Department of Chemistry, Wayne State University, Detroit, MI 48202, USA.
J Am Soc Mass Spectrom. 2012 Oct;23(10):1644-60. doi: 10.1007/s13361-012-0414-y. Epub 2012 Jul 12.
Mechanistic arguments relative to matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) address observations that predominately singly charged ions are detected. However, recently a matrix assisted laser ablation method, laserspray ionization (LSI), was introduced that can use the same sample preparation and laser as MALDI, but produce highly charged ions from proteins. In MALDI, ions are generated from neutral molecules by the photon energy provided to a matrix, while in LSI ions are produced inside a heated inlet tube linking atmospheric pressure and the first vacuum region of the mass spectrometer. Some LSI matrices also produce highly charged ions with MALDI ion sources operated at intermediate pressure or high vacuum. The operational similarity of LSI to MALDI, and the large difference in charge states observed by these methods, provides information of fundamental importance to proposed ionization mechanisms for LSI and MALDI. Here, we present data suggesting that the prompt and delayed ionization reported for vacuum MALDI are both fast processes relative to producing highly charged ions by LSI. The energy supplied to produce these charged clusters/droplets as well as their size and time available for desolvation are determining factors in the charge states of the ions observed. Further, charged droplets/clusters may be a common link for ionization of nonvolatile compounds by a variety of MS ionization methods, including MALDI and LSI.
相对基质辅助激光解吸/电离（MALDI）质谱（MS）的机制论点解决了主要检测到单电荷离子的观察结果。然而，最近引入了一种基质辅助激光烧蚀方法，即激光喷雾电离（LSI），它可以使用与 MALDI 相同的样品制备和激光，但可以从蛋白质中产生高电荷离子。在 MALDI 中，离子是通过基质提供的光子能量从中性分子中产生的，而在 LSI 中，离子是在连接大气压和质谱仪第一真空区的加热入口管内产生的。一些 LSI 基质也可以在用中间压力或高真空操作的 MALDI 离子源中产生高电荷离子。LSI 与 MALDI 的操作相似，以及这些方法观察到的电荷状态的巨大差异，为 LSI 和 MALDI 的提出的电离机制提供了至关重要的信息。在这里，我们提供的数据表明，真空 MALDI 报道的快速和延迟电离都是相对于 LSI 产生高电荷离子的快速过程。产生这些带电团簇/液滴的能量以及它们的大小和用于去溶剂化的时间是观察到的离子电荷状态的决定因素。此外，带电液滴/团簇可能是通过各种 MS 电离方法（包括 MALDI 和 LSI）电离非挥发性化合物的共同联系。
