MALDI Ionization: The Role of In-Plume Processes
MALDI Ionization: The Role of In-Plume Processes
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DOI：
10.1002/chin.200318288
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发表时间：
2003-05
期刊：
影响因子：
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通讯作者：
R. Knochenmuss;R. Zenobi
中科院分区：
文献类型：
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作者：
R. Knochenmuss;R. Zenobi
决定 MALDI 中观察到的离子分布的因素仍然引起人们极大的兴趣，1-15，希望该方法能够建立在合理和预测的基础上。已经有许多定性机制的建议，其中包括各种相当不同的过程。 16-18 Knochenmuss 等人。最近提出，MALDI 羽流中的次级反应在许多情况下可能是最终检测到的质谱的主要决定因素。 1 这一基于热力学的定量提议建立在早期的定性指示之上，即分析物离子主要或部分通过与基质或金属离子的二次反应形成。 16, 17, 19-27 热力学方法还扩展了早期对 MALDI 光谱的系统影响的研究。 10, 17, 24, 27-31 在 MALDI 中区分主要事件和次要事件的一个动机是时间尺度。激光脉冲通常持续 3-5 ns（N2 或 Nd:YAG 激光器），但扩展到无碰撞密度所需的时间要长得多，达到许多微秒。 32 除了稍后释放的预先形成的离子（例如，通过簇蒸发，见下文）之外，初级离子将在激光脉冲期间或在基质的激发态寿命内（也只有几纳秒）生成。然而，在膨胀的羽流中，只要存在碰撞，离子和中性物质之间的反应就会持续下去。如果这些碰撞的次数和能量足够高，任何热力学上有利的过程都可以达到平衡。请注意，初级和次级反应的时间尺度可能相似，也可能不相似；关键考虑因素是反应的顺序。举一个更具体的例子，常用的 UV-MALDI 基质（2, 5-二羟基苯甲酸，DHB）中的初级基质电离已被证明具有约 2 ns 的主导过程的时间尺度。 33 由于羽流速度约为 500-1000 m/s，因此向前膨胀为 1-2 微米。能量不足研究表明，一些分析物离子可能会在激光脉冲后在相当长的距离（数十微米）和很长的时间（10 至 100 纳秒）内形成。 4、20、25、34 通过脉冲提取 TOF 实验获得了类似的结果。 21, 35 对羽流的模拟表明，在此时间范围内，羽流密度仍高达解吸前固体的 10%。 36 还观测到约 500 K 的羽流温度。 37, 38 离子和分子的平均自由程相当短，只有几个分子直径，碰撞率很高。
The factors determining the observed ion distribution in MALDI continue to be of considerable interest, 1-15 in the hope that the method can be placed on a rational and predictive foundation. There have been a number of qualitative mechanistic proposals which include a wide variety of rather different processes. 16-18 Knochenmuss et al. have recently proposed that secondary reactions in the MALDI plume may in many cases be the dominant determinant of the final, detected mass spectrum. 1 This quantitative, thermodynamically based proposal was built on earlier qualitative indications that analyte ions are formed either predominantly or in part via secondary reactions with matrix or metal ions. 16, 17, 19-27 The thermodynamic approach also extends earlier studies of systematic influences on MALDI spectra. 10, 17, 24, 27-31One motivation for separating primary and secondary events in MALDI is one of time scale. The laser pulse typically lasts 3-5 ns (N2 or Nd: YAG lasers), but the time required for expansion to collision-free densities is much longer, many microseconds. 32 With the possible exception of preformed ions that are liberated later (eg, by cluster evaporation, vide infra), the primary ions will be generated during the laser pulse or within the excited state lifetime of the matrix (also only a few nanoseconds). In the expanding plume, however, reactions between ions and neutrals will continue as long as there are collisions. If the number and energies of these collisions are high enough, any thermodynamically favorable processes can proceed to equilibrium. Note that the time scales of primary and secondary reactions may or may not be similar; the key consideration is the sequence of the reactions. To give a more concrete example, primary matrix ionization in a commonly used UV-MALDI matrix (2, 5-dihydroxybenzoic acid, DHB) has been shown to have a time scale for the dominant process of about 2 ns. 33 Since plume velocities are about 500-1000 m/s, the forward expansion is 1-2 micrometers. Energy deficit studies suggest that some analyte ions may be formed later, at considerable distances (tens of micrometers), and long times (10s to 100s of nanoseconds) after the laser pulse. 4, 20, 25, 34 Similar results were obtained with pulsed extraction TOF experiments. 21, 35 Simulations of the plume show that in this time range the plume density is still up to 10% of the pre-desorption solid. 36 Plume temperatures of about 500 K have also been observed. 37, 38 The mean free path of ions and molecules is quite short, only a few molecular diameters, and the collision rate is high.