IMPROVED ACCURACY IN STEM MASS MEASUREMENTS

STEM质量测量的准确性受到计数的限制 统计数据。可以很容易地计算STEM的预期性能 从探测器大小、样品几何形状、探测器几何形状、原子 散射截面和剂量。明亮的视野，小角度 (SA)暗场和大角度(LA)暗场探测器各自具有 计数效率接近统一。对于典型的标本，我们可以 计算10个电子/A2时的预期质量精度：FAB 纤维蛋白(50 kDa Q 12%)、谷氨酰胺合成酶(620 kDa I：2YO)、 蚯蚓血红蛋白(3.6丙二醛Q 0.9%)和无尾T7颗粒(50 丙二醛0.2%)。质量精确度符合远高于2%的要求，但没有提高 就像预期的那样，更大的颗粒。我们已经采用了无尾T7Vinis (40丙二醛)作为测试样本，因为它有明确的结构和 稳定性。为此，我们正在调查影响经济增长的因素 质量测量的精确度，例如：洗涤缓冲液、衬底 参数、烘干条件和烘干参数。一个 PCMass程序查看STEM图像中粒子的质量分布 对比不同方向的模型以确定 扭曲或定位缺陷。有了STEM3，我们就有了 开始在两个显微镜下对相同的标本进行平行研究。 那些在STEM I中提供干净背景和均匀颗粒的是 在真空下转移到STEM3中并重新测量。在STEM3中，我们有 更好地控制束流剂量、样品温度(低至LHe 温度)和数据采集参数。冷冻的标本可以 也装入STEM3中，在阶段中冷冻干燥 观察到的。

The accuracy of STEM mass measurements is limited by counting statistics. Theexpected performance of STEM can be calculated easily from probe size, specimen geometry, detector geometry, atomic scattering cross-sections, and dose. The bright field, small angle (SA) dark field, and large angle (LA) dark field detectors each have counting efficiency close to unity. For typical specimens, we can calculate the expected mass accuracy at 10 electronS/A2 : Fab fi-agments (50 kDa q 12%), glutamine synthetase (620 kDa i: 2Yo), earthworm hemoglobin (3.6 MDa q 0.9%), and tailess T7 particles (50 MDa 0.2%). Mass accuracy agrees well above 2%, but does not improve as expected for largzer particles. We have adopted tailess T7 vinis (40 MDa) as a test specimen because of its well-defined structure and stability. With this we are investigating factors affecting the accuracy of mass measurements such as: wash buffers, substrate parameters, f1reezing conditions and fi-eeze drying parameters. A PCMass program views the mass profile of particles in the STEM image in comparison to models in various orientations to determine distortion or locate defects. With the availablfity of STEM3, we have begun parallel studies of the same specimens in both microscopes. Those giving clean backgrounds and homogereous particles in STEM I are transferred under vacuum to STEM3 and remeasured. In STEM3 we have more control of beam dose, specimen temperature (down to LHe temperature) and data acquisition parameters. Frozen specimens can also be loaded into STEM3 and freeze dried in the stage while being observed.