In beam-based ionization methods, the substrate plays an important role on the desorption mechanism of molecules from surfaces. Both the specific orientation that a molecule adopts at a surface and the strength of the molecule-surface interaction can greatly influence desorption processes, which in turn will affect the ion yield and the degree of in-source fragmentation of a molecule. In the beam-based method of secondary ion mass spectrometry (SIMS), in-source fragmentation can be significant and molecule specific due to the hard ionization method of using a primary ion beam for molecule desorption. To investigate the role of the substrate on orientation and in-source fragmentation, we have used atomistic simulations—molecular dynamics in combination with density functional theory calculations—to explore the desorption of a sphingolipid (palmitoylsphingomyelin) from a model surface (gold). We then compare SIMS data from this model system to our modeling findings. Using this approach, we found that the combined adsorption and binding energy of certain bonds associated with the headgroup fragments (C₃H₈N⁺, C₅H₁₂N⁺, C₅H₁₄NO⁺, and C₅H₁₅PNO₄⁺) was a good predictor for fragment intensities (as indicated by relative ion yields). This is the first example where atomistic simulations have been applied in beam-based ionization of lipids, and it presents a new approach to study biointerfacial lipid ordering effects on SIMS imaging.

中文翻译：

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二次离子质谱中底物对脂质源内碎裂影响研究的原子模拟

在基于束的电离方法中，基底对分子从表面的解吸机制起着重要作用。分子在表面采用的特定方向和分子-表面相互作用的强度都会极大地影响解吸过程，这反过来又会影响离子产率和分子的源内碎裂程度。在二次离子质谱 (SIMS) 的基于束的方法中，由于使用一次离子束进行分子解吸的硬电离方法，源内碎裂可能是显着的和分子特异性的。为了研究底物对方向和源内碎片的作用，我们使用原子模拟——分子动力学结合密度泛函理论计算——来探索模型表面（金）上鞘脂（棕榈酰鞘磷脂）的解吸。然后，我们将来自该模型系统的 SIMS 数据与我们的建模结果进行比较。使用这种方法，我们发现与头基片段相关的某些键的结合吸附和结合能 (C₃ H ₈ N ⁺、C ₅ H ₁₂ N ⁺、C ₅ H ₁₄ NO ⁺和C ₅ H ₁₅ PNO ₄ ⁺）是碎片强度的良好预测因子（如相对离子产率所示）。这是原子模拟应用于基于束的脂质电离的第一个例子，它提出了一种研究生物界面脂质有序对 SIMS 成像影响的新方法。