Cristadoro Anna, Räder Hans Joachim, Müllen Klaus
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.
Rapid Commun Mass Spectrom. 2007;21(16):2621-8. doi: 10.1002/rcm.3134.
The desorption/ionization behaviour of polycyclic aromatic hydrocarbons (PAHs) in matrix-assisted laser desorption/ionization (MALDI) and laser desorption (LD) mass spectrometry was studied by the solvent-free sample preparation method. As the understanding of the desorption/ionization mechanism in MALDI is normally hampered by the different ionization and desorption efficiencies of the analytes, this work was focused on the analyses of a homologous series of four hexabenzocoronenes (HBCs) possessing virtually the same ionization efficiency: HBC parent, hexamethyl-hexabenzocoronene (HBC-C1), hexapropyl-hexabenzocoronene (HBC-C3) and hexakis(dodecyl)-hexabenzocoronene (HBC-C12). The different signal intensities obtained in their mass spectra can be related to differences in their desorption efficiencies, which are attributed to the different strengths of the intermolecular interactions between unsubstituted and alkylated HBCs in the solid state. The influence of the aromatic structure of PAHs on their photoionization/desorption probability was investigated. As a model system, an equimolar mixture composed of HBC-C12 and hexakis(dodecyl)-hexaphenylbenzene (HPB-C12) was chosen. The aromatic structures of both molecules and thus their absorption coefficients at the laser wavelength differ substantially and have a huge influence on their photoionization efficiency. The combined effect of laser light absorption and intermolecular interactions on the desorption/ionization behaviour of giant PAHs was further studied by using an equimolar mixture composed of a larger PAH (C(222)H(42)) and its dendritic precursor (C(222)H(150)). This mixture shows the opposite behaviour to that of the former example, because the balance between desorption and ionization efficiency has changed significantly. The present investigation should be of interest for providing a better understanding of MALDI and LD spectra obtained from natural PAH-containing samples, such as heavy oils, asphaltenes or pitches, for which our artificial mixtures represent suitable model systems.
采用无溶剂样品制备方法，研究了多环芳烃（PAHs）在基质辅助激光解吸/电离（MALDI）和激光解吸（LD）质谱中的解吸/电离行为。由于MALDI中解吸/电离机制的理解通常受到分析物不同电离和解吸效率的阻碍，这项工作聚焦于分析具有几乎相同电离效率的四个六苯并蔻（HBCs）同系物：HBC母体、六甲基 - 六苯并蔻（HBC - C1）、六丙基 - 六苯并蔻（HBC - C3）和六（十二烷基） - 六苯并蔻（HBC - C12）。在它们的质谱中获得的不同信号强度可能与它们的解吸效率差异有关，这归因于固态下未取代和烷基化HBC之间分子间相互作用强度的不同。研究了PAHs的芳香结构对其光电离/解吸概率的影响。作为模型系统，选择了由HBC - C12和六（十二烷基） - 六苯基苯（HPB - C12）组成的等摩尔混合物。两种分子的芳香结构以及因此它们在激光波长处的吸收系数有很大差异，并且对它们的光电离效率有巨大影响。通过使用由更大的PAH（C(222)H(42)）及其树枝状前体（C(222)H(150)）组成的等摩尔混合物，进一步研究了激光吸收和分子间相互作用对巨型PAHs解吸/电离行为的综合影响。这种混合物表现出与前一个例子相反的行为，因为解吸和电离效率之间的平衡发生了显著变化。本研究对于更好地理解从含天然PAH的样品（如重油、沥青质或沥青）获得的MALDI和LD光谱应该是有意义的，对于这些样品，我们的人工混合物代表了合适的模型系统。
