The laser diode thermal desorption (LDTD) ionization source allows ultrafast and sensitive analysis of small molecules by mass spectrometry. Signal enhancement in LDTD has been observed when coating the surface of sample microwells with a solution of ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid. Here we present a quantitative analysis of signal enhancement using solutions of diverse commercial proteins (lysozyme, immunoglobulin G, albumin, and fibrinogen) as coatings. Results showed that compounds with polar chemical functions such as carboxylic acid, sulfonyl, and nitro had signal enhancement factors, in most cases higher than 10, when using any of the tested proteins as coating agent. Analysis of variance revealed that immunoglobulin G and fibrinogen gave the best results. However, the signal enhancement factors obtained with these proteins were not superior to those observed with EDTA. To explain the signal enhancement effect of proteins, analysis by scanning electron microscopy of dried samples on the microwell sample plates was carried out. Images showed that salicylic acid, one of the compounds with the highest observed signal enhancement, formed a thick layer when applied directly on the uncoated surface, but it formed small crystals (<1 μm) in the presence of protein or EDTA coatings. Further crystallographic studies using powder X-ray diffraction showed that the crystalline form of salicylic acid is modified in the presence of EDTA. Salicylic acid when mixed with EDTA had a higher percentage of amorphous phase (38.1%) than without EDTA (23.1%). These results appear to confirm that the diminution of crystal size of analytes and the increase of amorphous phase are implicated in signal enhancement effect observed in LDTD using microwell surface coatings. To design better coatings and completely elucidate the signal enhancement effect in LDTD, more studies are necessary to understand the effects of coatings on the ionization of analytes.

中文翻译：

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使用微孔表面涂层的激光二极管热解吸三重四极杆质谱中信号增强效应的进一步研究。

激光二极管热脱附（LDTD）电离源可通过质谱法对小分子进行超快速灵敏的分析。用乙二胺四乙酸（EDTA）或次氮基三乙酸溶液涂覆样品微孔表面时，观察到LDTD中的信号增强。在这里，我们介绍了使用多种商业蛋白质（溶菌酶，免疫球蛋白G，白蛋白和纤维蛋白原）的溶液作为涂层对信号增强的定量分析。结果表明，当使用任何一种测试蛋白作为包被剂时，具有极性化学功能的化合物（例如羧酸，磺酰基和硝基）具有信号增强因子，在大多数情况下高于10。方差分析表明，免疫球蛋白G和纤维蛋白原的效果最好。然而，这些蛋白质获得的信号增强因子并不优于EDTA所观察到的。为了解释蛋白质的信号增强作用，在微孔样品板上通过扫描电子显微镜对干燥样品进行了分析。图像显示，水杨酸是观察到的信号增强效果最高的化合物之一，当直接施用于未涂层的表面时会形成厚层，但在存在蛋白质或EDTA涂层的情况下会形成小晶体（<1μm）。使用粉末X射线衍射的进一步晶体学研究表明，水杨酸的晶体形式在EDTA存在下被修饰。与EDTA混合时，水杨酸的无定形相百分比（38.1％）比没有EDTA时更高（23.1％）。这些结果似乎证实了分析物晶体尺寸的减小和非晶相的增加与使用微孔表面涂层在LDTD中观察到的信号增强效应有关。为了设计更好的涂层并完全阐明LDTD中的信号增强效果，需要更多的研究来了解涂层对分析物电离的影响。