RATIONALE Hyphenation of atmospheric pressure chemical ionization (APCI) mass spectrometry with capillary and micro high-performance liquid chromatography (HPLC) is attractive for many applications, but reliable ion sources dedicated to these conditions are still missing. There are a number of aspects to consider when designing such an ion source, including the susceptibility of the ionization processes to ambient conditions. Here we discuss the importance of ion source housing for APCI at low flow rates. METHODS Selected compounds dissolved in various solvents were used to study ionization reactions at 10 μL/min flow rate. APCI spectra were generated using the Ion Max-S source (Thermo Fisher Scientific) operated with or without the ion source housing. RESULTS The APCI spectra of most compounds measured in the open and enclosed ion sources were markedly different. The differences were explained by water and oxygen molecules that entered the plasma region of the open ion source. Water tended to suppress charge transfer processes while oxygen diminished electron capture reactions and prevented the formation of acetonitrile-related radical cations useful for localizing double bonds in lipids. The effects associated with the ion source housing were significantly less important for compounds that are easy to protonate or deprotonate. CONCLUSIONS The use of ion source housing prevented alternative ionization channels leading to unwanted or unexpected ions. Compared with the conventional flow rate mode (1 mL/min), the effects of ambient air components were significantly higher at 10 μL/min, emphasizing the need for ion source housing in APCI sources dedicated to low flow rates.

中文翻译：

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低流速下的大气压化学电离质谱：离子源外壳的重要性。

原理毛细管化学和微型高效液相色谱（HPLC）联用的大气压化学电离（APCI）质谱联用技术在许多应用中具有吸引力，但仍缺乏专门用于这些条件的可靠离子源。设计此类离子源时，需要考虑多个方面，包括电离过程对环境条件的敏感性。在这里，我们讨论了低流速下APCI离子源外壳的重要性。方法使用溶解在各种溶剂中的选定化合物，以10μL/ min的流速研究电离反应。使用带或不带离子源外壳的Ion Max-S离子源（Thermo Fisher Scientific）生成APCI光谱。结果在打开和关闭的离子源中测得的大多数化合物的APCI光谱均存在明显差异。差异由进入开放离子源等离子体区域的水和氧分子解释。水往往会抑制电荷转移过程，而氧气会减少电子捕获反应，并阻止形成乙腈相关的自由基阳离子，这些阳离子可用于脂质中的双键定位。对于易于质子化或去质子化的化合物，与离子源外壳相关的作用不那么重要。结论离子源外壳的使用可防止产生其他离子化通道，从而导致有害离子或意外离子。与传统流速模式（1 mL / min）相比，在10μL/ min时，环境空气成分的影响明显更高，