Interfacing UV-IMS to time of flight mass spectrometer: simion analysis and experimental validation
ABSTRACT Ion mobility spectrometers are used to characterize chemical substances through the mobility of gas-phase ions in weak electric fields and provide details about the size and shape of ions but not unequivocally the mass of an ion. In order to fully understand ion behavior or reaction chemistry at ambient pressure, the union of a mobility spectrometer with a time-of-flight mass spectrometer (TOF-MS) was deemed useful for such mass-identification of simple ions, fragment ions, or ion clusters formed within the IMS. A critical component in such a combination of mobility and mass spectrometers is the interface between ambient pressure and high vacuum. Particularly important are the compromises between ion yield and maintenance of vacuum as found in the geometric and electrical aspects of pinhole and lens assemblies. In the present paper studies have been made using different size of pinhole and vacuum pumps and results will be compared to results from modeling studies. The ion paths from modeling will be compared to ions yields obtained using a Faraday plate inside the vacuum chamber. INTRODUCTION Several groups have used recently drift tubes of IMS coupled with TOF-MS instruments?1,2,3?. In these instances, a principal interest has been the measurement of bio-molecules through electrospray ion sources with low pressure drift tubes. In our laboratory, a main interest is the gas phase ion chemistry associated with photo-discharge ionization using ultraviolet discharge lamps with sample in air at ambient pressure. A particular configuration of these experiments has been a IMS drift tube with a 10.6 eV UV lamp. Since a continuous ion source at ambient pressure might be coupled directly to the TOF mass spectrometer, the ion injection geometry should be configured for orthogonal ion injection. In this paper, ion trajectories between ambient pressure and the lens assembly inside the interface region of a TOF-MS were simulated using SIMION 7.0. Simulations were compared to experimental findings with a vacuum chamber intended for eventual use as the design for a UV IMS-TOF-MS instrument.