在有机分析中，质谱仪是一种非常常用的仪器。质谱仪主要由进样系统、离子源、质量分析器、检测器和数据处理系统等几大部分组成。
质谱仪的主要组成部分
我们通常所说的质谱分类是按照质量分析器分类的，因为质量分析器反应了质谱的分辨力，决定了质谱的功能。今天我们来跟大家说说可以称之为质谱仪“心脏”的离子源。因为只有样品发生了离子化之后才能被质谱检测到，而离子源的作用就使被测样品分子电离，并把离子汇聚成具有一定能量和几何形状的离子束。由于分析样品的多样性和分析要求的差异性，物质电离的方法和原理也各不同，为满足多方位的分析要求，产生了一系列电离方法。因此，在实际实验过程中，我们需要根据物质的性质及其电离的原理选择合适的离子源进行实验。
最常用的离子源五种离子源为电子轰击源（EI）、化学电离源（CI）、电喷雾电离源（ESI）、大气压化学电离源（APCI）和基质辅助激光解吸电离源（MALDI）。目前我们所测试中心配备的主要是电子轰击源（EI）、电喷雾电离源（ESI）和大气压化学电离源（APCI）。那么我们配备的离子源的离子化原理及其适用的物质究竟是怎样的呢？
电子轰击源（EI）：EI源是用在气相色谱质谱上的，是一种“硬电离”。EI源主要由电离室（离子盒）、灯丝、离子聚焦透镜和一对磁极组成。其主要的工作原理是灯丝发射出具备70eV能量的电子，经聚焦并在磁场作用下穿过离子化室到达收集极。此时进入离子化室的样品分子在一定能量电子的作用下发生电离，内能较大的离子在与中性分子（如He）碰撞时能够自发裂解产生更多的碎片离子。所有的离子被聚焦、加速聚焦成离子束进入质谱分析器。
EI源电离过程
对于大部分有机物来说，EI源的这种硬电离方式不仅可以看到母离子，而且可以看到很多碎片离子，便于进行结构解析。而且标准谱库就是利用EI源在70eV的碰撞能量下轰击已知的纯有机化合物，电离后分子离子进一步破碎产生丰富的碎片离子，形成具有丰富“指纹”信息的标准质谱图，这些标准质谱图存储起来成为标准谱库。我们在相同的碰撞能量下进行实验获得的质谱可以与标准谱库进行对比进而对化合物进行定性分析。
但是当样品分子稳定性不高时，分子离子峰的强度弱，甚至没有分子离子峰。当样品不能气化或遇热分解时，则更看不见分子离子峰。
适用物质：可挥发的，热稳定的，沸点一般不超过500℃，分子量一般小于1000的有机物。
电喷雾电离源（ESI）：ESI源一般是用于液相色谱质谱联用仪器中，这种电离方式基本不产生碎片峰，故称为软电离。其主要的工作原理是：包裹着样品的溶剂进入电喷雾探头，通过加着高压的毛细管，高电压使得液体表面带上电荷，溶剂被周围加热的氮气气化从而挥发，随着溶剂蒸发，溶剂表面的库伦排斥力越来越大，引起液滴爆炸，最后生成单个离子进入质量分析器。
ESI电离原理
ESI源正是由于软电离的方式，因此适合做分子量确认。对于分子量大，稳定性差的化合物，也不会在电离过程中发生分解，它适合于分析极性强的大分子有机化合物，如药物、肽、糖等。它最大的特点就是可以生成多电荷离子，这样，一个分子量为10000Da的分子若带有10个电荷，则其质荷比只有1000Da，进入了一般质谱仪可以分析的范围之内。
但是ESI源要求待测样品在溶液中必须能够形成离子；流动相中缓冲盐的种类和浓度对灵敏度均有显著影响，因此流动相的选择非常重要；基质抑制现象较为明显。
大气压化学电离源（APCI）：APCI是介于ESI和EI源之间的一种离子源，主要应用于液相色谱质谱联用仪中，其也是产生(M+H)+或(M-H)-等准分子离子峰，几乎不产生碎片。其主要的工作原理是：样品流经热喷雾器，加热器辅助样品分子快速蒸发。电晕针持续放电使得源内O2或N2分子电离，O2或N2离子将电荷转移给溶剂分子，溶剂离子将电荷转移给目标分子，最终目标离子进入质量分析器。
APCI电离原理
大气压化学电离源主要用来分析中等极性或低极性的小分子化合物。有些分析物由于结构和极性方面的原因，用ESI不能产生足够强的离子，可以采用APCI方式增加离子产率，可以认为APCI是ESI的补充。APCI主要产生的是单电荷离子，所以分析的化合物分子量一般小于2000Da。用这种电离源得到的质谱很少有碎片离子，主要是准分子离子。
APCI源要求样品要有一定的挥发性，要能够进行气态离子化；热不稳定的化合物不能使用这种方式进行测定。
希望以上知识能帮助大家对配备不同离子源的质谱仪的检测适用范围有所了解，以便于今后在检测时选择正常的质谱仪类型。
原文链接：https://kepu.nimte.ac.cn/view-14332.html
（公共技术服务中心 韩莹莹）
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