A recently introduced technique based on MALDI with laser-induced postionization (PI), also named MALDI-2, increases the ion yields for numerous classes of lipids, metabolites, and carbohydrates in MALDI-MS imaging experiments under certain experimental conditions. Here, we used a semiautomatic LabVIEW-based protocol to investigate and optimize the efficiency of the PI process dependent on four relevant input parameters and a dense parameter grid: pulse energies of the two lasers, delay between the laser pulses, and buffer gas pressure in the ion source. All experiments were conducted with a modified MALDI-2 Synapt G2-S mass spectrometer (Waters) and use of a focal spot size on the sample of 15-17 μm. A wavelength-tunable optical parametric oscillator (OPO) laser served for PI at 260 or 280 nm. The investigated MALDI matrices were: 2,5-dihydroxybenzoic acid (positive ion mode, +), 2,5-dihydroxyacetophenone (+), α-cyano-4-hydroxycinnamic acid (+), norharmane (negative-ion mode, -), and 1,5-diaminonapthalene (-). A porcine brain extract served as lipid standard. In the positive-ion mode, a maximum boost for the generated [M + H]+ species was found with a N2 buffer gas pressure of ∼2 mbar and a delay between the laser emissions of ∼10 μs. Higher optimal delay settings of several 10 μs were registered for the two studied matrices in negative-ion mode. With regard to the laser fluences, best PI efficiencies were reached using maximum available ablation and PI laser pulse energies of up to 25 and 160 μJ, respectively. For analytes not profiting from MALDI-2, best ion signal yields were recorded for ablation laser pulse energies of around 7 μJ, depending on the MALDI matrix. At higher laser pulse energies, sizable fragmentation is observed for these ions. The PI laser pulse energy did not have any influence on the ion signals of these species. For optimal ion yield of all analyte species, best results were obtained with an ablation laser pulse energy of ∼7 μJ and a PI laser pulse energy of ∼160 μJ. Our comprehensive data set provides valuable insight into the mechanisms underlying the MALDI-2 processes and could help to further optimize this emerging technique.

中文翻译：

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MALDI-2中离子化效率作为相关输入参数的函数的详细表征。

最近引入的基于MALDI和激光诱导的去离子化（PI）的技术，也称为MALDI-2，在某些实验条件下，可以提高MALDI-MS成像实验中多种类脂，代谢物和碳水化合物的离子产率。在这里，我们使用基于LabVIEW的半自动协议来研究和优化PI过程的效率，该过程取决于四个相关的输入参数和密集的参数网格：两个激光的脉冲能量，两个激光脉冲之间的延迟以及缓冲气体压力。离子源。所有实验均使用改良的MALDI-2 Synapt G2-S质谱仪（Waters）进行，并且样品上的焦点尺寸为15-17μm。波长可调光学参量振荡器（OPO）激光器用于260或280 nm的PI。研究的MALDI矩阵为：2，5-二羟基苯甲酸（正离子模式，+），2,5-二羟基苯乙酮（+），α-氰基-4-羟基肉桂酸（+），去甲烷（负离子模式，-）和1,5-二氨基萘（ -）。猪脑提取物用作脂质标准。在阳离子模式下，发现N_2缓冲气体压力约为2 mbar，激光发射之间的延迟约为10μs，从而最大程度地增强了所产生的[M + H] +物质。在负离子模式下，两个研究矩阵的最佳最佳延迟设置约为10μs。关于激光通量，使用最大可用消融和分别高达25和160μJ的PI激光脉冲能量可达到最佳PI效率。对于不从MALDI-2获利的分析物，根据MALDI基质，对于约7μJ的消融激光脉冲能量，记录了最佳的离子信号产率。在较高的激光脉冲能量下，这些离子观察到相当大的碎片。PI激光脉冲能量对这些物质的离子信号没有任何影响。为了获得所有分析物种类的最佳离子产率，使用约7μJ的消融激光脉冲能量和约160μJ的PI激光脉冲能量可获得最佳结果。我们全面的数据集提供了对MALDI-2程序基础机制的宝贵见解，并可能有助于进一步优化这一新兴技术。