Origin of Peaks in Overtone Mobility Spectrometry

Submitted to J. Am. Soc. Mass Spectrom. Oct. 2010

Revised Dec. 1, 2010

Overtone Mobility Spectrometry: Part 3. On the Origin of Peaks

Stephen J. Valentine, Ruwan T. Kurulugama,ᶧand David E. Clemmer*

Department of Chemistry, Indiana University, Bloomington, IN 47405

Supplementary Information

Ion beam segmentation demonstrated by ion trajectory simulations. As described in the manuscript, OMS devices segment the continuous ion beam into transmitted portions separated by gaps in the overall ion beam. Supplementary Figure 1A shows the OMS spectrum obtained from ion trajectory simulations of the Φ = 4 system. Supplementary Figures1B to 1H showthe ion transmission efficiency for different starting points of the ions for the major peaks observed in the OMS spectrum (Supplementary Figure 1A). Supplementary Figure 1B shows the ion transmission at the ff (1700 Hz) revealing a duty cycle for aΦ = 4 system of~75%. Supplementary Figures 1C, 1D and 1E correspond to the ion transmission at 5ff, 9ff and 13ff. The plots show that the initial ion beam is divided into smaller transmitted portions that travel through the device (See Figure 4 and manuscript text for details). The numbers of these transmitted portions of the ion beam originating in the initial d regions are 5, 9, and 13 for the 5ff, 9ff and 13ffanalyses, respectively. This periodic division of the ion beam provides insight into the dependence of ROMS on the major harmonics peaks as well as their observed intensities. For example, the decreasing size of the transmitted portions of the ion beam with increasing major harmonic peakrequires more stringent matching of ion mobilities to field application frequency. That is, ROMS increases with increasing mfor major harmonic peaks. These peak intensities also decrease with increasing m as the smaller transmitted portions of the initial ion beam are more susceptible to ion loss due to diffusion.

Supplementary Figures1F and 1G correspond to the ion transmission at 3ff and 7ff. Here, the initial ion beam is divided into 6 and 14 small ion transmission packets,respectively. The transmission packets created at these frequencies are smaller than those created at the surrounding primary harmonic frequencies. Because the peak widths in the OMS spectrum scale with the size of the ion transmission packets, these peaks are narrower than the surrounding primary harmonic peaks (i.e., ROMS is higher for these features). Again, the intensities for these features are smaller than the surrounding primary harmonic peaks due to increased diffusional losses for the smaller transmission packets. Supplementary Figure1H shows the transmission of ions at 2.3ff revealing the creation of 7 small ion transmission packets. Again, the small ion transmission packets are indicators of increased ROMS and decreased intensity at this frequency. As noted in the article, different numbers of initial ion gates are responsible for the generation of transmitted portions of the ion beam for the peaks in the different overtone series.

Comparison of the same overtone peaks generated by different OMS systems. Supplementary Figures2A and 2B showplots of ion transmission efficiency for starting ion positions and final grid numbers versus starting positions, respectively, for the 3ffovertone peak from a Φ = 2 system. Supplementary Figures 2C and 2D show the same plots for the 3ff overtone peak from a Φ = 4 system. Three ion transmission packets are created (Supplementary Figure 2A) when the Φ = 2OMS instrument is operated at this frequency. In comparison, six ion transmission packets are created (Supplementary Figure 2C) for theΦ = 4 OMS instrument at this frequency. However, the initial length of the ion beam used for the Φ = 2system simulation is two times smaller than that used for the Φ = 4 system. Therefore, the relative widths of the ion packets created during the operation of these two systems are equal. ROMSis thus approximately equal for the two 3ff peaks (Figure 2)even though the gates responsible for creation of the ion transmission regions are different for the two systems. The first two gates (grid numbers 80 and 165) are responsible for the generation of the ion transmission packets for the Φ = 2 system (Supplementary Figure 2B). For the Φ = 4 system, the first five de regions produce the transmission packets (Supplementary Figure 2D).

Supplementary Figures2E and 2F showplots of ion transmission efficiency for starting ion positions and final grid numbers versus starting positions, respectively,for the5ffovertone peak from a Φ = 2 system. Supplementary Figures 2H and 2G show the same plots for the 5ffovertone peak from a Φ = 4 system. Five ion transmission packets are created when the Φ = 2 OMS instrument is operated at this frequency and five ion packets are also created for the Φ = 4 OMS instrument. Because of the difference in length of the initial ion beam, the relative width of the ion transmission packets created for the Φ = 4system is twice as large as that created for the Φ = 2 system. Hence, the resulting5ff overtone peak for the Φ = 2 OMS system shows a ROMS value that is approximately twice that obtained for the 5ff overtone peak from the Φ = 4 OMS system (Figure 2). The peak intensity for the 5ff overtone peak from the Φ = 2 OMS system drops off sharply due to the pronounced effect of ion diffusion in the smaller ion transmission packet size.

Figure Captions

Supplementary Figure 1. A) Plot of percent ion transmission as a function of field application frequency. Data are obtained from ion trajectory simulations of a Φ = 4 system (see article for details). Peaks in the primary harmonic series are labeled for each system. For this virtual OMS device, 11 d regions have been employed. A model mobility of 0.09 m2·V-1·s-1 has been used for ions in the simulations. Peaks in the primary harmonic series are labeled B, C, D, and E. Peaks from higher-order overtone series are labeled F, G, and H. B-H) Plots of percent ion transmission versus initial starting position for ions in the simulation. Each plot corresponds with the peak of the same letter in Supplementary Figure 2A. Percent ion transmission corresponds to the fraction of ions at each starting point that pass through the virtual OMS device expressed as a percentage.

Supplementary Figure 1

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