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RationaleFree fatty acids and lipid classes containing fatty acid esters are major components of lipidome. In the absence of a chemical derivatization step, FA anions do not yield all of the structural information that may be of interest under commonly used collision‐induced dissociation (CID) conditions. A line of work that avoids condensed‐phase derivatization takes advantage of gas‐phase ion/ion chemistry to charge invert FA anions to an ion type that provides the structural information of interest using conventional CID. This work was motivated by the potential for significant improvement in overall efficiency for obtaining FA chain structural information. MethodsA hybrid triple quadrupole/linear ion‐trap tandem mass spectrometer that has been modified to enable the execution of ion/ion reaction experiments was used to evaluate the use of 4,4′,4″‐tri‐tert‐butyl‐2,2′:6′,2″‐terpyridine (ttb‐Terpy) as the ligand in divalent magnesium complexes for charge inversion of FA anions. ResultsMg(ttb‐Terpy)₂²⁺complexes provide significantly improved efficiency in producing structurally informative products from FA ions relative to Mg(Terpy)₂²⁺complexes, as demonstrated for straight‐chain FAs, branched‐chain FAs, unsaturated FAs, and cyclopropane‐containing FAs. It was discovered that most of the structurally informative fragmentation from [FA‐H + Mg(ttb‐Terpy)]⁺results from the loss of a methyl radical from the ligand followed by radical‐directed dissociation (RDD), which stands in contrast to the charge‐remote fragmentation (CRF) believed to be operative with the [FA‐H + Mg(Terpy)]⁺ions. ConclusionsThis work demonstrates that a large fraction of product ions from the CID of ions of the form [FA‐H + Mg(ttb‐Terpy)]⁺are derived from RDD of the FA backbone, with a very minor fraction arising from structurally uninformative dissociation channels. This ligand provides an alternative to previously used ligands for the structural characterization of FAs via CRF. 

RationaleThe electrostatic linear ion trap (ELIT) can be operated as a multi‐reflection time‐of‐flight (MR‐TOF) or Fourier transform (FT) mass analyzer. It has been shown to be capable of performing high‐resolution mass analysis and high‐resolution ion isolations. Although it has been used in charge‐detection mass spectrometry (CDMS), it has not been widely used as a conventional mass spectrometer for ensemble measurements of ions, or for tandem mass spectrometer. The advantages of tandem mass spectrometer with high‐resolution ion isolations in the ELIT have thus not been fully exploited. MethodsA homebuilt ELIT was modified with BaF₂viewports to facilitate transmission of a laser beam at the turnaround point of the second ion mirror in the ELIT. Fragmentation that occurs at the turnaround point of these ion mirrors should result in minimal energy partitioning due to the low kinetic energy of ions at these points. The laser was allowed to irradiate ions for a period of many oscillations in the ELIT. ResultsDue to the low energy absorption of gas‐phase ions during each oscillation in the ELIT, fragmentation was found to occur over a range of oscillations in the ELIT generating a homogeneous ion beam. A mirror‐switching pulse is shown to create time‐varying perturbations in this beam that oscillate at the fragment ion characteristic frequencies and generate a time‐domain signal. This was found to recover FT signal for protonated pYGGFL and pSGGFL precursor ions. ConclusionsFragmentation at the turnaround point of an ELIT by continuous‐wave infrared multiphoton dissociation (cw‐IRMPD) is demonstrated. In cases where laser power absorption is low and fragmentation occurs over many laps, a mirror‐switching pulse may be used to recover varying time‐domain signal. The combination of laser activation at the turnaround points and mirror‐switching isolation allows for tandem MS in the ELIT.