Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080, USA.
J Am Soc Mass Spectrom. 2013 Aug;24(8):1288-95. doi: 10.1007/s13361-013-0666-1. Epub 2013 May 31.
The role of the ionic liquid (IL) anion structure on analyte signal enhancements has been systematically investigated in secondary ion mass spectrometry (SIMS) using a variety of samples, including lipids, sterols, polymers, and peptides. Twenty-four ILs were synthesized. The 12 matrix acids were cinnamic acid derivatives. Two bases were employed: 1-methylimidazole and tripropylamine. Three matrices, methylimmidazolium o-coumarate, tripropylammonium o-coumarate, and tripropylammonium 3,4,5-trimethoxycinnamate, were "universal" matrices enhancing all analytes tested. The pKa of the matrix acid does not appear to have a strong effect on analyte ion intensities. Rather, it is observed that a single hydroxyl group on the anion aromatic ring leads to significantly increased molecular ion intensities. No analyte signal enhancements were observed for -CH3, -CF3 and -OCH3 groups present on the aromatic ring. The position of the -OH group on the aromatic ring also alters molecular ion intensity enhancements. As well as the chemical identity and position of substituents, the number of moieties on the aromatic ring may affect the analyte signal enhancements observed. These observations suggest that the activation of the IL anion aromatic ring is important for optimizing analyte signal intensities. The implications for SIMS imaging of complex structures, such as biological samples, are discussed.
本文系统研究了离子液体（IL）阴离子结构在二次离子质谱（SIMS）中对分析物信号增强的作用，使用了多种样品，包括脂质、甾醇、聚合物和肽。合成了 24 种 IL。12 种基质酸为肉桂酸衍生物。使用了两种碱：1-甲基咪唑和三丙胺。三种基质为甲基咪唑邻香豆酸盐、三丙基铵邻香豆酸盐和三丙基铵 3,4,5-三甲氧基肉桂酸盐，它们是增强所有测试分析物的“通用”基质。基质酸的 pKa 似乎对分析物离子强度没有强烈影响。相反，观察到阴离子芳环上的单个羟基基团会导致分子离子强度显著增加。芳环上存在-CH3、-CF3 和-OCH3 基团时，没有观察到分析物信号增强。芳环上-OH 基团的位置也会改变分子离子强度增强。除了取代基的化学性质和位置外，芳环上的部分数量也可能影响观察到的分析物信号增强。这些观察结果表明，激活 IL 阴离子芳环对于优化分析物信号强度很重要。本文还讨论了这些结果对复杂结构（如生物样品）的 SIMS 成像的影响。
