EXPERIMENTS WITH A HIGH RESOLUTION FIELD EMISSION STEM

正在开发方法以使用电子光谱成像技术在配备有冷场发射枪的扫描透射电子显微镜（STEM）和耦合到并行检测系统的磁扇区电子光谱仪中绘制细胞和大分子组装体中的元素分布。绘制生物结构中的单原子图现在正成为这种仪器的范围之内。成像可以在高剂量分析之前用暗场STEM在低剂量下进行，使得大分子组装体的结构可以与它们所包含的特定原子的数量相关联。测量证实了理论预测，单原子检测磷可以实现通过使用纳米直径的探针。虽然磷原子可能由于光束诱导的结构退化而从其原始位置移动了几纳米，但受损分子仍然足够稳定，可以在1或2纳米分辨率下进行分析。这种分析只能通过校正样品漂移的光谱成像来实现。利用DNA和病毒的特征样本，研究了绘制少量磷原子的最佳策略。目前正在设计实验来绘制其他生物学上重要的元素（如钙和铁）的单原子图，这些元素也具有良好的电离截面。- STEM，扫描透射电子显微镜，磷，分子量，质谱图，微量分析

Methods are being developed to map the distribution of elements in cells and macromolecular assemblies using the technique of electron- spectroscopic imaging in a scanning transmission electron microscope (STEM) equipped with a cold field-emission gun and a magnetic sector electron spectrometer coupled to a parallel detection system. Mapping single atoms in biological structures is now becoming within the reach of this instrumentation. Imaging can be performed at low dose with dark-field STEM prior to analysis at high dose, so that structures of macromolecular assemblies can be correlated with the numbers of specific atoms that they contain. Measurements confirm theoretical predictions that single atom detection of phosphorus can be realized by using a nanometer diameter probe. Although phosphorus atoms may have moved several nanometers from their original positions due to beam- induced structural degradation, damaged molecules are nevertheless stable enough to be analyzed at 1 or 2 nanometer resolution. Such analyses can only be achieved by means of spectrum-imaging with correction for specimen drift. Optimal strategies for mapping small numbers of phosphorus atoms have been investigated using well characterized specimens of DNA and viruses. Experiments are being designed to map single atoms of other biologically important elements like calcium and iron that also have favorable ionization cross sections. - STEM, scanning transmission electron microscopy, phosphorus, molecular weights, mass maps, microanalysis