Imaging with SIMS

Two variations of conventional SIMS have evolved. In one, known as matrix-enhanced SIMS, the MALDI sample preparation protocol is used to coat the sample surface with an organic acid (e.g., 2,5-dihydroxybenzoic acid) or other suitable matrix material to improve ionization efficiency [24,25], This approach has been used to image brain tissue samples from freshwater snails (Lymnaea stagnalis). Another variation employs a coating of a thin layer of gold or other metals to enhance analytical signals [26], This method, known as metal-assisted SIMS, provides images with improved spatial and chemical resolution. 

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Fig. 3.24. Di rect-imaging mode SIMS image of a passivation layer on a niobium alloy [3.54], Boron enrichment at the interface is not visible with EPMA. Measurement time 10 s image diameter 150 pm primary ions OJ primary energy 5.5 keV.

Of the mass spectrometric techniques, SIMS has become a useful tool for the imaging of single cells.174 Human liver tumor cells poisoned with Cd, Cu, Cr, Hg and Zn were analyzed using imaging ToF-SIMS (ION-TOF) to visualize the metal distributions on a single cell basis. Among the five metal ions investigated, only Cr and Cu showed a preferential diffusion into the cell after... 

Finally, since it is possible to readily focus the probe ion beam, it has proven feasible to scan with SIMS and display images showing the elemental distributions. One example of such an analysis is shown in Figure 28. 

After chemical analysis with XPS at the various states of samples preparation, imaging ToF-SIMS analysis was applied to identify characteristic fragments of MAD-Gal at the surface. Figure 3 illustrates the negative F ion distribution on a masked-assisted patterned sample after photobonding. 

Another widely used application for SIMS is ion imaging, which shows secondary ion intensities as a function of spatial location on the sample surface (Figure 7.46). Further, if imaging is performed in tandem with depth profiling, a three-dimensional compositional map of a sample may also be generated (Figure 7.46). Two modes of imaging via SIMS are possible ... 

MALDI-IMS is still limited to about 20 pm spatial resolution (with 10-50 pm the most commonly achievable spatial resolution at this time), typically obtained with commercial ion sources (80). Recently, Zavalin et al. demonstrated that 5 pm spatial resolution can be achieved for MALDI-IMS instruments by spatial filtration of the laser beam by using a 25 pm ceramic pinhole filter (81). Secondary ion mass spectrometry (SIMS), developed in the 1960s (82), offers a complementary or alternative method to MALDI-IMS and other matrix-free LDI-MS methods for the acquisition of higher spatial resolution images. In SIMS, a focused high-energy primary ion beam (such as Cs+, Au3+, Bi5+, or C60+) is used to directly bombard the sample... 

A diagonal scan across the surface of the window was recorded with X-ray energy dispersive spectroscopy (XEDS) in the TEM equipment. The atomic ratios of Si 0 N were found to be 43% 13% 44%, which is not the expected stoichiometry of Si02 and Si3N4 (40% 20% 40%). However, we know from the TEM images and the SIMS data shown below that the thickness of the oxide layer is 7 nm and the total window thickness is about 42 nm from which we would expect atomic ratios of 42% 8% 50%. This is most likely indicative of a native oxide present on the backside of the windows, enriching the O content of the whole film (as seen with SIMS), since the escape depth of the photons is much greater than the window thickness. 

SIMS images. A simulated image with Poisson statistics and an SNR = 3.8 was created. The Anscombe transform was applied prior to filtering. 

Surface ion microscopy (SIM) can be nsed for speeies that desorb naturally as ions [92Stol]. Its use has therefore been restrieted so far to strongly electropositive elements sueh as potassimn, rubidimn, and cesium. The desorbing ions may be manipulated using appropriate ion opties to produee an enlarged image with a spatial resolution of abont 10 (tm. 

Secondary ion mass spectrometry (SIMS), when carried out in the static (i.e., zero damage) mode, is a uniquely powerful tool for the molecular characterization of polymer surfaces. Its molecular specificity also allows the detection and characterization of additives and contaminants, whose presence at the surface can cause major changes in surface behavior, at the submonolayer level. Modern instruments based on time-of-flight (ToF) mass analyzers and liquid-metal ion sources are capable of chemical imaging with a spatial resolution of well below 1 pm. 

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