, additives, as well as surface contaminants can be obtained. Basic concepts of TOF-SIMS will be described and typical analytical examples for the characterization of polymer materials will be presented.     

Method for improved secondary ion yields in cluster secondary ion mass spectrometry

A method to increase useful yields of organic molecules is investigated by cluster secondary ion mass spectrometry (SIMS). Glycerol drops were deposited onto various inkjet‐printed arrays and the organic molecules in the film were rapidly incorporated into the drop. The resulting glycerol/analyte drops were then probed with fullerene primary ions under dynamic SIMS conditions. High primary ion beam currents were shown to aid in the mixing of the glycerol drop, thus replenishing the probed area and sustaining high secondary ion yields. Integrated secondary ion signals for tetrabutylammonium iodide and cocaine in the glycerol drops were enhanced by more than a factor of 100 compared with an analogous area on the surface, and a factor of 1000 over the lifetime of the glycerol drop. Once the analyte of interest is incorporated into the glycerol microdrop, the solution chemistry can be tailored for enhanced secondary ion yields, with examples shown for cyclotrimethylenetrinitramine (RDX) chloride adduct formation. In addition, depositing localized glycerol drops may enhance analyte secondary ion count rates to high enough levels to allow for site‐specific chemical maps of molecules in complex matrices such as biological tissues. Published in 2010 by John Wiley & Sons, Ltd.     

Organic ion imaging of biological tissue with secondary ion mass spectrometry and matrix-assisted laser desorption/ionization

Organic secondary ion mass spectrometry (SIMS) and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry can be used to produce molecular images of samples. This is achieved through ionization from a clearly identified point on a flat sample, and performing a raster of the sample by moving the point of ionization over the sample surface. The unique analytical capabilities of mass spectrometry for mapping a variety of biological samples at the tissue level are discussed. SIMS provides information on the spatial distribution of the elements and low molecular mass compounds as well as molecular structures on these compounds, while MALDI yields spatial information about higher molecular mass compounds, including their distributions in tissues at very low levels, as well as information on the molecular structures of these compounds. Application of these methods to analytical problems requires appropriate instrumentation, sample preparation methodology, and a data presentation usually in a three-coordinate plot where x and y are physical dimensions of the sample and z is the signal amplitude. The use of imaging mass spectrometry is illustrated with several biological systems.     

Quantitative analysis by thin-layer chromatography with secondary ion mass spectrometry

Summary A direct combination of thin-layer chromatography with secondary ion mass spectrometry (TLC/SIMS) provides a method for the quantitative analysis of thermally unstable compounds or compounds of low volatility such as nicergoline. The method is very simple and has excellent precision. The analysis was performed by using an aluminium TLC plate and a mixture of methylene chloride, acetone, and distilled water as a developing solvent. After development the portion of the plate with the nicergoline and the internal standard spots was cut off the TLC plate, and was attached to the SIMS holder directly. The amount of nicergoline was determined from the ratio of the fragment ion intensity of the nicergoline to the internal standard. The calibration curve was linear, and the detection limit was 10 ng at a signal-to-noise ratio of 5. This method should be considered for application to the determination of drugs in biological samples and also for the determination of possible impurities and decomposition products in drugs.     

Rapid identification of trimethyl and triethyl amines using sulphonic acidic ionic liquids: A time-of-flight secondary ion mass spectrometry study of fragmentation reactions

The time-of-flight secondary ion mass spectrometry (TOF-SIMS) has emerged as a powerful tool for the unswerving detection of biomolecules, in particular, proteins and peptides. To date, there is very little information available on the direct determination of trimethyl/triethyl amines using TOF-SIMS. One major hurdle in this regard is an ultrahigh vacuum system, usually needed in TOF-SIMS, which hampers its usability to trimethyl/triethyl amines owing to their high evaporation rate. We designed an efficient and sensitive protocol for rapid identification and sensitive determination of tertiaryalkyl amines using TOF-SIMS. The amines were derivatized by reaction with 1,4-butane sultone and sulphuric acid sequentially to afford the corresponding sulphonic acidic ionic liquids (ILs). The TOF-SIMS analysis of these task-specific ILs (TSILs) was carried out in both positive and negative polarity. The positive ion mass spectra of TSILs showed sharp fragmented peaks for tertiaryalkyl amines at typical level and up to 10 ppm. The possible mechanism for different fragmentation pathways in positive polarity was discussed.