Imaging of Organic Samples with Megaelectron Volt Time-of-Flight Secondary Ion Mass Spectrometry Capillary Microprobe

Time-of-flight Secondary Ion Mass Spectrometry (TOF SIMS) with MeV primary ions offers a fine balance between secondary ion yield for molecules in the mass range from 100 to 1000 Da and beam spot size, both of which are critical for imaging applications of organic samples. Using conically shaped glass capillaries with an exit diameter of a few micrometers, a high energy heavy primary beam can be collimated to less than 10 μm. In this work, imaging capabilities of such a setup are presented for some organic samples (leucine-evaporated mesh, fly wing section, ink deposited on paper). Lateral resolution measurement and molecular distributions of selected mass peaks are shown. The negative influence of the beam halo, an unavoidable characteristic of primary beam collimation with a conical capillary, is also discussed. A new start trigger for TOF measurements based on the detection of secondary electrons released by the primary ion is presented. This method is applicable for a continuous primary ion beam, and for thick targets that are not transparent to the primary ion beam. The solution preserves the good mass resolution of the thin target setup, where the detection of primary ions with a PIN diode is used for a start trigger, reduces the background, and enables a wide range of samples to be analyzed.采用MeV级一次离子的飞行时间二次离子质谱（Time-of-flight Secondary Ion Mass Spectrometry, TOF SIMS），可在100至1000 Da质量范围的分子二次离子产率与束斑尺寸之间实现极佳平衡，二者均为有机样品成像应用的关键指标。借助出口直径为数微米的锥形玻璃毛细管，可将高能重一次离子束准直至10 μm以下。本研究展示了该装置针对三类有机样品的成像性能，分别为亮氨酸蒸发沉积网格、果蝇翅切片以及纸上沉积油墨。文中同时呈现了横向分辨率测试结果，以及选定质荷比峰的分子分布情况。此外，还讨论了锥形毛细管准直一次离子束时不可避免的束晕效应所带来的负面影响。本文提出了一种基于一次离子激发二次电子探测的飞行时间测量新起始触发方案。该方案适用于连续一次离子束，以及无法被一次离子束穿透的厚样品。该方法保留了采用PIN二极管探测一次离子作为起始触发的薄样品装置的优良质量分辨率，降低了背景噪声，并可实现对多种样品的分析。