质谱技术发展至今已逾百年,目前质谱已不仅是常规化学分析中的重要手段,逐渐也开始被用于生命科学、国土安全、食品安全、临床医学检测和空间技术等热门领域。但传统的实验室台式质谱仪昂贵、耗能、机体庞大。若要应用于临床、机场安检、食品安全等原位现场测量场景,仪器必须小型化。
在小型化质谱仪的设计中,最大的一个挑战在于真空系统。传统质谱仪的真空系统往往比较笨重,小型质谱仪也只能选择小型的真空泵,而泵速的下降,会直接导致系统真空度降低,这会严重影响质量分析器及探测器的正常运行。而从目前的研究结果来看,质谱的背景噪声主要来自探测器端。
常见的质谱探测器(如mcp、电子倍增器/em)都是将离子转化为电子;电子被电场加速、倍增并最终检出。而加速的电子会和残余气体分子碰撞,产生正离子。这些正离子在电场中会反向运动,再次轰击产生电子,这个过程称为离子反馈(Ion Feedback,IFB)。
由于正离子反向运动是需要时间的,所以离子反馈所产生的信号与真实信号本身并不会叠加,反而成为了噪声/杂峰的重要来源。
而低真空度下较高浓度的气体分子是客观存在的,因此相比于控制离子生成,更为明智的做法是控制生成离子的走向。但如今四级杆及离子阱质谱仪一般采用的电子倍增器(EM),却并没有办法解决这一问题。滨松最新研制的拥有三级结构的GEN3 MCP,通过实现控制离子走向的策略,成功解决了以上问题。
滨松GEN3 MCP采用了这样的结构设计:
在MCP出口和打拿极之间加入栅网电极构成三级结构,栅网电极作为阳极(负高压模式下接地),后端打拿极和MCP入口则被设置为等电位,这样残留的气体分子电离生成的正离子会从栅网电极向打拿极运动,并被打拿极俘获。这种三级的创新结构设计可以避免电离正离子返回MCP,从而在源头上解决了暗电流的问题。
下图是三级结构的滨松GEN3 MCP和传统两级MCP电流输出结构在不同真空度下的实验数据对比。
传统两片结构(Bi-planer Mode)和滨松最新三级结构(Triode Mode)MCP的实测噪声(暗电流)对比
可以明显的看出,在105增益下,传统的2片MCP电流输出型组件在真空度高于10-3pa的情况下即会发生离子反馈。而对于三级结构的GEN3 MCP,即使真空度降低到1pa,仍然不会发生离子反馈。
凭借在低真空度下的优异表现,加上小巧的尺寸(有效面积直径:14mm),滨松GEN3 MCP将会大大释放束缚在质谱仪真空系统上的缰绳,方便开发者开发更为灵活便携、功耗更低、更适合现场使用的小型质谱仪。
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