Ion-Neutral Collision Effects on Ion Trapping and Pseudopotential Depth in Ion Trap Mass Spectrometry
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J. Am. Soc. Mass Spectrom. (2019) DOI: 10.1007/s13361-019-02344-x
RESEARCH ARTICLE
Ion-Neutral Collision Effects on Ion Trapping and Pseudopotential Depth in Ion Trap Mass Spectrometry Ming Li,1,2 Xinwei Liu,2 Xiaoyu Zhou,2
Zheng Ouyang2
1
NCS Testing Technology CO., Ltd, Beijing, 100081, China State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing, 100084, China 2
D [V]
Abstract. Ion trapping using radio-frequency (RF) devices has been widely used in mass spectrom200 etry (MS). The pseudopotential well (PW) model enables the use of a pseudopotential depth, D, to evaluate the ion trapping capability of the RF 100 devices in the pure electric field. It remains unclear how gas pressures regulate the ion trapping and D. Here, we calculated the D of a linear ion 0 trap (LIT) from 1 mTorr to 2 Torr, a pressure range 0.0 0.5 1.0 critical for the operation of the RF devices, through ion cloud simulations. Compared with the case of pure electric field, ion-neutral collision effects at pressures of 1 to 100 mTorr were beneficial for the ion trapping and revealed an optimal trapping depth, D, at around 10 mTorr. We explained the mechanism and validated the observation via ion trapping experiments performed in a home-made dual LIT mass spectrometer. We also showed that near the stability boundary, the RF heating became comparable with the D, which led to the decrement of ion trapping capability characterized by the available D. Keywords: RF devices, Ion trap, Ion-neutral collisions, Pseudopotential depth
1 mTorr 10 mTorr 100 mTorr 2 Torr
q
0 Torr
Received: 25 July 2019/Revised: 7 September 2019/Accepted: 25 September 2019
Introduction
R
adio-frequency (RF) devices originated from the quadrupole ion trap are developed by Paul and Steinwedel in the 1950s [1]. They have been widely employed for ion trapping in mass spectrometry (MS) [2, 3], including ion trap [4–6], mass filter [4–6], ion guide [4–6], ion funnel [7], and ion mobility analyzer [8]. In the RF devices, the RF field varies with time periodically; the ions can be stably trapped within one half of the RF cycle and become unstable in the other half. To trap the ions in the entire RF cycle, the RF phases should change timely before the ions escape from the
Electronic supplementary material The online version of this article (https:// doi.org/10.1007/s13361-019-02344-x) contains supplementary material, which is available to authorized users. Correspondence to: Xiaoyu Zhou; e-mail: [email protected], Zheng Ouyang; e-mail: [email protected]
devices. Therefore, compared with that using direct current (DC), ion motion in the RF field is more complex. It includes a series of micro-harmonics which do not exist in the DC trapping, other than the secular (or macro-) harmonic. How to evaluate the ion trapping capability of the RF devices, in analogy to the DC counterpart, is a fundamental but very practical question. The pseudopotential well (PW) model [9–11]
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