The reactivity of a carbon‐centered σ,σ,σ,σ‐type singlet‐ground‐state tetraradical containing two meta‐benzyne moieties was examined in the gas phase. Surprisingly, the tetraradical showed higher reactivity than its individual meta‐benzyne counterparts. The reactivity of meta‐benzynes is controlled by their (calculated) distortion energy ΔE2.3, singlet–triplet spitting ΔES–T, and electron affinity (EA2.3) of the meta‐benzyne moiety at the transition state geometry for hydrogen‐atom abstraction reactions. The addition of a second meta‐benzyne moiety to a meta‐benzyne does not significantly change EA2.3. However, ΔE2.3 is substantially decreased for both meta‐benzyne moieties in the tetraradical, and this explains their higher reactivities. The decrease in ΔE2.3 for each meta‐benzyne moiety in the tetraradical is rationalized by stabilizing spin–spin coupling between one radical site in each meta‐benzyne moiety. Therefore, spin–spin coupling between the meta‐benzyne moieties in this tetraradical increases its reactivity, whereas spin–spin coupling within each meta‐benzyne moiety decreases its reactivity.

A two-dimensional tandem mass spectrometry (2D MS/MS) scan has been developed for the linear quadrupole ion trap. Precursor ions are mass-selectively excited using a nonlinear ac frequency sweep at constant rf voltage, while simultaneously, all product ions of the excited precursor ions are ejected from the ion trap using a broad-band waveform. The fragmentation time of the precursor ions correlates with the precursor m/z value (the first mass dimension) and also with the ejection time of the product ions, allowing the correlation between precursor and product ions. Additionally, the second mass dimension (product ions’ m/z values) is recovered through fast Fourier transform of each mass spectral peak, revealing either intentionally introduced “frequency tags” or the product ion micropacket frequencies, both of which can be converted to product ion m/z through the classical Mathieu parameters, thereby revealing a product ion mass spectrum for every precursor ion without prior isolation. We demonstrate the utility of this method for analyzing a broad range of structurally related precursor ions, including chemical warfare agent simulants, fentanyls and other opioids, amphetamines, cathinones, antihistamines, and tetracyclic antidepressants.

Ion trap mass spectrometers have emerged as powerful on-site analytical platforms, in spite of limited mass resolution, due to their compatibility with ambient ionization methods and ready implementation of tandem mass spectrometry (MS/MS). When operated at constant trapping voltage, ions can be activated at their secular frequencies and all MS/MS experiments can be performed, including the two-dimensional tandem mass scan (2D MS/MS scan) in which all precursor ions and their subsequent product ions are both identified and correlated. In the new method of performing this 2D MS/MS experiment presented here, the precursor ions are excited by a nonlinear (inverse Mathieu q) frequency sweep while the resulting product ions are identified by their ejection time within a repeating orthogonally applied nonlinear (inverse Mathieu q) frequency sweep. This resulting compact representation contains the total fragmentation behavior of a collection of ionized compounds and captures detailed chemical information efficiently (typically in 1 s). The approach is implemented using a simple single mass analyzer instrument. This methodology was tested on three different multicomponent mixtures: drugs of abuse, peptides, and fentanyl analogs. The data are compared with those obtained by more common MS/MS scan methods.

The gas-phase reactivities of several protonated quinoline-based σ-type (carbon-centered) mono-, bi-, and triradicals toward dimethyl disulfide (DMDS) were studied by using a linear quadrupole ion trap mass spectrometer. The mono- and biradicals produce abundant thiomethyl abstraction products and small amounts of DMDS radical cation, as expected. Surprisingly, all triradicals produce very abundant DMDS radical cations. A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the (experimental) adiabatic ionization energy of DMDS is almost 3 eV greater than the (calculated) adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical. Quantum chemical calculations as well as examination of deuterated and methylated triradicals provide support for this mechanism. The proton affinities of the neutral triradicals (and DMDS) influence the first step of the reaction while the vertical electron affinities and spin–spin coupling of the neutral triradicals influence the second step. The calculated total reaction exothermicities for the triradicals studied range from 27.6 up to 29.9 kcal mol–1.

Methods of performing precursor ion scans as well as neutral loss scans in a single linear quadrupole ion trap have recently been described. In this paper we report methodology for performing permutations of MS/MS scan modes, that is, ordered combinations of precursor, product, and neutral loss scans following a single ion injection event. Only particular permutations are allowed; the sequences demonstrated here are (1) multiple precursor ion scans, (2) precursor ion scans followed by a single neutral loss scan, (3) precursor ion scans followed by product ion scans, and (4) segmented neutral loss scans. (5) The common product ion scan can be performed earlier in these sequences, under certain conditions. Simultaneous scans can also be performed. These include multiple precursor ion scans, precursor ion scans with an accompanying neutral loss scan, and multiple neutral loss scans. We argue that the new capability to perform complex simultaneous and sequential MS operations on single ion populations represents a significant step in increasing the selectivity of mass spectrometry.

Described herein are novel sets of multiplexed MS/MS scans – termed ‘logical scans’ – performed on a linear ion trap.

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Implementation of orthogonal double resonance precursor and neutral loss scans on the Mini 12 miniature rectilinear ion trap mass spectrometer is described, and performance is compared to that of a commercial Thermo linear trap quadropole (LTQ) linear ion trap. The ac frequency scan version of the technique at constant rf voltage is used here because it is operationally much simpler to implement. Remarkably, the Mini 12 shows up to two orders of magnitude higher sensitivity compared to that of the LTQ. Resolution on the LTQ is better than unit at scan speeds of ~ 400 Th/s, whereas peak widths on the Mini 12, on average, range from 0.5 to 2.0 Th full width at half maximum and depend heavily on the precursor ion Mathieu q parameter as well as the pump down time that precedes the mass scan. Both sensitivity and resolution are maximized under higher pressure conditions (short pump down time) on the Mini 12. The effective mass range of the product ion ejection waveform was found to be 5.8 Th on the Mini 12 in the precursor ion scan mode vs. that of 3.9 Th on the LTQ. In the neutral loss scan mode, the product ion selectivity was between 8 and 11 Th on the Mini 12 and between 7 and 8 Th on the LTQ. The effects of nonlinear resonance lines on the Mini 12 were also explored. Graphical Abstract ᅟ.