Sheraz née Rabbani Sadia, Razo Irma Berrueta, Kohn Taylor, Lockyer Nicholas P, Vickerman John C
Manchester Institute of Biotechnology, The University of Manchester , Manchester, U.K.
Anal Chem. 2015 Feb 17;87(4):2367-74. doi: 10.1021/ac504191m. Epub 2015 Jan 27.
Following from our previous Letter on this topic, this Article reports a detailed study of time-of-flight-secondary ion mass spectrometry (TOF-SIMS) positive ion spectra generated from a set of model biocompounds (arginine, trehalose, DPPC, and angiotensin II) by water cluster primary ion beams in comparison to argon cluster beams over a range of cluster sizes and energies. Sputter yield studies using argon and water beams on arginine and Irganox 1010 have confirmed that the sputter yields using water cluster beams lie on the same universal sputtering curve derived by Seah for argon cluster beams. Thus, increased ion yield using water cluster beams must arise from increased ionization. The spectra and positive ion signals observed using cluster beams in the size range from 1,000 to 10,000 and the energy range 5-20 keV are reported. It is confirmed that water cluster beams enhance proton related ionization over against argon beams to a significant degree such that enhanced detection sensitivities from 1 μm(2) in the region of 100 to 1,000 times relative to static SIMS analysis with Ar2000 cluster beams appear to be accessible. These new studies show that there is an unexpected complexity in the ionization enhancement phenomenon. Whereas optimum ion yields under argon cluster bombardment occur in the region of E/n ≥ 10 eV (where E is the beam energy and n the number of argon atoms in the cluster) and fall rapidly when E/n < 10 eV; for water cluster beams, ion yields increase significantly in this E/n regime (where n is the number of water molecules in the cluster) and peak for 20 keV beams at a cluster size of 7,000 or E/n ∼3 eV. This important result is explored further using D2O cluster beams that confirm that in this low E/n regime protonation does originate to a large extent from the water molecules. The results, encouraging in themselves, suggest that for both argon and water cluster beams, higher energy beams, e.g., 40 and 80 keV, would enable larger cluster sizes to be exploited with significant benefit for ion yield and hence analytical capability.
继我们之前关于该主题的信函之后，本文报告了一项详细研究，该研究对比了在一系列团簇尺寸和能量条件下，水团簇初级离子束与氩团簇束作用于一组模型生物化合物（精氨酸、海藻糖、二棕榈酰磷脂酰胆碱和血管紧张素 II）所产生的飞行时间二次离子质谱（TOF-SIMS）正离子谱。使用氩束和水束对精氨酸和抗氧剂 1010 进行的溅射产率研究证实，使用水团簇束的溅射产率位于 Seah 为氩团簇束推导的同一条通用溅射曲线上。因此，使用水团簇束时离子产率的增加必定源于电离的增加。报告了在 1000 至 10000 的尺寸范围和 5 - 20 keV 的能量范围内使用团簇束观察到的谱图和正离子信号。已证实，相对于氩束，水团簇束在很大程度上增强了与质子相关的电离，以至于相对于使用 Ar2000 团簇束的静态 SIMS 分析，在 100 至 1000 倍的范围内，从 1 μm²区域可获得增强的检测灵敏度。这些新研究表明，电离增强现象存在意想不到的复杂性。在氩团簇轰击下，最佳离子产率出现在 E/n ≥ 10 eV 的区域（其中 E 是束能量，n 是团簇中氩原子的数量），当 E/n < 10 eV 时迅速下降；对于水团簇束，离子产率在该 E/n 区域（其中 n 是团簇中水分子的数量）显著增加，并在 20 keV 束、团簇尺寸为 7000 或 E/n ∼3 eV 时达到峰值。使用 D2O 团簇束进一步探究了这一重要结果，证实了在这个低 E/n 区域，质子化在很大程度上确实源于水分子。这些结果本身令人鼓舞，表明对于氩团簇束和水团簇束而言，更高能量的束，例如 40 和 80 keV，将能够利用更大的团簇尺寸，对离子产率以及分析能力有显著益处。
