A general method for in situ measurements of the energy of individual ions trapped and weighed using charge detection mass spectrometry (CDMS) is described. Highly charged (> 300 e), individual polyethylene glycol (PEG) ions are trapped and oscillate within an electrostatic trap, producing a time domain signal. A segmented Fourier transform (FT) of this signal yields the temporal evolution of the fundamental and harmonic frequencies of ion motion throughout the 500-ms trap time. The ratio of the fundamental frequency and second harmonic (HAR) depends on the ion energy, which is an essential parameter for measuring ion mass in CDMS. This relationship is calibrated using simulated ion signals, and the calibration is compared to the HAR values measured for PEG ion signals where the ion energy was also determined using an independent method that requires that the ions be highly charged (> 300 e). The mean error of 0.6% between the two measurements indicates that the HAR method is an accurate means of ion energy determination that does not depend on ion size or charge. The HAR is determined dynamically over the entire trapping period, making it possible to observe the change in ion energy that takes place as solvent evaporates from the ion and collisions with background gas occur. This method makes it possible to measure mass changes, either from solvent evaporation or from molecular fragmentation (MSⁿ), as well as the cross sections of ions measured using CDMS.

Open image in new window

描述了使用电荷检测质谱（CDMS）原位测量捕获和称重的单个离子的能量的一般方法。充满电（> 300 e），单个聚乙二醇（PEG）离子被捕获并在静电阱内振荡，从而产生时域信号。该信号的分段傅立叶变换（FT）会在整个500毫秒的捕获时间内产生离子运动的基波和谐波频率的时间演化。基频与二次谐波（HAR）之比取决于离子能量，离子能量是测量CDMS中离子质量的重要参数。使用模拟离子信号对这种关系进行校准，并将该校准与针对PEG离子信号测得的HAR值进行比较，其中还使用一种要求离子带高电荷（> 300 e）。两次测量之间的平均误差为0.6％，这表明HAR方法是一种准确的离子能量测定方法，与离子大小或电荷无关。HAR是在整个捕集期间内动态确定的，因此可以观察到随着溶剂从离子中蒸发以及与背景气体发生碰撞而发生的离子能量变化。这种方法可以测量由于溶剂蒸发或分子碎裂（MS ⁿ）引起的质量变化，以及使用CDMS测量的离子截面。

在新窗口中打开图像