IMPROVED ACCURACY IN STEM MASS MEASUREMENTS

STEM质量测量的准确性受到计数的限制 统计 可以很容易地计算STEM的预期性能 从探头尺寸、试样几何形状、检测器几何形状、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1。 对于典型的标本，我们可以 计算10电子S/A2：Fab下的预期质量准确度 谷氨酰胺合成酶（620 kDa i：2 Yo）， 血红蛋白（3.6MDaq0.9%）和无尾T7颗粒（50 MDa 0.2%）。 质量精度同意远高于2%，但没有改善 正如对于较大颗粒所预期的那样。 我们采用无尾T7葡萄酒 (40 MDa）作为测试样品，因为其结构明确， 稳定 有了这个，我们正在调查影响因素， 质量测量的准确度，例如：清洗缓冲液、底物 参数、冷冻条件和冷冻干燥参数。 一 PCMass程序查看STEM图像中颗粒的质量分布 与不同方向的模型进行比较，以确定 变形或定位缺陷。 有了STEM 3， 开始在两种显微镜下对相同的标本进行平行研究。 那些在STEM I中提供干净背景和同质粒子的人是 在真空下转移至STEM 3并重新测量。 在STEM 3中， 更好地控制射束剂量、样品温度（低至LHe 温度）和数据采集参数。 冷冻标本可以 也被加载到STEM 3中并在该阶段中冷冻干燥， 观察

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.