Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States.
Anal Chem. 2020 Jul 7;92(13):8883-8892. doi: 10.1021/acs.analchem.0c00681. Epub 2020 Jun 9.
Chemical characterization of complex mixtures of large saturated hydrocarbons is critically important for numerous fields, including petroleomics and renewable transportation fuels, but difficult to achieve. Atmospheric pressure chemical ionization (APCI) mass spectrometry has shown some promise in the analysis of saturated hydrocarbons. However, APCI causes extensive fragmentation to these compounds, which impedes its effectiveness. To prevent this fragmentation, its causes were examined via gas-phase ion-molecule reactions in vacuum in a linear quadrupole ion trap mass spectrometer. The results demonstrate that the mechanism proposed previously for ionization of saturated hydrocarbons upon APCI, hydride abstraction by carbocation reagent ions, is not correct. Instead, the fragmentation is caused by ionization of saturated hydrocarbons via exothermic proton-transfer reactions involving highly acidic, protonated atmospheric molecules, such as nitrogen and water. Accordingly, the extent of fragmentation was found to correlate with the proton affinities of the atmospheric molecules studied. Remarkably, controlled experiments involving isolated atmospheric ions and neat saturated hydrocarbons in vacuum yielded almost identical mass spectra as APCI involving atmospheric pressure conditions, the presence of many different chemicals, and an electrical discharge. In order to prevent or reduce the extent of fragmentation of saturated hydrocarbons upon APCI, and therefore enable accurate mass spectrometric characterization of complex mixtures of saturated hydrocarbons, the ion source should be purged of air to remove nitrogen and water and fill it with an inert gas with a substantially lower proton affinity.
复杂的大饱和烃混合物的化学特征对于包括石油学和可再生运输燃料在内的众多领域都至关重要，但却难以实现。大气压化学电离（APCI）质谱在分析饱和烃方面显示出了一些潜力。然而，APCI 会导致这些化合物广泛断裂，从而影响其效果。为了防止这种断裂，我们在直线四极离子阱质谱仪中通过真空气相离子-分子反应来检查其原因。结果表明，以前提出的关于 APCI 中饱和烃的电离机制，即碳正离子试剂离子夺取氢化物，是不正确的。相反，这种断裂是由饱和烃通过涉及高度酸性、质子化大气分子（如氮和水）的放热质子转移反应而引起的。因此，断裂程度与所研究的大气分子的质子亲和力相关。值得注意的是，在真空条件下进行的涉及分离的大气离子和纯饱和烃的对照实验与涉及大气压条件、许多不同化学物质和放电的 APCI 产生了几乎相同的质谱，在 APCI 中，氮气和水的存在会导致饱和烃的断裂程度大大增加。为了防止或减少 APCI 中饱和烃的断裂程度，从而能够准确地对饱和烃的复杂混合物进行质谱分析，离子源应被吹扫以去除氮气和水，并填充具有低得多的质子亲和力的惰性气体。
