Ion-molecule adduct formation in tandem mass spectrometry

Abstract

Nowadays most LC-MS methods rely on tandem mass spectrometry not only for quantitation and confirmation of compounds by multiple reaction monitoring (MRM), but also for the identification of unknowns from their product ion spectra. However, gas-phase reactions between charged and neutral species inside the mass analyzer can occur, yielding product ions at m/z values higher than that of the precursor ion, or at m/z values difficult to explain by logical losses, which complicate mass spectral interpretation. In this work, the formation of adduct ions in the mass analyzer was studied using several mass spectrometers with different mass analyzers (ion trap, triple quadrupole, and quadrupole-Orbitrap). Heterocyclic amines (AαC, MeAαC, Trp-P-1, and Trp-P-2), photo-initiators (BP and THBP), and pharmaceuticals (phenacetin and levamisole) were selected as model compounds and infused in LCQ Classic, TSQ Quantum Ultra AM, and Q-Exactive Orbitrap (ThermoFisher Scientific) mass spectrometers using electrospray as ionization method. The generation of ion-molecule adducts depended on the compound and also on the instrument employed. Adducts with neutral organic solvents (methanol and acetonitrile) were only observed in the ion trap instrument (LCQ Classic), because of the ionization source on-axis configuration and the lack of gas-phase barriers, which allowed inertial entrance of the neutrals into the analyzer. Adduct formation (only with water) in the triple quadrupole instruments was less abundant than in the ion trap and quadrupole-Orbitrap mass spectrometers, because of the lower residence time of the reactive product ions in the mass analyzer. The moisture level of the CID and/or damper gas had a great effect in beam-like mass analyzers such as triple quadrupole, but not in trap-like mass analyzers, probably because of the long residence time that allowed adduct formation even with very low concentrations of water inside the mass spectrometer.

Comprehensive investigation on synthetic cannabinoids: Metabolic behavior and potency testing, using 5F‐APP‐PICA and AMB‐FUBINACA as model compounds

“…An additional fragment is observed at m/z 271.0877 (Fragment 3, C 15 H 12 FN 2 O 2 + , −0.09 ppm), obtained after the addition of a water molecule to Fragment 4. The generation of adducts between unstable ions and neutral molecules, specially water molecules, has been previously reported for Q‐Orbitrap instruments . As stated by those authors, these adducts are less promoted in QTOF instruments, due to the different geometry of the collision cell.…”

Updating the list of known opioids through identification and characterization of the new opioid derivative 3,4-dichloro-N-(2-(diethylamino)cyclohexyl)-N-methylbenzamide (U-49900)

“…This fragment seems to be produced by an adduct formation with a neutral molecule present in the collision cell. The formation of fragment-adducts with small molecules in the collision cell has been recently described in literature 20 . According to literature, water adducts are the most commonly observed, due to water traces coming from the same instrument or CID gas impurities 20 .…”

“…The formation of fragment-adducts with small molecules in the collision cell has been recently described in literature 20 . According to literature, water adducts are the most commonly observed, due to water traces coming from the same instrument or CID gas impurities 20 . Based on this study, the fragment at m/z 127 would come from the dichlorophenyl fragment ( m/z 145), experiencing a neutral HCl loss followed by a water addition, and subsequent formation of the 2-chlorophenol fragment.…”

Identification of small molecules using accurate mass MS/MS search

“…These compound spectra should be acquired under different CID or HCD voltage settings (10, 20, 40, and 60 eV) in positive and negative ionization mode to acquire rich mass spectral fragmentations. Different molecular species beyond simply [M + H] + and [M − H] − should be included for each compound to capture the whole width and breadth of possible adducts . That includes a variety of mobile phase modifiers and solvent related ions as well as sodium and ammonium adducts or commonly observed water loss .…”