Jertz Roland, Friedrich Jochen, Kriete Claudia, Nikolaev Evgeny N, Baykut Gökhan
Bruker Daltonik GmbH, Bremen, Germany.
J Am Soc Mass Spectrom. 2015 Aug;26(8):1349-66. doi: 10.1007/s13361-015-1148-4. Epub 2015 May 14.
In Fourier transform ion cyclotron resonance spectrometry (FT-ICR MS) the ion magnetron motion is not usually directly measured, yet its contribution to the performance of the FT-ICR cell is important. Its presence is manifested primarily by the appearance of even-numbered harmonics in the spectra. In this work, the relationship between the ion magnetron motion in the ICR cell and the intensities of the second harmonic signal and its sideband peak in the FT-ICR spectrum is studied. Ion motion simulations show that during a cyclotron motion excitation of ions which are offset to the cell axis, a position-dependent radial drift of the cyclotron center takes place. This radial drift can be directed outwards if the ion is initially offset towards one of the detection electrodes, or it can be directed inwards if the ion is initially offset towards one of the excitation electrodes. Consequently, a magnetron orbit diameter can increase or decrease during a resonant cyclotron excitation. A method has been developed to study this behavior of the magnetron motion by acquiring a series of FT-ICR spectra using varied post-capture delay (PCD) time intervals. PCD is the delay time after the capture of the ions in the cell before the cyclotron excitation of the ion is started. Plotting the relative intensity of the second harmonic sideband peak versus the PCD in each mass spectrum leads to an oscillating "PCD curve". The position and height of minima and maxima of this curve can be used to interpret the size and the position of the magnetron orbit. Ion motion simulations show that an off-axis magnetron orbit generates even-numbered harmonic peaks with sidebands at a distance of one magnetron frequency and multiples of it. This magnetron offset is due to a radial offset of the electric field axis versus the geometric cell axis. In this work, we also show how this offset of the radial electric field center can be corrected by applying appropriate DC correction voltages to the mantle electrodes of the ICR cell while observing the signals of the second harmonic peak group. The field correction leads to a definite performance increase in terms of resolving power and mass accuracy, and the mass spectrum contains intensity-minimized even-numbered harmonics. This is very important in the case of high performance cells, particularly the dynamically harmonized cell, since the magnetron motion can severely impair the averaging effect for dynamic harmonization and can therefore reduce the resolving power.
在傅里叶变换离子回旋共振光谱法（FT-ICR MS）中，离子磁控管运动通常不直接测量，但其对FT-ICR池性能的影响很重要。其存在主要通过光谱中偶数谐波的出现来体现。在这项工作中，研究了ICR池中离子磁控管运动与FT-ICR光谱中二次谐波信号及其边带峰强度之间的关系。离子运动模拟表明，在对偏离池轴的离子进行回旋运动激发期间，回旋中心会发生与位置相关的径向漂移。如果离子最初偏向其中一个检测电极，这种径向漂移可以向外；如果离子最初偏向其中一个激发电极，它可以向内。因此，在共振回旋激发期间，磁控管轨道直径可以增加或减小。已经开发出一种方法，通过使用不同的捕获后延迟（PCD）时间间隔获取一系列FT-ICR光谱来研究磁控管运动的这种行为。PCD是在池中捕获离子后到开始对离子进行回旋激发之前的延迟时间。绘制每个质谱中二次谐波边带峰的相对强度与PCD的关系图会得到一条振荡的“PCD曲线”。该曲线的最小值和最大值的位置和高度可用于解释磁控管轨道的大小和位置。离子运动模拟表明，离轴磁控管轨道会产生偶数谐波峰，其边带间距为一个磁控管频率及其倍数。这种磁控管偏移是由于电场轴相对于几何池轴的径向偏移。在这项工作中，我们还展示了如何在观察二次谐波峰组信号时，通过向ICR池的屏蔽电极施加适当的直流校正电压来校正径向电场中心的这种偏移。场校正导致在分辨率和质量精度方面性能有明显提高，并且质谱中偶数谐波的强度最小。这在高性能池的情况下非常重要，特别是动态谐调池，因为磁控管运动会严重损害动态谐调中的平均效果，从而降低分辨率。
