ELEMENTAL MAPPING AT LOW DOSE

在132 eV能量损失处的磷L2边缘非常弱，但 提供了一个元素特定的信号。 磷直接显像 使用失去了这么多能量的电子， 比已知的破坏分子的剂量高得多。 无论多么低 剂量成像是可能的，在干使用同时记录的 用于定位用于成像的粒子的暗场环形检测器信号 平均 把几千个微弱的磷信号相加 排列的颗粒应足以获得 基本上没有受损的分子 核糖体的初步数据 分单位表示，该项目的许多部分是可行的， 大角度散射和小角度散射的差异 当平均时， 1,000个对齐的粒子图像，而TMV（烟草花叶病毒） 对照减去背景。 这可能表明RNA 子单元中的分布，但参数是间接的。 元素地图应该更容易解释。 一个关键 未知的是132 eV损失可能的局部化程度 电子。 这是-估计比'b.5 nra好，但需要 得到考验 对照标本为TMV和丝状病毒 它们是对齐的理想选择，并且已知或强烈预测 磷分布 一旦数据收集协议 建立和控制光谱匹配理论，磷作图将 在30 S和50 S核糖体亚基上进行尝试。 除了 区分核酸，核酸和蛋白质，这项技术应该是 可用于磷酸化蛋白质和磷脂。 相同的 技术可应用于B、C、N、O、F; Fe， Mg、Ca和其他元素在我们的标本中使用适当的 能量损失和控制信号。

The phosphorus L2, edge at 132 eV energy loss is very weak but provides an element-specificsignal. Direct imaging of phosphorus using electrons which have lost this amount of energy would require a dose much higher than that known to damage molecules. However low dose imaging is possible in the STEM using the simultaneously recorded dark field annular detector signal to locate particles for image averaging. Summing the weak phosphorus signal from several thousand aligned particles should be adequate to obtain a phosphorus map of essentially undamaged molecules. Preliminary data on ribosomal subunits indicated the feasibility -of many parts -of the project In that case the difference of large angle and small angle scattering gave a significant signal when averaged -over 1,000 aligned particle images, whereas the TMV (tobacco mosaic virus) control subtracted to background. This may be indicative of the RNA distribution in the subunits, but the arguments are indirect The elemental map should be much more straightforward to interpret. A key unknown is the degree of localization possible with the 132 eV loss electrons. This is -estimated to be better than'b.5 nra, but needs to be tested. Control specimens will be TMV and filamentous viruses which are ideal for alignment and have known or strongly predicted phosphorus distributions. Once the protocols for data collection are established and control spectra match theory, phosphorus mapping will be attempted on 30S and 50S ribosomal subunits. In addition to distinguishing nucleic, acid from protein, this technique should be useful for ing phosphorylated proteins and phospholipids. The same technique can be applied to elemental mapping of B, C, N, 0, F; Fe, Mg, Ca and other elements in our specimens using the appropriate energy loss and control signals.