Utility of Higher Harmonics in Electrospray Ionization Fourier Transform Electrostatic Linear Ion Trap Mass Spectrometry

Abstract

Mass resolution (M/ΔM fwhm) is observed to linearly increase with harmonic order in a Fourier transform electrostatic linear ion trap (ELIT) mass spectrometer. This behavior was predicted by Grosshans and Marshall for frequency-multiple detection in a Fourier transform ion cyclotron resonance mass spectrometer only for situations when the prominent mechanism for signal decay is ion ejection from the trap. As the analyzer pressure in our ELIT chamber is relatively high, such that collisional scattering and collision-induced dissociation are expected to underlie much of the ion loss, we sought to explore the relationship between harmonic order and mass resolution. Mass resolutions of 36 900 (fundamental), 75 850 (2nd harmonic), and 108 200 (3rd harmonic) were obtained for GdO (avg. m/z 173.919) with a transient length of 300 ms. To demonstrate that the mass resolution was truly increasing with harmonic order, the unresolved isotopes at the fundamental distribution of cytochrome c (m/z ∼ 1549) were nearly baseline, resolved at the third harmonic (mass resolution ≈ 23 000) with a transient length of only 200 ms. This experiment demonstrates that, when the ion density is sufficiently low, ions with frequency differences of less than 4 Hz remain uncoalesced. Higher harmonics can be used to increase the effective mass resolution for a fixed transient length and thereby may enable the resolution of closely spaced masses, determination of a protein ion's charge state, and study of the onset of peak coalescence when the resolution at the fundamental frequency is insufficient.

High-Capacity Electrostatic Ion Trap with Mass Resolving Power Boosted by High-Order Harmonics

“…The insets show the local zoomin spectrum regions for the second, fourth, and sixth harmonics (marked as H 2 , H 4 , H 6 ). In the H 2 inset, both m + 1 (right) and m + 2 (left) peaks are shown, where the two isotope fine structures of 13 C 2 (526.261 Th) and 34 S (526.272 Th) just start to be resolved. They are fully resolved in H 4 and H 6 .…”

“…were created by segmenting the detector electrodes. In the ELIT, ,, the relative smaller size of the pick-up electrode compared to the length of the trap resulted in an image charge signal in pulsed waveform which contains abundant higher harmonics. Both technologies gave alternative ways to boost the resolving power per transient time without fly path length reduction.…”

Fourier-Transform MS and Closed-Path Multireflection Time-of-Flight MS Using an Electrostatic Linear Ion Trap

“…The dashed blue line represents R versus time up to 300 ms for an ion of m/z 316 under the FT-MS conditions used in our apparatus. We note that R has been observed to increase with the order of the detected harmonic, at least up to the third harmonic, in our apparatus, 37 which increases the slope of the resolution versus time relationship by a corresponding factor.…”

“…The point is that R increases with measurement time until the signal is lost due to collisions/ dephasing. (Based on a previously reported R = 36900 at the fundamental frequency for ions at m/z 173.9 (i.e., the base peak of the GdO + isotopic envelope) at a T acq = 300 ms, 37 C can be estimated to be 1.622 × 10 6 for the present apparatus.) Figure 2 includes plots of R versus time for various scenarios.…”

“…The latter two developments enabled the execution of a tandem MS experiment using the ELIT for mass-selection, dissociation, and mass analysis of the products. In the course of this work, we have expended effort to optimize the FT-ELIT mass measurement in our apparatus. − However, a particularly attractive feature of the ELIT is that it is amenable to both FT-MS and closed-path MR-TOF MS measurements. Both forms of mass analysis can be performed simultaneously on the same population of ions as the two types of measurements are independent.…”