Non-resonant laser secondary neutral mass spectrometry (NR-laser-SNMS) has been used for examining human melanoma cells grown in cell cultures and human glioblastoma xenografts grown in NMRI nude mice. Cells were incubated and mice were treated with boron-containing drugs. Ion-induced electron images were used to identify individual cells. Elemental and molecular images were obtained from the cancer cells with very high sensitivity and subcellular resolution. The measurement of the K/Na ratio demonstrated that for both cell cultures and tissue materials the preparation techniques used were appropriate for preserving the chemical and structural integrity of the living cell. The boron images show intra-and extracellular boron signals with different intensities. Molecular images show distinct features partly correlated with the cell structure.
Analyzing different proteins with time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a big challenge owing to the absence of unique, identifying peaks between the protein spectra. One possibility for improving the interpretation of the protein spectra is the application of principal component analysis (PCA). The proteins fibronectin and collagen and three fibronectin-collagen mixtures, adsorbed on smooth silicon substrates, were analyzed with ToF-SIMS. The intensities of molecular protein fragment signals were used to perform a PCA calculation. It was shown that PCA could clearly distinguish the various protein mixtures. From the loadings, it was possible to draw conclusions about the peak intensity distribution. Furthermore, PCA was found to be an appropriate statistical tool to identify outlier.
The distribution of polyaromatic hydrocarbons (PAHs) in ambient aerosol particles is of importance to both human health and climate forcing. Although time-of-flight secondary ion mass spectrometry (ToF-SIMS) has proven useful for studying the distribution of organic compounds in individual aerosol particles, it is difficult to detect PAHs at relevant concentrations in individual aerosol particles because of their low ion yield. In this study, we explore the potential of using laser secondary neutral mass spectrometry (Laser-SNMS) to study three PAHs: pyrene, anthracene, and naphthalene. Because of the high volatility of PAHs, a cryostage was required for the analysis to prevent sublimation of the molecules into the vacuum chamber. We studied two laser systems, a 157 nm excimer laser, which is capable of single-photon ionization of the PAHs, and a 193 nm laser, which requires multiphoton ionization. Under optimized conditions for laser power density and primary ion pulse length, 193 nm postionization resulted in a 2-50-fold increase in ion yield over ToF-SIMS. Using the 157 nm laser, the yield was increased by more than 3 orders of magnitude for all 3 PAHs studied. The single-photon postionization process proved superior in terms of both yield enhancement and reduced fragmentation. By using the optimized 157 nm laser system and a cryostage, we were able to detect PAHs on the surface of 2 μm diameter ambient aerosol particles.
We have applied laser secondary neutral mass spectrometry (laser-SNMS) for examining different states of biomineralization in vitro. Additionally, time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyse fibronectin and collagen type I, the main matrix proteins involved in the mineralization process, in a model system.Primary osteoblast-like cells derived from bovine metacarpals were cultured for 5 weeks on clean smooth silicon substrates. For mass spectrometric investigations, cells and newly-formed minerals were cryofixed, freeze-fractured, and freeze-dried.The results indicate that extracellular enrichment of potassium typically occurs in the vicinity of single osteoblasts during the initial stages of mineralization. This interaction of potassium with matrix macromolecules may prevent uncontrolled apatite nucleation and control the transformation of the matrix into a nucleating system. Thus, monovalent cations seem to be involved in the regulation of mineral nucleation of the extracellular matrix. From the data obtained it can be concluded that ToF-SIMS and laser-SNMS are well suited for imaging trace element and molecule concentrations in biological samples as well as for identifying molecular fragments characteristic of matrix proteins.
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