Electron ionization (EI) mass spectra of 46 compounds from several different compound classes were measured. Their molecular ion abundances were compared as obtained with 70‐eV EI, with low eV EI (such as 14 eV), and with EI mass spectra of vibrationally cold molecules in supersonic molecular beams (Cold EI). We further compared these mass spectra in their National Institute of Standards and Technology (NIST) library identification probabilities. We found that

1. Low eV EI is not a soft ionization method, and it has little or no influence on the molecular ion relative abundances for large molecules and those with weak or no molecular ions.
2. Low eV EI for compounds with abundant or dominant molecular ions in their 70 eV mass spectra results in the reduction of low mass fragment ions abundances thereby reducing their NIST library identification probabilities thus rarely justifies its use in real‐world applications.
3. Cold EI significantly enhances the relative abundance of the molecular ions particularly for large compounds; yet, it retains the low mass fragment ions; hence, Cold EI mass spectra can be effectively identified by the NIST library.
4. Different standard EI ion sources provide different 70 eV EI mass spectra. Among the Agilent technologies ion sources, the “Extractor” exhibits relatively abundant molecular ions compared with the “Inert” ion source, while the “High efficiency source” (HES) provides mass spectra with depleted molecular ions compared with the “Inert” ion source or NIST library mass spectra.

中文翻译：

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在70 eV，低电子能量和冷EI下获得的电子电离质谱的比较及其NIST库识别概率

测量了来自几种不同化合物类别的46种化合物的电子电离（EI）质谱。比较了它们的分子离子丰度，分别是70 eV EI，低eV EI（例如14 eV）和超音速分子束中冷态振动分子的EI质谱（冷EI）。我们在其美国国家标准技术研究院（NIST）库识别概率中进一步比较了这些质谱图。我们发现

1. 低eV EI不是一种软电离方法，它对大分子以及分子离子弱或无分子离子的分子离子相对丰度几乎没有影响。
2. 对于在其70 eV质谱图中具有大量或占主导地位的分子离子的化合物，低eV EI会降低低质量碎片离子的丰度，从而降低其NIST文库鉴定概率，因此很少有理由在现实应用中使用它。
3. 冷EI显着增强了分子离子的相对丰度，特别是对于大型化合物。然而，它保留了低质量碎片离子；因此，NIST库可有效识别冷EI质谱。
4. 不同的标准EI离子源提供不同的70 eV EI质谱。在安捷伦技术离子源中，与“惰性”离子源相比，“萃取器”显示出相对丰富的分子离子，而与“惰性”离子源相比，“高效离子源”（HES）提供了具有耗尽分子离子的质谱图或NIST库质谱图。