Evaluation of ultraviolet photodissociation tandem mass spectrometry for the structural assignment of unsaturated fatty acid double bond positional isomers

Shedding light on isomeric FAHFA lipid structures using 213 nm ultraviolet photodissociation mass spectrometry

“…This is consistent with a recently described report on the non-competitive formation of diagnostic C ¼ C double bond product ions from [M þ NH 4 ] þ adducts of unsaturated fatty acids using 193 nm UVPD-MS/MS. 57 In contrast, and consistent with our recent report on the UVPD fragmentation behaviour of unsaturated fatty acids, 57 These UVPD-MS 3 product ions allow for localization of C ¼ C double bonds at the D8 and D9 positions. Since the D8 and D9 18:1 unsaturated fatty acids produced by HCD both share a carbon-carbon double bond featuring the 9th carbon in the fatty acid chain, the site of esterification within the original PAHSA lipid can therefore be unambiguously be assigned at the 9th carbon.…”

“…This effect appears to be less pronounced with the [MÀH þ 2Li] þ adduct, since it produces a greater abundance of UVPD specific product ions when compared with the [M þ Li] þ adduct. Therefore, despite an overall decrease in absolute sensitivity, and consistent with that recently reported for UVPD-MS/MS of fatty acids, 57 dissociation of the [MÀH þ 2Li] þ adduct compared to [M þ Li] þ adduct is preferred for complete FAHFA lipid structure determination using the UVPD-MS 3 workflow.…”

“…This differs from our earlier report on 193 UVPD-MS/MS of di-lithiated fatty acids. 57 We attribute this as being due to the ester linkage within the FAHFA lipid being the more favored site of photon absorption, and/or the energetically preferable fragmentation pathway. Despite this, the position of esterification, and the endogenous site of unsaturation in the fatty acid feature of 9-OAHSA could be determined through HCD-UVPD-MS 3 .…”

“…49 193 nm and 213 nm UVPD-MS/MS have been shown to facilitate near complete structural characterisation across multiple lipid categories and classes, including identification of positions of unsaturation via the observation of pairs of product ions formed by cleavage of allylic bond positions that differ in mass by 24 Da. [50][51][52][53][54][55][56][57][58][59][60] Sequential 'hybrid' MS n dissociation using HCD-MS/MS followed by UVPD-MS 3 has been shown to identify both unsaturation and sn-linkage positions in certain glycerophospholipids, [52][53][54] while most recently a reverse 'hybrid' UVPD-MS/MS and HCD-MS 3 approach has been shown to enable the improved structural characterisation of cholesterol lipids, including assignment of the cholesterol backbone, differentiation of isomeric of oxy-cholesterols, and localization of the site(s) of unsaturation within the acyl chains of cholesterol esters. 60 To date, however, UVPD-MS/MS or -MS n workflows have not been applied to FAHFA lipids.…”

In situ detection of fatty acid C=C positional isomers by coupling on‐tissue mCPBA epoxidation with infrared matrix‐assisted laser desorption electrospray ionization mass spectrometry

“…Several advanced fragmentation techniques are capable of providing richer fragmentation spectra from which more thorough structural information can be derived. Ultraviolet photodissociation (UVPD) at 193 nm of unsaturated lipids enables a high‐energy photoactivation process, resulting in the cleavage of C‐C bonds adjacent to a C=C bond and yielding a diagnostic pair of ions with 24.0000 Da difference 9,14 . Ozone‐induced dissociation (OzID) involves trapping of isolated ions with ozone vapor in an ion trap mass spectrometer, allowing for gas‐phase ozonolysis of unsaturated bonds 15,16 .…”