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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Pumping Rate and Surface Morphology Dependence of Ionization processes in Matrix-Assisted Laser Desorption Ionization
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Pumping Rate and Surface Morphology Dependence of Ionization processes in Matrix-Assisted Laser Desorption Ionization

机译：基质辅助激光解吸电离中电离过程的抽速和表面形貌依赖性

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摘要

The quantitative photophysical description of matrix-assisted laser desorption ionization (MALDI) is explored by using a wide range of optical pumping rates established by 22 ps and 4 ns laser pulse lengths. Furthermore, the sample morphology dependence of ionization was tested on pellets compacted at different pressures, yielding surfaces of varying roughness. Mass spectra acquired from 2, 5-dihydroxybenzoic acid pellets showed an increase in the matrix ionization threshold fluence with increasing pressure during pellet formation. Sample surfaces produced at higher pressure exhibited lower fractal dimension. This fractal coarsening is suggested to be behind the elevated ion formation threshold in these samples. Compared with conventional nanosecond pulses, the ionization threshold values were consistently higher by a factor of - 2 for the picosecond laser. The DHB matrix was dramatically more likely to yield fragment ions when irradiated with the longer laser pulse, indicating significant differences in the primary ionization mechanism. Furthermore, fragmentation escalated with the increase in laser fluence for both lasers, although this effect was significantly more pronounced for nanosecond pulses. Dried droplet samples of sinapinic acid and α-cyano-4-hydroxycinnamic acid also showed higher fragmentation with the longer laser pulse. Different optical pumping rates and relaxation channels are thought to be responsible for the significant difference in the yield of matrix molecular ions. For small analytes (≤ 2000 Da), molecular ion signal was easily acquired with both lasers, whereas for large molecules (≥ 5000 Da), the analyte ion yield was low or even vanishing with the picosecond laser. The nanosecond laser produced ions from the small and large analytes with comparable yields. These observations are compatible with the assumption that the longer laser pulse can sequentially desorb and ionize analyte molecules due to the extended interaction of the laser pulse and the MALDI plume, whereas the shorter pulse can only desorb the analyte molecules but cannot promote their ionization due to its limited temporal overlap with the plume.
机译：通过使用由22 ps和4 ns激光脉冲长度建立的各种光泵浦速率，探索了基质辅助激光解吸电离（MALDI）的定量光物理描述。此外，在不同压力下压实的小球上测试了电离的样品形态学依赖性，产生了粗糙度不同的表面。从2，5-二羟基苯甲酸小球获得的质谱图显示，在小球形成过程中，基质电离阈值通量随压力的增加而增加。在较高压力下产生的样品表面表现出较低的分形维数。在这些样品中，这种分形粗化被认为是在离子形成阈值升高之后。与传统的纳秒脉冲相比，皮秒激光的电离阈值始终高出-2倍。当用更长的激光脉冲照射时，DHB基质极有可能产生碎片离子，这表明主要电离机理存在显着差异。此外，对于两个激光器，碎片随着激光通量的增加而增加，尽管这种影响在纳秒脉冲中更为明显。干燥的芥子酸和α-氰基-4-羟基肉桂酸液滴样品在较长的激光脉冲下也显示出较高的碎片化程度。人们认为，不同的光泵浦速率和弛豫通道是造成基质分子离子产率显着不同的原因。对于较小的分析物（≤2000 Da），两个激光都可以轻松获取分子离子信号，而对于较大的分子（≥5000 Da），皮秒激光的分析物离子产率较低甚至消失。纳秒激光以可比的产率从大小分析物产生离子。这些观察结果与以下假设是一致的，即较长的激光脉冲由于激光脉冲和MALDI羽流的扩展相互作用而可以依次解吸和电离分析物分子，而较短的脉冲只能解吸分析物分子，而不能促进其电离。其与羽流的有限时间重叠。

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