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

An electrostatic linear ion trap (ELIT) has been configured to allow for the simultaneous acquisition of mass spectra via Fourier transform (FT) techniques (frequency measurement) and via time-of-flight (TOF; time measurement). In the former case, the time-domain image charge derived from a pick-up electrode in the field-free region of the ELIT is converted to frequency-domain data via Fourier transformation (i.e., FT-ELIT MS). In the latter case, the time difference between ion injection into the ELIT and ion detection after release from the ELIT using a microchannel plate (MCP) enables the acquisition of multireflection time-of-flight mass spectra (MR-TOF MS). The ELIT geometry facilitates the acquisition of both types of data simultaneously because the detection schemes are independent and do not preclude one another. The two MS approaches exhibit a degree of complementarity. Resolution increases much faster with time with the MR-TOF approach, for example, but the closed-path nature of executing MR-TOF in an ELIT limits both the <i>m</i>/<i>z</i> range and the peak capacity. For this reason, the FT-ELIT MS approach is most appropriate for wide <i>m</i>/<i>z</i> range applications, whereas MR-TOF MS can provide advantages in a “zoom-in” mode in which moderate resolution (<i>M</i>/Δ<i>M</i>_fwhm ≈ 10000) at short analysis times (10 ms) is desirable

Alkali Cation Chelation in Cold β‑O‑4 Tetralignol Complexes

We employ cold ion spectroscopy (UV action and IR–UV double resonance) in the gas phase to unravel the qualitative structural elements of G-type alkali metal cationized (X = Li⁺, Na⁺, K⁺) tetralignol complexes connected by β-O-4 linkages. The conformation-specific spectroscopy reveals a variety of conformers, each containing distinct infrared spectra in the OH stretching region, building on recent studies of the neutral and alkali metal cationized β-O-4 dimers. The alkali metal ion is discovered to bind in penta-coordinate pockets to ether and OH groups involving at least two of the three β-O-4 linkages. Different binding sites are distinguished from one another by the number of M⁺···OH···O interactions present in the binding pocket, leading to characteristic IR transitions appearing below 3550 cm^–1. This interaction is mitigated in the major conformer of the K⁺ adduct, demonstrating a clear impact of the size of the charge center on the three-dimensional structure of the tetramer