Development of Standards for NanoSIMS Analyses of Biological Materials
NanoSIMS is a powerful analytical technique for investigating element distributions at the nanometer scale, but quantifying elemental abundances requires appropriate standards, which are not readily available for biological materials. Standards for trace element analyses have been extensively developed for secondary ion mass spectrometry (SIMS) in the semiconductor industry and in the geological sciences. The three primary approaches for generating standards for SIMS are: (1) ion implantation (2) using previously characterized natural materials, and (3) preparing synthetic substances. Ion implantation is a reliable method for generating trace element standards, but it is expensive, which limits investigation of the analytical issues discussed above. It also requires low background levels of the elements of interest. Finding or making standard materials has the potential to provide more flexibility than ion implantation, but realizing homogeneity at the nano-scale is in itself a significant challenge. In this study, we experiment with all three approaches, but with an emphasis toward synthetic organic polymers in order to reduce costs, increase flexibility, and achieve a wide dynamic concentration range. This emphasis serves to meet the major challenge for biological samples of identifying matrix matched, homogeneous material. Biological samples themselves are typically heterogeneous at the scale of microns to 100s of microns, and therefore they are poor SIMS standards. Therefore, we focused on identifying 'biological-like' materials--either natural or synthetic--that can be used for standards. The primary criterion is that the material be as compositionally similar to biological samples as possible (primarily C, H, O, and N). For natural material we adsorbed organic colloids consisting of peptidoglycan (i.e., amino sugars), activated charcoal, and humic acids. Experiments conducted with Si on peptidoglycan showed low affinity as SiO{sub 2}, yet its distribution in the matrix was similar to that observed in spores. In experiments with Mo on humic acid, homogeneity was achieved and a sensitivity factor relative to C was determined. For synthetic material, we successfully prepared polyacrylic acid containing complexed elements of Mo, Ca, Sr, and Ba at low abundance. These were prepared as aqueous mixtures of dissolved elements and polyacrylic resin, followed by thin film drying. The Mo was homogeneously distributed and yielded a relative sensitivity factor nearly identical to that calculated for humic acid. This approach shows great promise for most water soluble metals. Poly(methacrylate) thin films were prepared that contained different low-level concentrations of Si introduced as a silane compound. Although homogeneity was not fully achieved, the analytical results did validate our previous quantitative methodology for Si. In addition, Commercial plastics were also examined for suitability for F and Cl. We found food-grade polyvinyl tubing produced high precision Cl determinations. For ion implantation, we used epoxy as the substrate and successfully extracted depth profiles and sensitivity factors for F and Cu.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 945782
- Report Number(s):
- LLNL-TR-406039; TRN: US200904%%153
- Country of Publication:
- United States
- Language:
- English
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