Shvartsburg Alexandre A, Li Fumin, Tang Keqi, Smith Richard D
Biological Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA.
Anal Chem. 2007 Feb 15;79(4):1523-8. doi: 10.1021/ac061306c.
Field asymmetric waveform ion mobility spectrometry (FAIMS) is emerging as a major analytical tool, especially in conjunction with mass spectrometry (MS), conventional ion mobility spectrometry (IMS), or both. In particular, FAIMS is used to separate protein or peptide conformers prior to characterization by IMS, MS/MS, or H/D exchange. High electric fields in FAIMS induce ion heating, previously estimated at <10 degrees C on average and believed too weak to affect ion geometries. Here we use a FAIMS/IMS/MS system to compare the IMS spectra for ESI-generated ubiquitin ions that have and have not passed FAIMS and find that some unfolding occurs for most charge states. These data and their comparison with the reported protein unfolding in a Paul trap imply that at least some structural transitions observed in FAIMS, or previously in an ion trap, are not spontaneous. The observed unfolding is similar to that produced by heating of approximately 50 degrees C above room temperature, consistent with the calculated heating of ions at FAIMS waveform peaks. Hence, the ion isomerization in FAIMS likely proceeds in steps during the "hot" periods, especially right after entering the device. The process distorts ion geometries and causes ion losses by a "self-cleaning" mechanism and thus should be suppressed as much as possible. We propose achieving that via cooling FAIMS by the amount of ion heating; in most cases, cooling by approximately 75 degrees C should suffice.
场不对称波形离子迁移谱（FAIMS）正在成为一种主要的分析工具，尤其是与质谱（MS）、传统离子迁移谱（IMS）联用，或同时与两者联用。特别地，FAIMS用于在通过IMS、MS/MS或H/D交换进行表征之前分离蛋白质或肽构象异构体。FAIMS中的高电场会导致离子加热，此前估计平均升温<10摄氏度，并且认为该升温太弱以至于不会影响离子几何形状。在此，我们使用FAIMS/IMS/MS系统比较了经过和未经过FAIMS的电喷雾电离（ESI）产生的泛素离子的IMS谱图，发现大多数电荷态都会发生一些去折叠现象。这些数据以及它们与在保罗阱中报道的蛋白质去折叠情况的比较表明，在FAIMS中观察到的至少一些结构转变，或者此前在离子阱中观察到的结构转变，并非自发产生。观察到的去折叠现象类似于在室温以上大约50摄氏度加热所产生的情况，这与在FAIMS波形峰值处计算得到的离子加热情况一致。因此，FAIMS中的离子异构化可能在“热”时段分步进行，尤其是在刚进入仪器之后。该过程会扭曲离子几何形状，并通过“自清洁”机制导致离子损失，因此应尽可能加以抑制。我们建议通过将FAIMS冷却至离子加热的程度来实现这一点；在大多数情况下，冷却大约75摄氏度应该就足够了。
