Chemical imaging of organic materials with MeV SIMS using a continuous collimated ion beam

MeV SIMS is a type of secondary ion mass spectrometry technique (SIMS) where molecules are desorbed from the sample surface with ions of MeV energies. In this work, we present a novel system for molecular imaging of organic materials using a continuous analytical beam and a start trigger for timing based on the detection of secondary electrons. The sample is imaged by the collimated primary ion beam and scanning of the target with a lateral resolution of ~ 20 µm. The mass of the analysed molecules is determined with a reflectron type time-of-flight (TOF) analyser, where the START signal for the TOF measurement is generated by the secondary electrons emitted from the thin carbon foil (~ 5 nm) placed over the beam collimator. With this new configuration of the MeV SIMS setup, a primary ion beam with the highest possible electronic stopping can be used (i.e., highest secondary molecular yield), and samples of any thickness can be analysed. Since the electrons are collected from the thin foil rather than from the sample surface, the detection efficiency of secondary electrons is always the same for any type of analysed material. Due to the ability to scan the samples by piezo stage, samples of a few cm in surface size can be imaged. The imaging capabilities of MeV SIMS are demonstrated on crossing ink lines deposited on the paper, a thin section of a mouse brain and on a fingerprint deposited on a thick Si wafer to show the potential application of the presented technique for analytical purposes in biology and forensics science.兆电子伏级二次离子质谱（MeV SIMS）是二次离子质谱（SIMS）的一类技术，其通过兆电子伏能量的离子从样品表面解吸分子。本研究提出了一套全新的有机材料分子成像系统，该系统采用连续式分析离子束，并基于二次电子检测生成计时起始触发信号。样品通过准直后的一次离子束以及对靶标的扫描完成成像，横向分辨率约为20 µm。待分析分子的质量通过反射式飞行时间（TOF）分析仪进行测定，飞行时间测量的起始信号由置于束流准直器上方的约5纳米（~5 nm）薄碳箔发射的二次电子产生。通过该MeV SIMS装置的全新配置，可使用电子阻止本领尽可能高的一次离子束（即获得最高的二次分子产率），且可对任意厚度的样品进行分析。由于二次电子采集自薄碳箔而非样品表面，因此无论待分析材料的类型如何，二次电子的检测效率始终保持一致。借助压电样品台的扫描能力，可对表面尺寸达数厘米的样品进行成像。本研究通过沉积于纸张上的交叉墨线、小鼠脑组织薄切片以及沉积于厚硅片（Si wafer）上的指纹三组样品，验证了MeV SIMS的成像能力，以此展示该技术在生物学与法医学分析领域的应用潜力。