Chemistry Department, Indiana University, Bloomington, Indiana 47405 United States.
J Am Soc Mass Spectrom. 2020 Jun 3;31(6):1241-1248. doi: 10.1021/jasms.0c00081. Epub 2020 Apr 30.
Charge detection mass spectrometry (CDMS) depends on the measurement of the charge induced on a cylinder by individual ions by means of a charge-sensitive amplifier. For high-accuracy charge measurements, the detection cylinder is embedded in an electrostatic linear ion trap (ELIT), and the ions oscillate back and forth through the cylinder so that multiple measurements are made. To assign the charge state with a low error rate, the charge of each ion must be determined with an uncertainty (root-mean-square deviation) of around 0.2 elementary charges. We show here that high-accuracy charge measurements can be achieved for large ions by dynamic calibration of the charge measurement using an internal standard. The internal standard is generated by irradiating the detection cylinder, by means of a small antenna, with a radiofrequency signal. Using this approach, we have obtained a relative charge uncertainty of around 5 × 10, allowing charge-state resolution to be achieved for single ions with up to 500 charges. In another application of this approach, the detection cylinder is irradiated with a signal that counteracts the transients generated when the potentials on the ELIT end-caps are switched to trapping mode. Using this approach, the dead time after switching (during which the signal cannot be analyzed) has been reduced by more than an order of magnitude. With charge-state resolution for ions with up to 500 charges, we were able to calibrate the charges precisely. The results show that the response of the charge-sensitive amplifier with dynamic calibration is linear to within a small fraction of an elementary charge.
电荷检测质谱（CDMS）依赖于通过电荷灵敏放大器测量单个离子在圆柱上感应的电荷。为了进行高精度的电荷测量，检测圆柱被嵌入静电线性离子阱（ELIT）中，离子在圆柱中来回振荡，从而进行多次测量。为了以低错误率分配电荷状态，必须以约 0.2 个基本电荷的不确定度（均方根偏差）确定每个离子的电荷。我们在这里表明，通过使用内部标准对电荷测量进行动态校准，可以为大离子实现高精度的电荷测量。内部标准是通过用小天线照射检测圆柱，用射频信号产生的。使用这种方法，我们已经获得了约 5×10 的相对电荷不确定性，从而可以实现单个离子高达 500 个电荷的电荷状态分辨率。在这种方法的另一个应用中，检测圆柱被照射以抵消在 ELIT 端盖上的电势切换到捕获模式时产生的瞬变的信号。使用这种方法，切换后的死区时间（在此期间信号无法分析）减少了一个数量级以上。通过对高达 500 个电荷的离子进行电荷状态分辨率测量，我们能够精确地校准电荷。结果表明，在小部分基本电荷内，电荷灵敏放大器的动态校准响应是线性的。
