01 生物质谱法
质谱（MS）是先将物质离子化，按离子的质荷比分离，然后测量各种离子谱峰的强度而实现分析目的的一种分析方法。不同类型的质谱仪均利用电场或磁场来操纵离子运动并确定其质荷比。
质谱仪的基本组件是离子源，质量分析器，检测器以及数据和真空系统。离子源是在MS系统中引入的样品组分通过电子束，光子束（UV光），激光束或电晕放电电离的地方。在电喷雾电离的情况下，离子源将存在于液体溶液中的离子移动到气相中，将中性样品分子转化并分裂成气相离子，然后送入质量分析器。当质量分析器应用电场和磁场通过质量对离子进行分类时，检测器测量并放大离子电流以计算每个质量分辨离子的丰度。以检测器检测到的离子信号强度为纵坐标，离子质荷比为横坐标所作的条状图就是我们常见的质谱图。
其主要术语包括：质荷比：离子质量(以相对原子量单位计)与它所带电荷(以电子电量为单位计)的比值，写作m/Z；峰：质谱图中的离子信号通常称为离子峰或简称峰；离子丰度：检测器检测到的离子信号强度；基峰：在质谱图中，指定质荷比范围内强度最大的离子峰称作基峰。
图1 Thermo Fisher质谱仪.
02 液相色谱-质谱联用
LC-MS，全称Liquid Chromatography-Mass Spectrometry，即液相色谱-质谱联用，是一种结合了液相色谱（或高效液相色谱技术）的物理分离能力和质谱（MS）的质量分析能力的分析化学技术。其中液相色谱技术将混合物与多种组分分离，质谱技术则负责检测分析具有高分子特异性和检测灵敏度的各组分的结构特征，两种技术得到了协同增强。其与气相色谱-质谱联用（GC-MS）同属于色谱质谱联用技术范畴。目前已广泛的应用于生物技术，环境监测，食品加工、制药等领域。
液相色谱-质谱联用（LC-MS）为常温操作，主要针对不挥发性化合物、极性化合物、热不稳定性化合物及大分子量化合物（蛋白、多肽、聚合物等）的分析测定。其分析数据暂无商品化的谱库用于对比查询，目前只能通过建立谱库或自己解析谱图分析数据。而气质联用（GC-MS）为加温操作，主要适用于易挥发性化合物、热稳定性化合物、小分子量化合物、可气化化合物的分析测定；谱图获取采用电子轰击方式（EI），可与标准谱库进行对比。气质联用仪是最早商品化的联用仪器，但因其适用范围窄，应用受到一定程度的限制，据统计只有大约20%的有机物能用气相色谱分析。两种联用方式互为补充，可用于不同性质化合的分析。本文主要针对液相色谱-质谱联用进行介绍。
图2. LC-MS技术原理解析图。
03 常见质谱技术横向比较
四极杆质谱仪（QMS）是最常见的质谱仪，在GC-MS中占绝大多数，其优点在于结构简单，成本低且易于维护，但其定性能力欠佳，速度较慢且质量上限低（小于1200u）。三重四极杆质谱仪（QQQ）在保留QMS原有定量能力的特点上，还引入了串级功能，加强了质谱的定性能力，但其分辨力不足，容易受m/z近似值的离子干扰，且售价较高，需要定期维护。
目前的生物质谱仪，除了GC-MS的EI和CI源，离子化方式还包括大气压电离（API）与基质辅助激光解吸电离（MALDI）。前者常采用四极杆或离子阱质量分析器，统称API-MS。后者常用飞行时间作为质量分析器，所构成的仪器称为基质辅助激光解吸电离飞行时间质谱仪（MALDI-TOF-MS）。
MALDI-TOF测样速度快，操作简单，但一般来说蛋白往往能水解出几十种肽，并且经常含有修饰基团。用MALDI-TOF质谱测量多肽混合物时，待测组分不能完全离子化，对于一个蛋白得不到足够的肽信息，故其识别结果较不可靠，无法搜索肽数据库进行蛋白的识别。而LC-MS-MS联用能够检测到相当多的肽段数量和氨基酸序列，这样进行蛋白质识别就有了较高的可信度。
MALDI-TOF也不能进行翻译后修饰蛋白质的分析。具有离子阱的LC-MS-MS技术能够通过解析修饰导致的肽谱质量漂移提供修饰信息，并且能够提供修饰类型和修饰位点的结构信息。
另外的，相较于必须在液氦的温度和超高真空系统下工作的傅立叶变换离子回旋共振质谱(FTICR-MS)，一种根据离子在磁场中会进行回旋运动的特性设计的质谱技术，LC-MS则拥有更广泛的适用性。类似的磁质谱（Sector MS）虽然具备所有质谱中最强的定量能力，但因其仪器体积、重量大，造价昂贵，现已较少使用，仅用于地质元素和痕量二恶英的检测。
而LC-MS-MS在LC-MS的基础上将经过第一次质谱检测的离子以某种方式碎裂后再进行质谱检测，较LC-MS更加精密，且无需高条件环境或成本即可以分离和检测溶解在溶液中的微量物质。是目前来说常用的质谱分析技术。
参考文献
1.李卫中, et al. 液质联用技术在医药领域的发展与应用[J]. 現代食品與藥品雜誌, 2007, 17(6): 66-70.
2. Vogeser M, et al. Liquid chromatography tandem-mass spectrometry (LC-MS/MS)-technique and applications in endocrinology[J]. Experimental and clinical endocrinology & diabetes, 2007, 115(09): 559-570.
3. Tsikas D, et al. Analysis of eicosanoids by LC-MS/MS and GC-MS/MS: a historical retrospect and a discussion[J]. Journal of Chromatography B, 2014, 964: 79-88.
4. Keevil B G. LC–MS/MS analysis of steroids in the clinical laboratory[J]. Clinical biochemistry, 2016, 49(13-14): 989-997.
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