Principles and operation methods of high performance, portable ion trap mass spectrometry

Wei Xu, Purdue University

Abstract

Mass spectrometry (MS) is widely used for chemical analysis in many areas, such as homeland security, medical diagnosis, pollution and environment study, molecule detection and identification from space, oil deposits detection, etc. Recently, lot efforts have been put on the miniature ion trap MS system development. Miniature ion trap MS system has the advantages of small dimension, light weight, low cost and easy to use, which makes it a good candidate for fast, onsite chemical detection. Basically there are two ways to miniaturize the ion trap MS system: (1) Decrease the ion trap dimension to lower the radio frequency (RF) voltage requirement and thereby the RF generation circuit; (2) Operate the ion trap at higher pressure regions, which will lower the requirements for the vacuum pumping system. By modeling the ion trajectory as a harmonic pseudo motion, theoretical models are developed, which provide us the ability to characterize the ion trap MS system performances at different operation conditions. Ion trap MS system performances, such as ion capture ability, ion trapping capacity and ion ejection resolution, of scaled ion traps under different buffer gas pressure are calculated. Results show that ion traps on millimeter scale have reasonable performances at relatively high pressure region (1-200 mtorr). Smaller ion traps (on the level of micrometer) can further lower down the RF voltage requirement, but surface roughness effects and array effects will become dominant failure mechanisms. To improve the performance and simplify the MS system, a discontinuous atmospheric pressure interface ion trap mass spectrometer (DAPI-MS) is built. By using high conductance, discrete ion introduction methods, high efficient ion transfer and high sensitivity is achieved on the instrument. The ion trapping and desolvation are further enhanced by using dual DAPI operations. Ion-molecule and ion-ion reactions (electron transfer dissociation, ETD) are demonstrated on the setup. A new method which is suitable to operate ion traps at high pressure is also developed based on the high order ion harmonic motion, image current detection method. Pico to Nano ampere level current detector, which can detect ion image current with the presence of AC and RF signal is designed and fabricated. The detector is then integrated with the DAPI-MS system built in house. Ion high order harmonic motion frequency is detected.

Degree

Ph.D.

Advisors

Ouyang, Purdue University.

Subject Area

Analytical chemistry|Electrical engineering

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