Shinholt Deven L, Anthony Staci N, Alexander Andrew W, Draper Benjamin E, Jarrold Martin F
Department of Chemistry, Indiana University, 800 East Kirkwood Ave., Bloomington, Indiana 47405, USA.
Rev Sci Instrum. 2014 Nov;85(11):113109. doi: 10.1063/1.4900627.
Quadrupole mass filters (QMFs) are usually not used to analyze high m/z ions, due to the low frequency resonant circuit that is required to drive them. Here we describe a new approach to generating waveforms for QMFs. Instead of scanning the amplitude of a sine wave to measure the m/z spectrum, the frequency of a trapezoidal wave is digitally scanned. A synchronous, narrow-range (<0.2%) amplitude scan overlays the frequency scan to improve the sampling resolution. Because the frequency is the primary quantity that is scanned, there is, in principle, no upper m/z limit. The frequency signal is constructed from a stabilized base clock using a field programmable gate array. This signal drives integrating amplifiers which generate the trapezoidal waves. For a trapezoidal wave the harmonics can be minimized by selecting the appropriate rise and fall times. To achieve a high resolving power, the digital signal has low jitter, and the trapezoidal waveform is generated with high fidelity. The QMF was characterized with cesium iodide clusters. Singly and multiply charged clusters with z up to +5 were observed. A resolving power of ∼1200 (FWHM) was demonstrated over a broad m/z range. Resolution was lost above 20,000 Th, partly because of congestion due to overlapping multiply charged clusters. Ions were observed for m/z values well in excess of 150,000 Th.
四极杆质量过滤器（QMFs）通常不用于分析高m/z离子，这是因为驱动它们需要低频谐振电路。在此，我们描述一种为QMFs生成波形的新方法。不是扫描正弦波的幅度来测量m/z谱，而是对梯形波的频率进行数字扫描。同步的窄范围（<0.2%）幅度扫描叠加在频率扫描上以提高采样分辨率。由于频率是被扫描的主要量，原则上不存在m/z上限。频率信号由使用现场可编程门阵列的稳定基准时钟构建而成。该信号驱动产生梯形波的积分放大器。对于梯形波，通过选择合适的上升和下降时间可使谐波最小化。为实现高分辨能力，数字信号具有低抖动，并且以高保真度产生梯形波形。使用碘化铯团簇对QMF进行了表征。观察到了z高达+5的单电荷和多电荷团簇。在很宽的m/z范围内展示了约1200（半高宽）的分辨能力。在高于20,000 Th时分辨率下降，部分原因是多电荷团簇重叠导致的峰拥挤。观察到了m/z值远超过150,000 Th的离子。
