Doped gelatin films as a model matrix for molecular secondary ion mass spectrometry studies of biological soft tissue

Carbon-13 labeling for improved tracer depth profiling of organic materials using secondary ion mass spectrometry

13C labeling is introduced as an alternative to deuterium labeling for analysis of organic materials using secondary ion mass spectrometry (SIMS). A model macromolecular system composed of polystyrene (PS) and poly(methyl methacrylate) (PMMA) was used to compare the effects of isotopic labeling using both deuterium substitution (dPS) and 13C labeling (13C-PS). Clear evidence is shown that deuterium labeling does introduce changes in the thermodynamic properties of the system, with the observation of segregation of dPS to an hPS:dPS/hPMMA interface. This type of behavior could significantly impact many types of investigations due to the potential for improper interpretation of experimental results as a consequence of labeling-induced artifacts. 13C labeling is shown to provide a true tracer for analysis using SIMS.

Comparative study of organic dyes with time-of-flight static secondary ion mass spectrometry and related techniques

A series of cationic, zwitterionic and anionic fluorinated carbocyanine dyes, spin-coated on Si substrates, were measured with time-of-flight static secondary ion mass spectrometry (TOF-S-SIMS) under Ga(+) primary ion bombardment. Detailed fragmentation patterns were developed for all dyes measured. In the positive mode, the resulting spectra showed very intense signals for the precursor ions of the cationic dyes, whereas the protonated signals of the anionic dyes were hardly detected. Differences of three orders of magnitude were repeatedly observed for the secondary ion signal intensities of cationic and anionic dyes, respectively. All measured dyes yielded mass spectra containing several characteristic fragment ions. Although the secondary ion yields were still higher for the cationic than the anionic dye fragments, the difference was reduced to a factor of < or =10. This result and the fact that M(+), [M + H](+) or [M + 2H](+) are even-electron species make it very likely that the recorded fragments were not formed directly out of the (protonated) parent ions M(+), [M + H](+) or [M + 2H](+). In the negative mode, none of the recorded spectra contained molecular information. Only signals originating from some characteristic elements of the molecules (F, Cl), the anionic counter ion signal and some low-mass organic ions were detected. A comparative study was made between TOF-S-SIMS, using Ga(+) primary ions, and other mass spectrometric techniques, namely fast atom bombardment (FAB), electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). The measurements showed that MALDI, ESI and FAB all give rise to spectra containing molecular ion signals. ESI and FAB produced M(+) and [M + H](+) signals, originating from the cationic and zwitterionic dyes, in the positive mode and M(-) and [M - H](-) signals of the anionic and zwitterionic dyes in the negative mode. With MALDI, molecular ion signals were measured in both modes for all the dyes. Structural fragment ions were detected for FAB, ESI and MALDI in both the positive and negative modes. Compared with the other techniques, TOF-S-SIMS induced a higher degree of fragmentation.

Degradation kinetics of VX on concrete by secondary ion mass spectrometry

At trace coverages on concrete surfaces, the nerve agent VX (O-ethyl S-2-diisopropylaminoethyl methyl phosphonothiolate) degrades by cleavage of the P-S and S-C bonds, as revealed by periodic secondary ion mass spectrometry (SIMS). The observed kinetics were (pseudo-) first-order, with a half-life of 2-3 h at room temperature. The rate increased with surface pH and temperature, with an apparent second-order constant of k(OH) = 0.64 M(-1) min(-1) at 25 degrees C and an activation energy of 50-60 kJ mol(-1). These values are consistent with a degradation mechanism of alkaline hydrolysis within the adventitious water film on the concrete surface. Degradation of bulk VX on concrete would proceed more slowly.

Metal-Assisted Secondary Ion Mass Spectrometry: Influence of Ag and Au Deposition on Molecular Ion Yields

A series of organic dyes and a pharmaceutical are used to study secondary ion yield enhancement by metal deposition. The molecules were dissolved in methanol and spin-casted on silicon substrates. Subsequently, silver or gold was evaporated onto the samples to produce a very thin coating. The coated samples, when measured with TOF-SIMS, showed a considerable increase in characteristic secondary ion intensity. Gold evaporated samples appeared to exhibit the highest signal enhancement. A major advantage of the metallization technique is that it can be used on real world samples. In particular, additive containing organic crystals could be studied; however, the observed signal increase does not occur at any given moment. The time between metal deposition on the sample surface and the measurement of the sample with TOf-SIMS appears to have an important influence on the enhancement of the secondary ion intensities. Therefore, the metal-coated samples were measured at different times after sample preparation. The results show that, depending on the sample and the metal deposited, the secondary ion signals reach their maximum at different times. Further study will be necessary to reveal the mechanism responsible for the observed enhancement effect.

Secondary ion formation of low molecular weight organic dyes in time-of-flight static secondary ion mass spectrometry

Time-of-flight static secondary ion mass spectrometry (TOF-S-SIMS) was used to characterize thin layers of oxy- and thiocarbocyanine dyes on Ag and Si. Apart from adduct ions a variety of structural fragment ions were detected for which a fragmentation pattern is proposed. Peak assignments were confirmed by comparing spectra of dyes with very similar structures. All secondary ions were assigned with a mass accuracy better than 50 ppm. The intensity of molecular ions as well as fragment ions has been studied as a function of the type of organic dye, the substrate, the layer thickness and the type of primary ion. A large yield difference of two orders of magnitude was observed between the precursor ions of cationic carbocyanine dyes and the protonated molecules of the anionic dyes. Fragment ions, on the other hand, yielded similar intensities for both types of dye. As the dye layers deposited on an Ag substrate yielded higher secondary ion intensities than those deposited on a Si substrate, the Ag metal clearly acts as a promoting agent for secondary ion formation. The effect was more pronounced for precursor signals than for fragment ions. The promoting effect decreased as the deposited layer thickness of the organic dye layer was increased.

Hydrolysis of VX on concrete: rate of degradation by direct surface interrogation using an ion trap secondary ion mass spectrometer

The nerve agent VX (O-ethyl S-2-diisopropylaminoethyl methylphosphonothiolate) is lethal at very low levels of exposure, which can occur by dermal contact with contaminated surfaces. Hence, behavior of VX in contact with common urban or industrial surfaces is a subject of acute interest. In the present study, VX was found to undergo complete degradation when in contact with concrete surfaces. The degradation was directly interrogated at submonolayer concentrations by periodically performing secondary ion mass spectrometry (SIMS) analyses after exposure of the concrete to VX. The abundance of the [VX + H]+ ion in the SIMS spectra was observed to decrease in an exponential fashion, consistent with first-order or pseudo-first-order behavior. This phenomenon enabled the rate constant to be determined at 0.005 min(-1) at 25 degrees C, which corresponds to a half-life of about 3 h on the concrete surface. The decrease in [VX + H]+ was accompanied by an increase in the abundance of the principal degradation product diisopropylaminoethanethiol (DESH), which arises by cleavage of the P-S bond. Degradation to form DESH is accompanied by the formation of ethyl methylphosphonic acid, which is observable only in the negative ion spectrum. A second degradation product was also implicated, which corresponded to a diisopropylvinylamine isomer (perhaps N,N-diisopropyl aziridinium) that arose via cleavage of the S-C bond. No evidence was observed for the formation of the toxic S-2-diisopropylaminoethyl methylphosphonothioic acid. The degradation rate constants were measured at four different temperatures (24-50 degrees C), which resulted in a linear Arrhenius relationship and an activation energy of 52 kJ mol(-1). This value agrees with previous values observed for VX hydrolysis in alkaline solutions, which suggests that the degradation of submonolayer VX is dominated by alkaline hydrolysis within the adventitious water film on the concrete surface.

Elemental and molecular imaging of human hair using secondary ion mass spectrometry

Secondary ion mass spectrometry (SIMS) is used to image the spatial distribution of elemental and molecular species on the surface and in cross sections of doped human hair using a magnetic sector SIMS instrument operated as an ion microprobe. Analysis of electrically insulating, non-planar hair samples requires one of two different methods of charge compensation to be used depending on the polarity of the sputtered secondary ions. For detection of positive secondary ions, the hair is imaged using a approximately 0.5 micron diameter, 19.5 keV impact energy, O- microbeam with no auxiliary electron bombardment. For detection of negative secondary ions, a approximately 0.2 micron diameter, 14.5 keV impact energy Cs+ microbeam is used in conjunction with normal incidence, low-energy electron bombardment. Both of these methods allow submicrometer spatial resolution elemental and molecular secondary ion images to be obtained from hair samples without metallic coating of the sample surface prior to analysis. Several examples are presented that reflect potential application areas for these analytical methods.

Charge compensation for imaging large insulating samples by using secondary ion tandem mass spectrometry

A charge compensation technique has been developed for secondary ion mass spectrometry and imaging of insulating samples as large as 1 cm(2) using a triple quadrupole-based microprobe. The microprobe secondary ion extraction field is synchronized with a periodic primary Cs(+) beam to allow a sheetlike beam of 5-eV electrons to pass over the sample surface when the extraction field is zeroed. Electrons are attracted to, and neutralize, any points on the sample that have accumulated positive charge. Positive secondary ion images from Teflon®, a well-known insulator, illustrate the effectiveness of charge compensation. Locating and identifying analytes on dry filter paper by using tandem mass spectrometry are also demonstrated.

Formation and emission of tetraalkylammonium salt molecular ions sputtered from a gelatin matrix

A gelatin matrix was simultaneously doped with nine equimolar, homologous, tetraalkylammonium salts ranging in mass from 210 to 770 Da. Bombardment of the sample with kiloelectronvolt ions resulted in a nonidentical distribution of relative cation intensities with a maximum at m/z 242 for samples with a total salt concentration of 0.004 g of salt/g of gelatin. A rapid increase in relative intensities with increasing mass is observed for the low mass salts and is believed to be linked to changes in the ionization efficiencies. The changes in ionization efficiencies are likely related to decreasing coulombic attractive forces between the organic cation and the counterion. Disappearance cross sections, determined from decay curves, indicate that sputter-induced damage increases with increasing mass of the cation. Fragment-to-intact cation ratios also suggest that damage accumulates fastest in the heaviest salts. These observations indicate that desorption yields of the organic salts in a gelatin matrix decrease with increasing mass. In addition, suppression of lower mass tetraalkylammonium salt intact cation intensities was observed for salt-in-gelatin concentrations greater than 10(-3) g/g.