Overtone mobility spectrometry: Part 1. Experimental observations

Abstract

A new method that allows a linear drift tube to be operated as a continuous ion mobility filter is described. Unlike conventional ion mobility instruments that use an electrostatic gate to introduce a packet of ions into a drift region, the present approach uses multiple segmented drift regions with modulated drift fields to produce conditions that allow only ions with appropriate mobilities to pass through the instrument. In this way, the instrument acts as a mobility filter for continuous ion sources. By changing the frequency of the applied drift fields it is possible to tune this instrument to transmit ions having different mobilities. A scan over a wide range of drift field frequencies for a single ion species shows a peak corresponding to the expected resonance time of the ions in one drift region segment and a series of peaks at higher frequencies that are overtones of the resonant frequency. The measured resolving power increases for higher overtones, making it possible to resolve structures that were unresolved in the region of the fundamental frequency. We demonstrate the approach by examining oligosaccharide isomers, raffinose and melezitose as well as a mixture of peptides obtained from enzymatic digestion of myoglobin. S eparation of a mixture of ions by ion mobility spectrometry (IMS) in linear drift tubes is initiated by gating short packets of ions into the front of a drift region containing a static buffer gas. Different species then migrate through the drift region under the influence of an applied field according to their mobilities in the buffer gas, such that ions with high mobilities reach the detector before those with low mobilities [1][2][3][4][5][6]. This approach has attracted considerable attention as a means of analyzing complex mixtures [7][8][9][10][11][12][13][14][15][16]. Additionally, measured mobilities can be converted to experimental cross sections and comparison of these values with cross sections for model geometries that are calculated can be used to characterize ion structures [17][18][19][20][21][22][23][24][25][26][27][28].In this report we introduce a new approach for isolating ions having specific mobilities (or collision cross sections). Ions from a continuous source enter a drift tube with segmented drift regions. The drift fields are modulated at a frequency that allows only those ions having mobilities that are resonant with the experimental conditions to be transmitted through all drift regions. In this way, this device filters away all ions except those with mobilities over the selected narrow range. An unanticipated feature of this approach is the observation that ions can be passed at overtone frequencies; moreover, the resolving power in higher overtone regions is greater than that observed in the fundamental frequency range. Because of the ability to select ions in different frequency regions, including those that are associated with higher overtones, we refer to the approach as overtone mobility spectrometry (OMS).To demonstrate this approach we have examin...

Towards measuring ion mobilities in non-stationary gases and non-uniform and dynamic electric fields (I). Transport equation

“…Recent advances in ion mobility and mass spectrometry instrumentation [1][2][3][4][5][6][7][8][9] have transformed IMS-MS into a tool that is now routinely used to determine structures of molecules in a wide range of sizes and disciplines.…”

Altering the mobility-time continuum: nonlinear scan functions for targeted high resolution trapped ion mobility-mass spectrometry

“…Alternatively, the experimental setup for overtone mobility spectrometry (OMS) represents a hybrid approach [20]. Introduced by Clemmer and coworkers, OMS operates a drift tube in a filter mode by supplanting the conventional electrostatic uniform electric field with an electrodynamic field, thus restricting transmission of all ions except those contained in a narrow range of mobilities that are resonant with the field application frequency ( f ).…”

Selected Overtone Mobility Spectrometry

“…The method of ion selection and transmission and of spectrum acquisition in OMS has been described in more detail previously. [24][25][26]30 One limitation of OMS is that the frequencies used to transmit the same ions at different values of m may overlap.…”

“…23−30 OMS operates under the same conditions as linear drift tube IMS, separating ions by a mobility from which a collision cross section can be calculated, a parameter directly correlated to structure. 23,25 The resolving power of OMS scales nearly linearly with the length of the drift tube 24,25 (compared with the square root dependence upon length that is observed for IMS) 31 while also acting as a filter in a manner analogous to a quadrupole mass spectrometer opening up potential analogous experiments with drift tubes. OMS uses alternating fields and a segmented drift tube with each segment containing a transmission region of length ( t ) and an elimination region of length ( e ).…”

“…The method of ion selection and transmission and of spectrum acquisition in OMS has been described in more detail previously. [24][25][26]30 One limitation of OMS is that the frequencies used to transmit the same ions at different values of m may overlap. This is especially problematic when working in high-overtone regions, which is desirable because of increases in resolving power that occur with increasing m. 24,25 The inability to unambiguously assign an overtone to specific peaks in regions where they may overlap complicates the interpretation of data, especially when analyzing complex mixtures.…”