A new type of photoionization ion source was developed for the ionization of cold molecules in supersonic molecular beams (named Cold PI). The system was based on a GC–MS with supersonic molecular beams and its fly‐through EI of cold molecules ion source (Cold EI) plus quadrupole mass analyzer. A continuously operated deuterium VUV photoionization lamp was added and placed above and between the supersonic nozzle and skimmer whereas the Cold EI ion source served only as a portion of the ion transfer ion optics. The supersonic nozzle and skimmer were voltage biased and the VUV light crossed the supersonic expansion about 10 mm from the nozzle. We obtained over three orders of magnitude enhancement in the relative abundance of the molecular ion of squalane in Cold PI versus in photoionization of this compound as a thermal compound. Accordingly, we also proved that standard photoionization is not as soft ionization method as previously perceived for large compounds. We found that Cold PI is as soft as and possibly softer than field ionization; thus, it could be the softest known ionization method. The ionization yield was about 200–300 times weaker than with Cold EI yet our limit of detection was about 200 femtogram in SIM mode for cholesterol and pyrene which is reasonable. Practically, all hydrocarbons gave only molecular ions with rather uniform response whereas alcohols gave some molecular ions plus major fragment ions particularly with a loss of water (similarly to field ionization). We tested Cold PI in the GC–MS analysis of diesel fuels and analyzed the time averaged data for group type information. We also found that we can analyze the diesel fuels by fast under 20‐s flow injection analysis in which the generated averaged mass spectrum of molecular ions only could serve for the characterization of fuels.

中文翻译：

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GC-MS在超音速分子束中对冷分子进行电离-接近最软的电离方法。

开发了一种新型的光电离离子源，用于使超音速分子束中的冷分子电离（称为Cold PI）。该系统基于具有超音速分子束的GC-MS及其冷分子离子源（冷EI）和四极杆质量分析仪的飞越EI。添加了一个连续运行的氘VUV光电离灯，并将其放置在超声波喷嘴和分离器上方和之间，而Cold EI离子源仅用作离子转移离子光学系统的一部分。对超音速喷嘴和撇渣器施加电压偏置，VUV光穿过超音速膨胀管，距喷嘴约10 mm。与该化合物作为热化合物的光电离相比，我们在冷PI中角鲨烷分子离子的相对丰度提高了三个数量级以上。因此，我们还证明了标准的光电离方法不像以前对大型化合物所感知的那样软。我们发现，冷PI与场电离一样软，甚至比其软。因此，它可能是最软的已知电离方法。电离产率比Cold EI弱约200-300倍，但是我们在SIM模式下检测到的胆固醇和limit的检测极限约为200飞克，这是合理的。实际上，所有烃仅给出具有相当均匀响应的分子离子，而醇给出一些分子离子加上主要碎片离子，尤其是失水（类似于场电离）。我们在柴油的GC-MS分析中测试了Cold PI，并分析了时间平均数据以获取组类型信息。