Kaiser Nathan K, Bruce James E
Department of Chemistry, Washington State University, Pullman Washington 99164-4630, USA.
Anal Chem. 2005 Sep 15;77(18):5973-81. doi: 10.1021/ac050606b.
Ion motion in Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) is complex and the subject of ongoing theoretical and experimental studies. Two predominant pathways for the loss of ICR signals are thought to include damping of cyclotron motion, in which ions lose kinetic energy and radially damp toward the center of the ICR cell, and dephasing of ion coherence, in which ions of like cyclotron frequency become distributed out of phase at similar cyclotron radii. Both mechanisms result in the loss of induced ion image current in FTICR-MS measurements and are normally inseparable during time-domain signal analysis. For conventional ICR measurements which take advantage of ion ensembles, maximization of the ion population size and density can produce the desired effect of increasing phase coherence of ions during cyclotron motion. However, this approach also presents the risk of coalescence of ion packets of similar frequencies. In general, ICR researchers in the past have lacked the tools necessary to distinguish or independently control dephasing and damping mechanisms for ICR signal loss. Nonetheless, the ability to impart greater phase coherence of ions in ICR measurements will allow significant advances in FTICR-MS research by improving the current understanding of ICR signal loss contributions of dephasing and damping of ion ensembles, increasing overall time-domain signal length, and possibly, resulting in more routine ultrahigh resolution measurements. The results presented here demonstrate the ability to employ a high density electron beam to perturb electric fields within the ICR cell during detection of cyclotron motion, in an approach we call electron-promoted ion coherence (EPIC). As such, EPIC reduces ICR signal degradation through loss of phase coherence, and much longer time-domain signals can be obtained. Our results demonstrate that time-domain signals can be extended by more than a factor of 4 with the implementation of EPIC, as compared to conventional experiments with otherwise identical conditions. The application of EPIC has also been observed to reduce the appearance of peak coalescence. These capabilities are not yet fully optimized nor fully understood in terms of the complex physics that underlies the enhancement. However, the enhanced time-domain signals can result in improved resolution in frequency-domain signals, and as such, this result is important for more efficient utilization of FTICR-MS. High resolution and accurate mass analysis are prime motivating factors in the application of advanced FTICR technology. We believe the approach presented here and derivatives from it may have significant benefit in future applications of advanced FTICR technology.
在傅里叶变换离子回旋共振质谱（FTICR-MS）中，离子运动十分复杂，一直是理论和实验研究的主题。ICR信号损失的两个主要途径被认为包括回旋运动的阻尼，即离子失去动能并径向朝着ICR池中心阻尼，以及离子相干性的失相，即具有相同回旋频率的离子在相似的回旋半径处变得不同相分布。这两种机制都会导致FTICR-MS测量中感应离子镜像电流的损失，并且在时域信号分析过程中通常是不可分割的。对于利用离子团的传统ICR测量，离子数量大小和密度的最大化可以产生在回旋运动期间增加离子相位相干性的预期效果。然而，这种方法也存在相似频率离子包合并的风险。一般来说，过去的ICR研究人员缺乏区分或独立控制ICR信号损失的失相和阻尼机制所需的工具。尽管如此，在ICR测量中赋予离子更大的相位相干性的能力将通过改善目前对离子团失相和阻尼对ICR信号损失贡献的理解、增加总时域信号长度以及可能实现更常规的超高分辨率测量，从而在FTICR-MS研究中取得重大进展。此处展示的结果证明了在检测回旋运动期间利用高密度电子束扰动ICR池内电场的能力，我们将这种方法称为电子促进离子相干（EPIC）。因此，EPIC通过减少相位相干性的损失来降低ICR信号降解，并且可以获得长得多的时域信号。我们的结果表明，与在其他条件相同的传统实验相比，实施EPIC后时域信号可以延长四倍以上。还观察到EPIC的应用减少了峰合并的出现。就增强背后的复杂物理而言，这些能力尚未得到充分优化或完全理解。然而，增强的时域信号可以导致频域信号分辨率的提高，因此，这一结果对于更有效地利用FTICR-MS很重要。高分辨率和精确质量分析是先进FTICR技术应用的主要推动因素。我们相信此处提出的方法及其衍生方法可能会在先进FTICR技术的未来应用中带来显著益处。
